diff mbox series

[v6] block: disable iopoll for split bio

Message ID 20201125064147.25389-1-jefflexu@linux.alibaba.com (mailing list archive)
State New, archived
Headers show
Series [v6] block: disable iopoll for split bio | expand

Commit Message

Jeffle Xu Nov. 25, 2020, 6:41 a.m. UTC
iopoll is initially for small size, latency sensitive IO. It doesn't
work well for big IO, especially when it needs to be split to multiple
bios. In this case, the returned cookie of __submit_bio_noacct_mq() is
indeed the cookie of the last split bio. The completion of *this* last
split bio done by iopoll doesn't mean the whole original bio has
completed. Callers of iopoll still need to wait for completion of other
split bios.

Besides bio splitting may cause more trouble for iopoll which isn't
supposed to be used in case of big IO.

iopoll for split bio may cause potential race if CPU migration happens
during bio submission. Since the returned cookie is that of the last
split bio, polling on the corresponding hardware queue doesn't help
complete other split bios, if these split bios are enqueued into
different hardware queues. Since interrupts are disabled for polling
queues, the completion of these other split bios depends on timeout
mechanism, thus causing a potential hang.

iopoll for split bio may also cause hang for sync polling. Currently
both the blkdev and iomap-based fs (ext4/xfs, etc) support sync polling
in direct IO routine. These routines will submit bio without REQ_NOWAIT
flag set, and then start sync polling in current process context. The
process may hang in blk_mq_get_tag() if the submitted bio has to be
split into multiple bios and can rapidly exhaust the queue depth. The
process are waiting for the completion of the previously allocated
requests, which should be reaped by the following polling, and thus
causing a deadlock.

To avoid these subtle trouble described above, just disable iopoll for
split bio.

Suggested-by: Ming Lei <ming.lei@redhat.com>
Signed-off-by: Jeffle Xu <jefflexu@linux.alibaba.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
---
 block/bio.c               |  2 ++
 block/blk-merge.c         | 12 ++++++++++++
 block/blk-mq.c            |  3 +++
 include/linux/blk_types.h |  1 +
 4 files changed, 18 insertions(+)

Comments

Ming Lei Nov. 25, 2020, 7:19 a.m. UTC | #1
On Wed, Nov 25, 2020 at 02:41:47PM +0800, Jeffle Xu wrote:
> iopoll is initially for small size, latency sensitive IO. It doesn't
> work well for big IO, especially when it needs to be split to multiple
> bios. In this case, the returned cookie of __submit_bio_noacct_mq() is
> indeed the cookie of the last split bio. The completion of *this* last
> split bio done by iopoll doesn't mean the whole original bio has
> completed. Callers of iopoll still need to wait for completion of other
> split bios.
> 
> Besides bio splitting may cause more trouble for iopoll which isn't
> supposed to be used in case of big IO.
> 
> iopoll for split bio may cause potential race if CPU migration happens
> during bio submission. Since the returned cookie is that of the last
> split bio, polling on the corresponding hardware queue doesn't help
> complete other split bios, if these split bios are enqueued into
> different hardware queues. Since interrupts are disabled for polling
> queues, the completion of these other split bios depends on timeout
> mechanism, thus causing a potential hang.
> 
> iopoll for split bio may also cause hang for sync polling. Currently
> both the blkdev and iomap-based fs (ext4/xfs, etc) support sync polling
> in direct IO routine. These routines will submit bio without REQ_NOWAIT
> flag set, and then start sync polling in current process context. The
> process may hang in blk_mq_get_tag() if the submitted bio has to be
> split into multiple bios and can rapidly exhaust the queue depth. The
> process are waiting for the completion of the previously allocated
> requests, which should be reaped by the following polling, and thus
> causing a deadlock.
> 
> To avoid these subtle trouble described above, just disable iopoll for
> split bio.
> 
> Suggested-by: Ming Lei <ming.lei@redhat.com>
> Signed-off-by: Jeffle Xu <jefflexu@linux.alibaba.com>
> Reviewed-by: Christoph Hellwig <hch@lst.de>
> ---
>  block/bio.c               |  2 ++
>  block/blk-merge.c         | 12 ++++++++++++
>  block/blk-mq.c            |  3 +++
>  include/linux/blk_types.h |  1 +
>  4 files changed, 18 insertions(+)
> 
> diff --git a/block/bio.c b/block/bio.c
> index fa01bef35bb1..7f7ddc22a30d 100644
> --- a/block/bio.c
> +++ b/block/bio.c
> @@ -684,6 +684,8 @@ void __bio_clone_fast(struct bio *bio, struct bio *bio_src)
>  	bio_set_flag(bio, BIO_CLONED);
>  	if (bio_flagged(bio_src, BIO_THROTTLED))
>  		bio_set_flag(bio, BIO_THROTTLED);
> +	if (bio_flagged(bio_src, BIO_SPLIT))
> +		bio_set_flag(bio, BIO_SPLIT);
>  	bio->bi_opf = bio_src->bi_opf;
>  	bio->bi_ioprio = bio_src->bi_ioprio;
>  	bio->bi_write_hint = bio_src->bi_write_hint;
> diff --git a/block/blk-merge.c b/block/blk-merge.c
> index bcf5e4580603..a2890cebf99f 100644
> --- a/block/blk-merge.c
> +++ b/block/blk-merge.c
> @@ -279,6 +279,18 @@ static struct bio *blk_bio_segment_split(struct request_queue *q,
>  	return NULL;
>  split:
>  	*segs = nsegs;
> +
> +	/*
> +	 * Bio splitting may cause subtle trouble such as hang when doing sync
> +	 * iopoll in direct IO routine. Given performance gain of iopoll for
> +	 * big IO can be trival, disable iopoll when split needed. We need
> +	 * BIO_SPLIT to identify bios need this workaround. Since currently
> +	 * only normal IO under mq routine may suffer this issue, BIO_SPLIT is
> +	 * only marked here.
> +	 */
> +	bio->bi_opf &= ~REQ_HIPRI;
> +	bio_set_flag(bio, BIO_SPLIT);
> +
>  	return bio_split(bio, sectors, GFP_NOIO, bs);
>  }
>  
> diff --git a/block/blk-mq.c b/block/blk-mq.c
> index 55bcee5dc032..ce1f3628e4c2 100644
> --- a/block/blk-mq.c
> +++ b/block/blk-mq.c
> @@ -2265,6 +2265,9 @@ blk_qc_t blk_mq_submit_bio(struct bio *bio)
>  		blk_mq_sched_insert_request(rq, false, true, true);
>  	}
>  
> +	if (bio_flagged(bio, BIO_SPLIT))
> +		return BLK_QC_T_NONE;
> +

