diff mbox series

[bitmap-for-next,1/5] blk_mq: Fix cpumask_check() warning in blk_mq_hctx_next_cpu()

Message ID 20221003153420.285896-2-vschneid@redhat.com (mailing list archive)
State New, archived
Headers show
Series bitmap,cpumask: Add for_each_cpu_andnot() | expand

Commit Message

Valentin Schneider Oct. 3, 2022, 3:34 p.m. UTC 
A recent commit made cpumask_next*() trigger a warning when passed
n = nr_cpu_ids - 1. This means extra care must be taken when feeding CPU
numbers back into cpumask_next*().

The warning occurs nearly every boot on QEMU:

[    5.398453] WARNING: CPU: 3 PID: 162 at include/linux/cpumask.h:110 __blk_mq_delay_run_hw_queue+0x16b/0x180
[    5.399317] Modules linked in:
[    5.399646] CPU: 3 PID: 162 Comm: ssh-keygen Tainted: G                 N 6.0.0-rc4-00004-g93003cb24006 #55
[    5.400135] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel-1.14.0-0-g155821a1990b-prebuilt.qemu.org 04/01/2014
[    5.405430] Call Trace:
[    5.406152]  <TASK>
[    5.406452]  blk_mq_sched_insert_requests+0x67/0x150
[    5.406759]  blk_mq_flush_plug_list+0xd0/0x280
[    5.406987]  ? bit_wait+0x60/0x60
[    5.407317]  __blk_flush_plug+0xdb/0x120
[    5.407561]  ? bit_wait+0x60/0x60
[    5.407765]  io_schedule_prepare+0x38/0x40
[    5.407974]  io_schedule+0x6/0x40
[    5.408283]  bit_wait_io+0x8/0x60
[    5.408485]  __wait_on_bit+0x72/0x90
[    5.408668]  out_of_line_wait_on_bit+0x8c/0xb0
[    5.408879]  ? swake_up_one+0x30/0x30
[    5.409158]  ext4_read_bh+0x7a/0x80
[    5.409381]  ext4_get_branch+0xc0/0x130
[    5.409583]  ext4_ind_map_blocks+0x1ac/0xb30
[    5.409844]  ? __es_remove_extent+0x61/0x6d0
[    5.410128]  ? blk_account_io_merge_bio+0x67/0xd0
[    5.410416]  ? percpu_counter_add_batch+0x59/0xb0
[    5.410720]  ? percpu_counter_add_batch+0x59/0xb0
[    5.410949]  ? _raw_read_unlock+0x13/0x30
[    5.411323]  ext4_map_blocks+0xc2/0x590
[    5.411609]  ? xa_load+0x7c/0xa0
[    5.411859]  ext4_mpage_readpages+0x4a2/0xaa0
[    5.412192]  read_pages+0x69/0x2b0
[    5.412477]  ? folio_add_lru+0x4f/0x70
[    5.412696]  page_cache_ra_unbounded+0x11d/0x170
[    5.412960]  filemap_get_pages+0x109/0x5d0
[    5.413340]  ? __vma_adjust+0x348/0x930
[    5.413576]  filemap_read+0xb7/0x380
[    5.413805]  ? vma_merge+0x2e9/0x330
[    5.414067]  ? vma_set_page_prot+0x43/0x80
[    5.414309]  ? __inode_security_revalidate+0x5e/0x80
[    5.414581]  ? selinux_file_permission+0x107/0x130
[    5.414889]  vfs_read+0x205/0x2e0
[    5.415162]  ksys_read+0x54/0xd0
[    5.415348]  do_syscall_64+0x3a/0x90
[    5.415584]  entry_SYSCALL_64_after_hwframe+0x63/0xcd

Fixes: 78e5a3399421 ("cpumask: fix checking valid cpu range")
Suggested-by: Yury Norov <yury.norov@gmail.com>
Signed-off-by: Valentin Schneider <vschneid@redhat.com>
---
 block/blk-mq.c | 9 +++++++--
 1 file changed, 7 insertions(+), 2 deletions(-)

Comments

Yury Norov Oct. 3, 2022, 5:54 p.m. UTC | #1 
On Mon, Oct 03, 2022 at 04:34:16PM +0100, Valentin Schneider wrote:
> A recent commit made cpumask_next*() trigger a warning when passed
> n = nr_cpu_ids - 1. This means extra care must be taken when feeding CPU
> numbers back into cpumask_next*().
> 
> The warning occurs nearly every boot on QEMU:

[...]
 