Not sure the new bio flag is really required for this case, just wondering
why not take the following simple way? BTW we are really going to run
out of bio flag.

diff --git a/block/blk-mq.c b/block/blk-mq.c
index 55bcee5dc032..1139b1efd712 100644
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -2157,6 +2157,7 @@ blk_qc_t blk_mq_submit_bio(struct bio *bio)
 	unsigned int nr_segs;
 	blk_qc_t cookie;
 	blk_status_t ret;
+	struct bio *orig_bio = bio;
 
 	blk_queue_bounce(q, &bio);
 	__blk_queue_split(&bio, &nr_segs);
@@ -2265,6 +2266,10 @@ blk_qc_t blk_mq_submit_bio(struct bio *bio)
 		blk_mq_sched_insert_request(rq, false, true, true);
 	}
 
+	/* don't poll splitted bio */
+	if (orig_bio != bio)
+		return BLK_QC_T_NONE;
+
 	return cookie;
 queue_exit:
 	blk_queue_exit(q);

Thanks,
Ming
Christoph Hellwig Nov. 25, 2020, 7:48 a.m. UTC | #2
On Wed, Nov 25, 2020 at 03:19:44PM +0800, Ming Lei wrote:
> Not sure the new bio flag is really required for this case, just wondering
> why not take the following simple way? BTW we are really going to run
> out of bio flag.
> 
> diff --git a/block/blk-mq.c b/block/blk-mq.c
> index 55bcee5dc032..1139b1efd712 100644
> --- a/block/blk-mq.c
> +++ b/block/blk-mq.c
> @@ -2157,6 +2157,7 @@ blk_qc_t blk_mq_submit_bio(struct bio *bio)
>  	unsigned int nr_segs;
>  	blk_qc_t cookie;
>  	blk_status_t ret;
> +	struct bio *orig_bio = bio;
>  
>  	blk_queue_bounce(q, &bio);
>  	__blk_queue_split(&bio, &nr_segs);
> @@ -2265,6 +2266,10 @@ blk_qc_t blk_mq_submit_bio(struct bio *bio)
>  		blk_mq_sched_insert_request(rq, false, true, true);
>  	}
>  
> +	/* don't poll splitted bio */
> +	if (orig_bio != bio)
> +		return BLK_QC_T_NONE;
> +
>  	return cookie;
>  queue_exit:
>  	blk_queue_exit(q);

Yeah, this looks simpler and more obvious, and it also catches the
bounce buffering case (not that polling + bounce buffering should ever
happen).
Jeffle Xu Nov. 25, 2020, 8:05 a.m. UTC | #3
On 11/25/20 3:19 PM, Ming Lei wrote:
> On Wed, Nov 25, 2020 at 02:41:47PM +0800, Jeffle Xu wrote:
>> iopoll is initially for small size, latency sensitive IO. It doesn't
>> work well for big IO, especially when it needs to be split to multiple
>> bios. In this case, the returned cookie of __submit_bio_noacct_mq() is
>> indeed the cookie of the last split bio. The completion of *this* last
>> split bio done by iopoll doesn't mean the whole original bio has
>> completed. Callers of iopoll still need to wait for completion of other
>> split bios.
>>
>> Besides bio splitting may cause more trouble for iopoll which isn't
>> supposed to be used in case of big IO.
>>
>> iopoll for split bio may cause potential race if CPU migration happens
>> during bio submission. Since the returned cookie is that of the last
>> split bio, polling on the corresponding hardware queue doesn't help
>> complete other split bios, if these split bios are enqueued into
>> different hardware queues. Since interrupts are disabled for polling
>> queues, the completion of these other split bios depends on timeout
>> mechanism, thus causing a potential hang.
>>
>> iopoll for split bio may also cause hang for sync polling. Currently
>> both the blkdev and iomap-based fs (ext4/xfs, etc) support sync polling
>> in direct IO routine. These routines will submit bio without REQ_NOWAIT
>> flag set, and then start sync polling in current process context. The
>> process may hang in blk_mq_get_tag() if the submitted bio has to be
>> split into multiple bios and can rapidly exhaust the queue depth. The
>> process are waiting for the completion of the previously allocated
>> requests, which should be reaped by the following polling, and thus
>> causing a deadlock.
>>
>> To avoid these subtle trouble described above, just disable iopoll for
>> split bio.
>>
>> Suggested-by: Ming Lei <ming.lei@redhat.com>
>> Signed-off-by: Jeffle Xu <jefflexu@linux.alibaba.com>
>> Reviewed-by: Christoph Hellwig <hch@lst.de>
>> ---
>>  block/bio.c               |  2 ++
>>  block/blk-merge.c         | 12 ++++++++++++
>>  block/blk-mq.c            |  3 +++
>>  include/linux/blk_types.h |  1 +
>>  4 files changed, 18 insertions(+)
>>
>> diff --git a/block/bio.c b/block/bio.c
>> index fa01bef35bb1..7f7ddc22a30d 100644
>> --- a/block/bio.c
>> +++ b/block/bio.c
>> @@ -684,6 +684,8 @@ void __bio_clone_fast(struct bio *bio, struct bio *bio_src)
>>  	bio_set_flag(bio, BIO_CLONED);
>>  	if (bio_flagged(bio_src, BIO_THROTTLED))
>>  		bio_set_flag(bio, BIO_THROTTLED);
>> +	if (bio_flagged(bio_src, BIO_SPLIT))
>> +		bio_set_flag(bio, BIO_SPLIT);
>>  	bio->bi_opf = bio_src->bi_opf;
>>  	bio->bi_ioprio = bio_src->bi_ioprio;
>>  	bio->bi_write_hint = bio_src->bi_write_hint;
>> diff --git a/block/blk-merge.c b/block/blk-merge.c
>> index bcf5e4580603..a2890cebf99f 100644
>> --- a/block/blk-merge.c
>> +++ b/block/blk-merge.c
>> @@ -279,6 +279,18 @@ static struct bio *blk_bio_segment_split(struct request_queue *q,
>>  	return NULL;
>>  split:
>>  	*segs = nsegs;
>> +
>> +	/*
>> +	 * Bio splitting may cause subtle trouble such as hang when doing sync
>> +	 * iopoll in direct IO routine. Given performance gain of iopoll for
>> +	 * big IO can be trival, disable iopoll when split needed. We need
>> +	 * BIO_SPLIT to identify bios need this workaround. Since currently
>> +	 * only normal IO under mq routine may suffer this issue, BIO_SPLIT is
>> +	 * only marked here.
>> +	 */
>> +	bio->bi_opf &= ~REQ_HIPRI;
>> +	bio_set_flag(bio, BIO_SPLIT);
>> +
>>  	return bio_split(bio, sectors, GFP_NOIO, bs);
>>  }
>>  
>> diff --git a/block/blk-mq.c b/block/blk-mq.c
>> index 55bcee5dc032..ce1f3628e4c2 100644
>> --- a/block/blk-mq.c
>> +++ b/block/blk-mq.c
>> @@ -2265,6 +2265,9 @@ blk_qc_t blk_mq_submit_bio(struct bio *bio)
>>  		blk_mq_sched_insert_request(rq, false, true, true);
>>  	}
>>  
>> +	if (bio_flagged(bio, BIO_SPLIT))
>> +		return BLK_QC_T_NONE;
>> +
> 
> Not sure the new bio flag is really required for this case, just wondering
> why not take the following simple way? BTW we are really going to run
> out of bio flag.
> 

Please consider the following case:

One big bio got split into two split bios. At the first call of
blk_mq_submit_bio(), the input @bio (actually the original big bio)
indeed gets split. The split bio gets enqueued to hw queue and the
returned cookie is BLK_QC_T_NONE, while the remained bio gets buffered
in bio_list. So far so good.

Then when calling blk_mq_submit_bio() the second time, the input @bio is
indeed the remained bio. At this time, it will not get split and you
will get a *valid* cookie. And since the cookie of last split bio will
actually overrides the previous cookie, you will get a *valid* cookie as
a result.