> Fixes: 78e5a3399421 ("cpumask: fix checking valid cpu range")

No! It fixes blk-mq bug, which has been revealed after 78e5a3399421.

> Suggested-by: Yury Norov <yury.norov@gmail.com>

OK, maybe I suggested something like this. But after looking into the code
of blk_mq_hctx_next_cpu() code for more, I have a feeling that this should
be overridden deeper. 

Can you check - did this warning raise because hctx->next_cpu, or
because cpumask_next_and() was called twice after jumping on
select_cpu label?

> Signed-off-by: Valentin Schneider <vschneid@redhat.com>
> ---
>  block/blk-mq.c | 9 +++++++--
>  1 file changed, 7 insertions(+), 2 deletions(-)
> 
> diff --git a/block/blk-mq.c b/block/blk-mq.c
> index c96c8c4f751b..30ae51eda95e 100644
> --- a/block/blk-mq.c
> +++ b/block/blk-mq.c
> @@ -2046,8 +2046,13 @@ static int blk_mq_hctx_next_cpu(struct blk_mq_hw_ctx *hctx)
>  
>  	if (--hctx->next_cpu_batch <= 0) {
>  select_cpu:

Because we have backward looking goto, I have a strong feeling that the
code should be reorganized.

> -		next_cpu = cpumask_next_and(next_cpu, hctx->cpumask,
> -				cpu_online_mask);
> +		if (next_cpu == nr_cpu_ids - 1)
> +			next_cpu = nr_cpu_ids;
> +		else
> +			next_cpu = cpumask_next_and(next_cpu,
> +						    hctx->cpumask,
> +						    cpu_online_mask);
> +
>  		if (next_cpu >= nr_cpu_ids)
>  			next_cpu = blk_mq_first_mapped_cpu(hctx);

This simply means 'let's start from the beginning', and should be
replaced with cpumask_next_and_wrap().

>  		hctx->next_cpu_batch = BLK_MQ_CPU_WORK_BATCH;


Maybe something like this would work?

        if (--hctx->next_cpu_batch > 0 && cpu_online(next_cpu)) {
                hctx->next_cpu = next_cpu;
                return next_cpu;
        }

        next_cpu = cpumask_next_and_wrap(next_cpu, hctx->cpumask, cpu_online_mask)
        if (next_cpu < nr_cpu_ids) {
                hctx->next_cpu_batch = BLK_MQ_CPU_WORK_BATCH;
                hctx->next_cpu = next_cpu;
                return next_cpu;
        }

        /*
         * Make sure to re-select CPU next time once after CPUs
         * in hctx->cpumask become online again.
         */
        hctx->next_cpu = next_cpu;
        hctx->next_cpu_batch = 1;
        return WORK_CPU_UNBOUND;

I didn't test it and likely screwed some corner case. I'm just
trying to say that picking next cpu should be an easier thing.

Thanks,
Yury
Valentin Schneider Oct. 5, 2022, 12:19 p.m. UTC | #2 
On 03/10/22 10:54, Yury Norov wrote:
> On Mon, Oct 03, 2022 at 04:34:16PM +0100, Valentin Schneider wrote:
>> A recent commit made cpumask_next*() trigger a warning when passed
>> n = nr_cpu_ids - 1. This means extra care must be taken when feeding CPU
>> numbers back into cpumask_next*().
>>
>> The warning occurs nearly every boot on QEMU:
>
> [...]
>
>> Fixes: 78e5a3399421 ("cpumask: fix checking valid cpu range")
>
> No! It fixes blk-mq bug, which has been revealed after 78e5a3399421.
>
>> Suggested-by: Yury Norov <yury.norov@gmail.com>
>
> OK, maybe I suggested something like this. But after looking into the code
> of blk_mq_hctx_next_cpu() code for more, I have a feeling that this should
> be overridden deeper.
>
> Can you check - did this warning raise because hctx->next_cpu, or
> because cpumask_next_and() was called twice after jumping on
> select_cpu label?
>

It seems to always happen when hctx->next_cpu == nr_cpu_ids-1 at the start
of the function - no jumping involved.