> diff --git a/block/blk-mq.c b/block/blk-mq.c
> index 55bcee5dc032..1139b1efd712 100644
> --- a/block/blk-mq.c
> +++ b/block/blk-mq.c
> @@ -2157,6 +2157,7 @@ blk_qc_t blk_mq_submit_bio(struct bio *bio)
>  	unsigned int nr_segs;
>  	blk_qc_t cookie;
>  	blk_status_t ret;
> +	struct bio *orig_bio = bio;
>  
>  	blk_queue_bounce(q, &bio);
>  	__blk_queue_split(&bio, &nr_segs);
> @@ -2265,6 +2266,10 @@ blk_qc_t blk_mq_submit_bio(struct bio *bio)
>  		blk_mq_sched_insert_request(rq, false, true, true);
>  	}
>  
> +	/* don't poll splitted bio */
> +	if (orig_bio != bio)
> +		return BLK_QC_T_NONE;
> +
>  	return cookie;
>  queue_exit:
>  	blk_queue_exit(q);
> 
> Thanks,
> Ming
>
Ming Lei Nov. 25, 2020, 8:19 a.m. UTC | #4
On Wed, Nov 25, 2020 at 04:05:10PM +0800, JeffleXu wrote:
> 
> 
> On 11/25/20 3:19 PM, Ming Lei wrote:
> > On Wed, Nov 25, 2020 at 02:41:47PM +0800, Jeffle Xu wrote:
> >> iopoll is initially for small size, latency sensitive IO. It doesn't
> >> work well for big IO, especially when it needs to be split to multiple
> >> bios. In this case, the returned cookie of __submit_bio_noacct_mq() is
> >> indeed the cookie of the last split bio. The completion of *this* last
> >> split bio done by iopoll doesn't mean the whole original bio has
> >> completed. Callers of iopoll still need to wait for completion of other
> >> split bios.
> >>
> >> Besides bio splitting may cause more trouble for iopoll which isn't
> >> supposed to be used in case of big IO.
> >>
> >> iopoll for split bio may cause potential race if CPU migration happens
> >> during bio submission. Since the returned cookie is that of the last
> >> split bio, polling on the corresponding hardware queue doesn't help
> >> complete other split bios, if these split bios are enqueued into
> >> different hardware queues. Since interrupts are disabled for polling
> >> queues, the completion of these other split bios depends on timeout
> >> mechanism, thus causing a potential hang.
> >>
> >> iopoll for split bio may also cause hang for sync polling. Currently
> >> both the blkdev and iomap-based fs (ext4/xfs, etc) support sync polling
> >> in direct IO routine. These routines will submit bio without REQ_NOWAIT
> >> flag set, and then start sync polling in current process context. The
> >> process may hang in blk_mq_get_tag() if the submitted bio has to be
> >> split into multiple bios and can rapidly exhaust the queue depth. The
> >> process are waiting for the completion of the previously allocated
> >> requests, which should be reaped by the following polling, and thus
> >> causing a deadlock.
> >>
> >> To avoid these subtle trouble described above, just disable iopoll for
> >> split bio.
> >>
> >> Suggested-by: Ming Lei <ming.lei@redhat.com>
> >> Signed-off-by: Jeffle Xu <jefflexu@linux.alibaba.com>
> >> Reviewed-by: Christoph Hellwig <hch@lst.de>
> >> ---
> >>  block/bio.c               |  2 ++
> >>  block/blk-merge.c         | 12 ++++++++++++
> >>  block/blk-mq.c            |  3 +++
> >>  include/linux/blk_types.h |  1 +
> >>  4 files changed, 18 insertions(+)
> >>
> >> diff --git a/block/bio.c b/block/bio.c
> >> index fa01bef35bb1..7f7ddc22a30d 100644
> >> --- a/block/bio.c
> >> +++ b/block/bio.c
> >> @@ -684,6 +684,8 @@ void __bio_clone_fast(struct bio *bio, struct bio *bio_src)
> >>  	bio_set_flag(bio, BIO_CLONED);
> >>  	if (bio_flagged(bio_src, BIO_THROTTLED))
> >>  		bio_set_flag(bio, BIO_THROTTLED);
> >> +	if (bio_flagged(bio_src, BIO_SPLIT))
> >> +		bio_set_flag(bio, BIO_SPLIT);
> >>  	bio->bi_opf = bio_src->bi_opf;
> >>  	bio->bi_ioprio = bio_src->bi_ioprio;
> >>  	bio->bi_write_hint = bio_src->bi_write_hint;
> >> diff --git a/block/blk-merge.c b/block/blk-merge.c
> >> index bcf5e4580603..a2890cebf99f 100644
> >> --- a/block/blk-merge.c
> >> +++ b/block/blk-merge.c
> >> @@ -279,6 +279,18 @@ static struct bio *blk_bio_segment_split(struct request_queue *q,
> >>  	return NULL;
> >>  split:
> >>  	*segs = nsegs;
> >> +
> >> +	/*
> >> +	 * Bio splitting may cause subtle trouble such as hang when doing sync
> >> +	 * iopoll in direct IO routine. Given performance gain of iopoll for
> >> +	 * big IO can be trival, disable iopoll when split needed. We need
> >> +	 * BIO_SPLIT to identify bios need this workaround. Since currently
> >> +	 * only normal IO under mq routine may suffer this issue, BIO_SPLIT is
> >> +	 * only marked here.
> >> +	 */
> >> +	bio->bi_opf &= ~REQ_HIPRI;
> >> +	bio_set_flag(bio, BIO_SPLIT);
> >> +
> >>  	return bio_split(bio, sectors, GFP_NOIO, bs);
> >>  }
> >>  
> >> diff --git a/block/blk-mq.c b/block/blk-mq.c
> >> index 55bcee5dc032..ce1f3628e4c2 100644
> >> --- a/block/blk-mq.c
> >> +++ b/block/blk-mq.c
> >> @@ -2265,6 +2265,9 @@ blk_qc_t blk_mq_submit_bio(struct bio *bio)
> >>  		blk_mq_sched_insert_request(rq, false, true, true);
> >>  	}
> >>  
> >> +	if (bio_flagged(bio, BIO_SPLIT))
> >> +		return BLK_QC_T_NONE;
> >> +
> > 
> > Not sure the new bio flag is really required for this case, just wondering
> > why not take the following simple way? BTW we are really going to run
> > out of bio flag.
> > 
> 
> Please consider the following case:
> 
> One big bio got split into two split bios. At the first call of
> blk_mq_submit_bio(), the input @bio (actually the original big bio)
> indeed gets split. The split bio gets enqueued to hw queue and the
> returned cookie is BLK_QC_T_NONE, while the remained bio gets buffered
> in bio_list. So far so good.

When this bio gets splitted, REQ_HIPRI is cleared for this bio, and
all splitted bios won't set this flag too.

> 
> Then when calling blk_mq_submit_bio() the second time, the input @bio is
> indeed the remained bio. At this time, it will not get split and you
> will get a *valid* cookie. And since the cookie of last split bio will
> actually overrides the previous cookie, you will get a *valid* cookie as
> a result.

Then valid cookie can be returned only for bio with REQ_HIPRI.


Thanks, 
Ming
Ming Lei Nov. 25, 2020, 8:29 a.m. UTC | #5
On Wed, Nov 25, 2020 at 02:41:47PM +0800, Jeffle Xu wrote:
> iopoll is initially for small size, latency sensitive IO. It doesn't
> work well for big IO, especially when it needs to be split to multiple
> bios. In this case, the returned cookie of __submit_bio_noacct_mq() is
> indeed the cookie of the last split bio. The completion of *this* last
> split bio done by iopoll doesn't mean the whole original bio has
> completed. Callers of iopoll still need to wait for completion of other
> split bios.
> 
> Besides bio splitting may cause more trouble for iopoll which isn't
> supposed to be used in case of big IO.
> 
> iopoll for split bio may cause potential race if CPU migration happens
> during bio submission. Since the returned cookie is that of the last
> split bio, polling on the corresponding hardware queue doesn't help
> complete other split bios, if these split bios are enqueued into
> different hardware queues. Since interrupts are disabled for polling
> queues, the completion of these other split bios depends on timeout
> mechanism, thus causing a potential hang.
> 
> iopoll for split bio may also cause hang for sync polling. Currently
> both the blkdev and iomap-based fs (ext4/xfs, etc) support sync polling
> in direct IO routine. These routines will submit bio without REQ_NOWAIT
> flag set, and then start sync polling in current process context. The
> process may hang in blk_mq_get_tag() if the submitted bio has to be
> split into multiple bios and can rapidly exhaust the queue depth. The
> process are waiting for the completion of the previously allocated
> requests, which should be reaped by the following polling, and thus
> causing a deadlock.
> 
> To avoid these subtle trouble described above, just disable iopoll for
> split bio.
> 
> Suggested-by: Ming Lei <ming.lei@redhat.com>
> Signed-off-by: Jeffle Xu <jefflexu@linux.alibaba.com>
> Reviewed-by: Christoph Hellwig <hch@lst.de>
> ---
>  block/bio.c               |  2 ++
>  block/blk-merge.c         | 12 ++++++++++++
>  block/blk-mq.c            |  3 +++
>  include/linux/blk_types.h |  1 +
>  4 files changed, 18 insertions(+)
> 
> diff --git a/block/bio.c b/block/bio.c
> index fa01bef35bb1..7f7ddc22a30d 100644
> --- a/block/bio.c
> +++ b/block/bio.c
> @@ -684,6 +684,8 @@ void __bio_clone_fast(struct bio *bio, struct bio *bio_src)
>  	bio_set_flag(bio, BIO_CLONED);
>  	if (bio_flagged(bio_src, BIO_THROTTLED))
>  		bio_set_flag(bio, BIO_THROTTLED);
> +	if (bio_flagged(bio_src, BIO_SPLIT))
> +		bio_set_flag(bio, BIO_SPLIT);
>  	bio->bi_opf = bio_src->bi_opf;
>  	bio->bi_ioprio = bio_src->bi_ioprio;
>  	bio->bi_write_hint = bio_src->bi_write_hint;
> diff --git a/block/blk-merge.c b/block/blk-merge.c
> index bcf5e4580603..a2890cebf99f 100644
> --- a/block/blk-merge.c
> +++ b/block/blk-merge.c
> @@ -279,6 +279,18 @@ static struct bio *blk_bio_segment_split(struct request_queue *q,
>  	return NULL;
>  split:
>  	*segs = nsegs;
> +
> +	/*
> +	 * Bio splitting may cause subtle trouble such as hang when doing sync
> +	 * iopoll in direct IO routine. Given performance gain of iopoll for
> +	 * big IO can be trival, disable iopoll when split needed. We need
> +	 * BIO_SPLIT to identify bios need this workaround. Since currently
> +	 * only normal IO under mq routine may suffer this issue, BIO_SPLIT is
> +	 * only marked here.
> +	 */
> +	bio->bi_opf &= ~REQ_HIPRI;
> +	bio_set_flag(bio, BIO_SPLIT);

You may need to put the above into one helper, and call the helper for
other splitted cases(discard, write zero and write same) too.