>> Signed-off-by: Valentin Schneider <vschneid@redhat.com>
>> ---
>>  block/blk-mq.c | 9 +++++++--
>>  1 file changed, 7 insertions(+), 2 deletions(-)
>>
>> diff --git a/block/blk-mq.c b/block/blk-mq.c
>> index c96c8c4f751b..30ae51eda95e 100644
>> --- a/block/blk-mq.c
>> +++ b/block/blk-mq.c
>> @@ -2046,8 +2046,13 @@ static int blk_mq_hctx_next_cpu(struct blk_mq_hw_ctx *hctx)
>>
>>      if (--hctx->next_cpu_batch <= 0) {
>>  select_cpu:
>
> Because we have backward looking goto, I have a strong feeling that the
> code should be reorganized.
>
>> -		next_cpu = cpumask_next_and(next_cpu, hctx->cpumask,
>> -				cpu_online_mask);
>> +		if (next_cpu == nr_cpu_ids - 1)
>> +			next_cpu = nr_cpu_ids;
>> +		else
>> +			next_cpu = cpumask_next_and(next_cpu,
>> +						    hctx->cpumask,
>> +						    cpu_online_mask);
>> +
>>              if (next_cpu >= nr_cpu_ids)
>>                      next_cpu = blk_mq_first_mapped_cpu(hctx);
>
> This simply means 'let's start from the beginning', and should be
> replaced with cpumask_next_and_wrap().

I hadn't looked in depth there, but that's a strange behaviour.
If we get to the end of the cpumask, blk_mq_first_mapped_cpu() does:

        int cpu = cpumask_first_and(hctx->cpumask, cpu_online_mask);

        if (cpu >= nr_cpu_ids)
                cpu = cpumask_first(hctx->cpumask);
        return cpu;

That if branch means the returned CPU is offline, which then triggers:

        if (!cpu_online(next_cpu)) {
                if (!tried) {
                        tried = true;
                        goto select_cpu;
                }

but going back to select_cpu doesn't make much sense, we've already checked
that hctx->cpumask and cpu_online_mask were disjoint.

>
>>              hctx->next_cpu_batch = BLK_MQ_CPU_WORK_BATCH;
>
>
> Maybe something like this would work?
>
>         if (--hctx->next_cpu_batch > 0 && cpu_online(next_cpu)) {
>                 hctx->next_cpu = next_cpu;
>                 return next_cpu;
>         }
>
>         next_cpu = cpumask_next_and_wrap(next_cpu, hctx->cpumask, cpu_online_mask)
>         if (next_cpu < nr_cpu_ids) {
>                 hctx->next_cpu_batch = BLK_MQ_CPU_WORK_BATCH;
>                 hctx->next_cpu = next_cpu;
>                 return next_cpu;
>         }
>
>         /*
>          * Make sure to re-select CPU next time once after CPUs
>          * in hctx->cpumask become online again.
>          */
>         hctx->next_cpu = next_cpu;
>         hctx->next_cpu_batch = 1;
>         return WORK_CPU_UNBOUND;
>
> I didn't test it and likely screwed some corner case. I'm just
> trying to say that picking next cpu should be an easier thing.
>

Agreed, your suggestion looks sane, let me play with that a bit.

> Thanks,
> Yury
diff mbox series

Patch

diff --git a/block/blk-mq.c b/block/blk-mq.c
index c96c8c4f751b..30ae51eda95e 100644
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -2046,8 +2046,13 @@  static int blk_mq_hctx_next_cpu(struct blk_mq_hw_ctx *hctx)
 
 	if (--hctx->next_cpu_batch <= 0) {
 select_cpu:
-		next_cpu = cpumask_next_and(next_cpu, hctx->cpumask,
-				cpu_online_mask);
+		if (next_cpu == nr_cpu_ids - 1)
+			next_cpu = nr_cpu_ids;
+		else
+			next_cpu = cpumask_next_and(next_cpu,
+						    hctx->cpumask,
+						    cpu_online_mask);
+
 		if (next_cpu >= nr_cpu_ids)
 			next_cpu = blk_mq_first_mapped_cpu(hctx);
 		hctx->next_cpu_batch = BLK_MQ_CPU_WORK_BATCH;