thanks,
Ming
Jeffle Xu Nov. 25, 2020, 9:15 a.m. UTC | #6
On 11/25/20 4:19 PM, Ming Lei wrote:
> On Wed, Nov 25, 2020 at 04:05:10PM +0800, JeffleXu wrote:
>>
>>
>> On 11/25/20 3:19 PM, Ming Lei wrote:
>>> On Wed, Nov 25, 2020 at 02:41:47PM +0800, Jeffle Xu wrote:
>>>> iopoll is initially for small size, latency sensitive IO. It doesn't
>>>> work well for big IO, especially when it needs to be split to multiple
>>>> bios. In this case, the returned cookie of __submit_bio_noacct_mq() is
>>>> indeed the cookie of the last split bio. The completion of *this* last
>>>> split bio done by iopoll doesn't mean the whole original bio has
>>>> completed. Callers of iopoll still need to wait for completion of other
>>>> split bios.
>>>>
>>>> Besides bio splitting may cause more trouble for iopoll which isn't
>>>> supposed to be used in case of big IO.
>>>>
>>>> iopoll for split bio may cause potential race if CPU migration happens
>>>> during bio submission. Since the returned cookie is that of the last
>>>> split bio, polling on the corresponding hardware queue doesn't help
>>>> complete other split bios, if these split bios are enqueued into
>>>> different hardware queues. Since interrupts are disabled for polling
>>>> queues, the completion of these other split bios depends on timeout
>>>> mechanism, thus causing a potential hang.
>>>>
>>>> iopoll for split bio may also cause hang for sync polling. Currently
>>>> both the blkdev and iomap-based fs (ext4/xfs, etc) support sync polling
>>>> in direct IO routine. These routines will submit bio without REQ_NOWAIT
>>>> flag set, and then start sync polling in current process context. The
>>>> process may hang in blk_mq_get_tag() if the submitted bio has to be
>>>> split into multiple bios and can rapidly exhaust the queue depth. The
>>>> process are waiting for the completion of the previously allocated
>>>> requests, which should be reaped by the following polling, and thus
>>>> causing a deadlock.
>>>>
>>>> To avoid these subtle trouble described above, just disable iopoll for
>>>> split bio.
>>>>
>>>> Suggested-by: Ming Lei <ming.lei@redhat.com>
>>>> Signed-off-by: Jeffle Xu <jefflexu@linux.alibaba.com>
>>>> Reviewed-by: Christoph Hellwig <hch@lst.de>
>>>> ---
>>>>  block/bio.c               |  2 ++
>>>>  block/blk-merge.c         | 12 ++++++++++++
>>>>  block/blk-mq.c            |  3 +++
>>>>  include/linux/blk_types.h |  1 +
>>>>  4 files changed, 18 insertions(+)
>>>>
>>>> diff --git a/block/bio.c b/block/bio.c
>>>> index fa01bef35bb1..7f7ddc22a30d 100644
>>>> --- a/block/bio.c
>>>> +++ b/block/bio.c
>>>> @@ -684,6 +684,8 @@ void __bio_clone_fast(struct bio *bio, struct bio *bio_src)
>>>>  	bio_set_flag(bio, BIO_CLONED);
>>>>  	if (bio_flagged(bio_src, BIO_THROTTLED))
>>>>  		bio_set_flag(bio, BIO_THROTTLED);
>>>> +	if (bio_flagged(bio_src, BIO_SPLIT))
>>>> +		bio_set_flag(bio, BIO_SPLIT);
>>>>  	bio->bi_opf = bio_src->bi_opf;
>>>>  	bio->bi_ioprio = bio_src->bi_ioprio;
>>>>  	bio->bi_write_hint = bio_src->bi_write_hint;
>>>> diff --git a/block/blk-merge.c b/block/blk-merge.c
>>>> index bcf5e4580603..a2890cebf99f 100644
>>>> --- a/block/blk-merge.c
>>>> +++ b/block/blk-merge.c
>>>> @@ -279,6 +279,18 @@ static struct bio *blk_bio_segment_split(struct request_queue *q,
>>>>  	return NULL;
>>>>  split:
>>>>  	*segs = nsegs;
>>>> +
>>>> +	/*
>>>> +	 * Bio splitting may cause subtle trouble such as hang when doing sync
>>>> +	 * iopoll in direct IO routine. Given performance gain of iopoll for
>>>> +	 * big IO can be trival, disable iopoll when split needed. We need
>>>> +	 * BIO_SPLIT to identify bios need this workaround. Since currently
>>>> +	 * only normal IO under mq routine may suffer this issue, BIO_SPLIT is
>>>> +	 * only marked here.
>>>> +	 */
>>>> +	bio->bi_opf &= ~REQ_HIPRI;
>>>> +	bio_set_flag(bio, BIO_SPLIT);
>>>> +
>>>>  	return bio_split(bio, sectors, GFP_NOIO, bs);
>>>>  }
>>>>  
>>>> diff --git a/block/blk-mq.c b/block/blk-mq.c
>>>> index 55bcee5dc032..ce1f3628e4c2 100644
>>>> --- a/block/blk-mq.c
>>>> +++ b/block/blk-mq.c
>>>> @@ -2265,6 +2265,9 @@ blk_qc_t blk_mq_submit_bio(struct bio *bio)
>>>>  		blk_mq_sched_insert_request(rq, false, true, true);
>>>>  	}
>>>>  
>>>> +	if (bio_flagged(bio, BIO_SPLIT))
>>>> +		return BLK_QC_T_NONE;
>>>> +
>>>
>>> Not sure the new bio flag is really required for this case, just wondering
>>> why not take the following simple way? BTW we are really going to run
>>> out of bio flag.
>>>
>>
>> Please consider the following case:
>>
>> One big bio got split into two split bios. At the first call of
>> blk_mq_submit_bio(), the input @bio (actually the original big bio)
>> indeed gets split. The split bio gets enqueued to hw queue and the
>> returned cookie is BLK_QC_T_NONE, while the remained bio gets buffered
>> in bio_list. So far so good.
> 
> When this bio gets splitted, REQ_HIPRI is cleared for this bio, and
> all splitted bios won't set this flag too.
> 
That's true, at least in terms of my patch.

>>
>> Then when calling blk_mq_submit_bio() the second time, the input @bio is
>> indeed the remained bio. At this time, it will not get split and you
>> will get a *valid* cookie. And since the cookie of last split bio will
>> actually overrides the previous cookie, you will get a *valid* cookie as
>> a result.
> 
> Then valid cookie can be returned only for bio with REQ_HIPRI.
Sounds reasonable. The side effect is that original IO without REQ_HIPRI
now also returns BLK_QC_T_NONE. But it may not be a big problem, since
the returned cookie is not used at all for IO without REQ_HIPRI.
Jeffle Xu Nov. 25, 2020, 9:17 a.m. UTC | #7
On 11/25/20 4:29 PM, Ming Lei wrote:
> On Wed, Nov 25, 2020 at 02:41:47PM +0800, Jeffle Xu wrote:
>> iopoll is initially for small size, latency sensitive IO. It doesn't
>> work well for big IO, especially when it needs to be split to multiple
>> bios. In this case, the returned cookie of __submit_bio_noacct_mq() is
>> indeed the cookie of the last split bio. The completion of *this* last
>> split bio done by iopoll doesn't mean the whole original bio has
>> completed. Callers of iopoll still need to wait for completion of other
>> split bios.
>>
>> Besides bio splitting may cause more trouble for iopoll which isn't
>> supposed to be used in case of big IO.
>>
>> iopoll for split bio may cause potential race if CPU migration happens
>> during bio submission. Since the returned cookie is that of the last
>> split bio, polling on the corresponding hardware queue doesn't help
>> complete other split bios, if these split bios are enqueued into
>> different hardware queues. Since interrupts are disabled for polling
>> queues, the completion of these other split bios depends on timeout
>> mechanism, thus causing a potential hang.
>>
>> iopoll for split bio may also cause hang for sync polling. Currently
>> both the blkdev and iomap-based fs (ext4/xfs, etc) support sync polling
>> in direct IO routine. These routines will submit bio without REQ_NOWAIT
>> flag set, and then start sync polling in current process context. The
>> process may hang in blk_mq_get_tag() if the submitted bio has to be
>> split into multiple bios and can rapidly exhaust the queue depth. The
>> process are waiting for the completion of the previously allocated
>> requests, which should be reaped by the following polling, and thus
>> causing a deadlock.
>>
>> To avoid these subtle trouble described above, just disable iopoll for
>> split bio.
>>
>> Suggested-by: Ming Lei <ming.lei@redhat.com>
>> Signed-off-by: Jeffle Xu <jefflexu@linux.alibaba.com>
>> Reviewed-by: Christoph Hellwig <hch@lst.de>
>> ---
>>  block/bio.c               |  2 ++
>>  block/blk-merge.c         | 12 ++++++++++++
>>  block/blk-mq.c            |  3 +++
>>  include/linux/blk_types.h |  1 +
>>  4 files changed, 18 insertions(+)
>>
>> diff --git a/block/bio.c b/block/bio.c
>> index fa01bef35bb1..7f7ddc22a30d 100644
>> --- a/block/bio.c
>> +++ b/block/bio.c
>> @@ -684,6 +684,8 @@ void __bio_clone_fast(struct bio *bio, struct bio *bio_src)
>>  	bio_set_flag(bio, BIO_CLONED);
>>  	if (bio_flagged(bio_src, BIO_THROTTLED))
>>  		bio_set_flag(bio, BIO_THROTTLED);
>> +	if (bio_flagged(bio_src, BIO_SPLIT))
>> +		bio_set_flag(bio, BIO_SPLIT);
>>  	bio->bi_opf = bio_src->bi_opf;
>>  	bio->bi_ioprio = bio_src->bi_ioprio;
>>  	bio->bi_write_hint = bio_src->bi_write_hint;
>> diff --git a/block/blk-merge.c b/block/blk-merge.c
>> index bcf5e4580603..a2890cebf99f 100644
>> --- a/block/blk-merge.c
>> +++ b/block/blk-merge.c
>> @@ -279,6 +279,18 @@ static struct bio *blk_bio_segment_split(struct request_queue *q,
>>  	return NULL;
>>  split:
>>  	*segs = nsegs;
>> +
>> +	/*
>> +	 * Bio splitting may cause subtle trouble such as hang when doing sync
>> +	 * iopoll in direct IO routine. Given performance gain of iopoll for
>> +	 * big IO can be trival, disable iopoll when split needed. We need
>> +	 * BIO_SPLIT to identify bios need this workaround. Since currently
>> +	 * only normal IO under mq routine may suffer this issue, BIO_SPLIT is
>> +	 * only marked here.
>> +	 */
>> +	bio->bi_opf &= ~REQ_HIPRI;
>> +	bio_set_flag(bio, BIO_SPLIT);
> 
> You may need to put the above into one helper, and call the helper for
> other splitted cases(discard, write zero and write same) too.
It could be, though currently only normal IO could be marked with REQ_HIPRI.
Ming Lei Nov. 25, 2020, 9:52 a.m. UTC | #8
On Wed, Nov 25, 2020 at 05:17:07PM +0800, JeffleXu wrote:
> 
> 
> On 11/25/20 4:29 PM, Ming Lei wrote:
> > On Wed, Nov 25, 2020 at 02:41:47PM +0800, Jeffle Xu wrote:
> >> iopoll is initially for small size, latency sensitive IO. It doesn't
> >> work well for big IO, especially when it needs to be split to multiple
> >> bios. In this case, the returned cookie of __submit_bio_noacct_mq() is
> >> indeed the cookie of the last split bio. The completion of *this* last
> >> split bio done by iopoll doesn't mean the whole original bio has
> >> completed. Callers of iopoll still need to wait for completion of other
> >> split bios.
> >>
> >> Besides bio splitting may cause more trouble for iopoll which isn't
> >> supposed to be used in case of big IO.
> >>
> >> iopoll for split bio may cause potential race if CPU migration happens
> >> during bio submission. Since the returned cookie is that of the last
> >> split bio, polling on the corresponding hardware queue doesn't help
> >> complete other split bios, if these split bios are enqueued into
> >> different hardware queues. Since interrupts are disabled for polling
> >> queues, the completion of these other split bios depends on timeout
> >> mechanism, thus causing a potential hang.
> >>
> >> iopoll for split bio may also cause hang for sync polling. Currently
> >> both the blkdev and iomap-based fs (ext4/xfs, etc) support sync polling
> >> in direct IO routine. These routines will submit bio without REQ_NOWAIT
> >> flag set, and then start sync polling in current process context. The
> >> process may hang in blk_mq_get_tag() if the submitted bio has to be
> >> split into multiple bios and can rapidly exhaust the queue depth. The
> >> process are waiting for the completion of the previously allocated
> >> requests, which should be reaped by the following polling, and thus
> >> causing a deadlock.
> >>
> >> To avoid these subtle trouble described above, just disable iopoll for
> >> split bio.
> >>
> >> Suggested-by: Ming Lei <ming.lei@redhat.com>
> >> Signed-off-by: Jeffle Xu <jefflexu@linux.alibaba.com>
> >> Reviewed-by: Christoph Hellwig <hch@lst.de>
> >> ---
> >>  block/bio.c               |  2 ++
> >>  block/blk-merge.c         | 12 ++++++++++++
> >>  block/blk-mq.c            |  3 +++
> >>  include/linux/blk_types.h |  1 +
> >>  4 files changed, 18 insertions(+)
> >>
> >> diff --git a/block/bio.c b/block/bio.c
> >> index fa01bef35bb1..7f7ddc22a30d 100644
> >> --- a/block/bio.c
> >> +++ b/block/bio.c
> >> @@ -684,6 +684,8 @@ void __bio_clone_fast(struct bio *bio, struct bio *bio_src)
> >>  	bio_set_flag(bio, BIO_CLONED);
> >>  	if (bio_flagged(bio_src, BIO_THROTTLED))
> >>  		bio_set_flag(bio, BIO_THROTTLED);
> >> +	if (bio_flagged(bio_src, BIO_SPLIT))
> >> +		bio_set_flag(bio, BIO_SPLIT);
> >>  	bio->bi_opf = bio_src->bi_opf;
> >>  	bio->bi_ioprio = bio_src->bi_ioprio;
> >>  	bio->bi_write_hint = bio_src->bi_write_hint;
> >> diff --git a/block/blk-merge.c b/block/blk-merge.c
> >> index bcf5e4580603..a2890cebf99f 100644
> >> --- a/block/blk-merge.c
> >> +++ b/block/blk-merge.c
> >> @@ -279,6 +279,18 @@ static struct bio *blk_bio_segment_split(struct request_queue *q,
> >>  	return NULL;
> >>  split:
> >>  	*segs = nsegs;
> >> +
> >> +	/*
> >> +	 * Bio splitting may cause subtle trouble such as hang when doing sync
> >> +	 * iopoll in direct IO routine. Given performance gain of iopoll for
> >> +	 * big IO can be trival, disable iopoll when split needed. We need
> >> +	 * BIO_SPLIT to identify bios need this workaround. Since currently
> >> +	 * only normal IO under mq routine may suffer this issue, BIO_SPLIT is
> >> +	 * only marked here.
> >> +	 */
> >> +	bio->bi_opf &= ~REQ_HIPRI;
> >> +	bio_set_flag(bio, BIO_SPLIT);
> > 
> > You may need to put the above into one helper, and call the helper for
> > other splitted cases(discard, write zero and write same) too.
> It could be, though currently only normal IO could be marked with REQ_HIPRI.

You are right, so far RWF_HIPRI is only applied on RW IO, then no need to
do that for other non-RW split.

Thanks,
Ming
diff mbox series

Patch

diff --git a/block/bio.c b/block/bio.c
index fa01bef35bb1..7f7ddc22a30d 100644
--- a/block/bio.c
+++ b/block/bio.c
@@ -684,6 +684,8 @@  void __bio_clone_fast(struct bio *bio, struct bio *bio_src)
 	bio_set_flag(bio, BIO_CLONED);
 	if (bio_flagged(bio_src, BIO_THROTTLED))
 		bio_set_flag(bio, BIO_THROTTLED);
+	if (bio_flagged(bio_src, BIO_SPLIT))
+		bio_set_flag(bio, BIO_SPLIT);
 	bio->bi_opf = bio_src->bi_opf;
 	bio->bi_ioprio = bio_src->bi_ioprio;
 	bio->bi_write_hint = bio_src->bi_write_hint;
diff --git a/block/blk-merge.c b/block/blk-merge.c
index bcf5e4580603..a2890cebf99f 100644
--- a/block/blk-merge.c
+++ b/block/blk-merge.c
@@ -279,6 +279,18 @@  static struct bio *blk_bio_segment_split(struct request_queue *q,
 	return NULL;
 split:
 	*segs = nsegs;
+
+	/*
+	 * Bio splitting may cause subtle trouble such as hang when doing sync
+	 * iopoll in direct IO routine. Given performance gain of iopoll for
+	 * big IO can be trival, disable iopoll when split needed. We need
+	 * BIO_SPLIT to identify bios need this workaround. Since currently
+	 * only normal IO under mq routine may suffer this issue, BIO_SPLIT is
+	 * only marked here.
+	 */
+	bio->bi_opf &= ~REQ_HIPRI;
+	bio_set_flag(bio, BIO_SPLIT);
+
 	return bio_split(bio, sectors, GFP_NOIO, bs);
 }
 
diff --git a/block/blk-mq.c b/block/blk-mq.c
index 55bcee5dc032..ce1f3628e4c2 100644
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -2265,6 +2265,9 @@  blk_qc_t blk_mq_submit_bio(struct bio *bio)
 		blk_mq_sched_insert_request(rq, false, true, true);
 	}
 
+	if (bio_flagged(bio, BIO_SPLIT))
+		return BLK_QC_T_NONE;
+
 	return cookie;
 queue_exit:
 	blk_queue_exit(q);
diff --git a/include/linux/blk_types.h b/include/linux/blk_types.h
index d9b69bbde5cc..528054c5e0bd 100644
--- a/include/linux/blk_types.h
+++ b/include/linux/blk_types.h
@@ -284,6 +284,7 @@  enum {
 				 * of this bio. */
 	BIO_CGROUP_ACCT,	/* has been accounted to a cgroup */
 	BIO_TRACKED,		/* set if bio goes through the rq_qos path */
+	BIO_SPLIT,		/* bio got split because of queue limits*/
 	BIO_FLAG_LAST
 };