| Message ID | 20171127050721.5884-2-ming.lei@redhat.com (mailing list archive) |
|---|---|
| State | Superseded, archived |
| Delegated to: | Mike Snitzer |
| Headers | show |
On Mon, 2017-11-27 at 13:07 +0800, Ming Lei wrote: > If .queue_rq() returns BLK_STS_RESOURCE, blk-mq will rerun the queue in > the three situations: > > 1) if BLK_MQ_S_SCHED_RESTART is set > - queue is rerun after one rq is completed, see blk_mq_sched_restart() > which is run from blk_mq_free_request() > > 2) run out of driver tag > - queue is rerun after one tag is freed > > 3) otherwise > - queue is run immediately in blk_mq_dispatch_rq_list() > > This random dealy of running hw queue is introduced by commit 6077c2d706097c0 > (dm rq: Avoid that request processing stalls sporadically), which claimed > one request processing stalling is fixed, but never explained the behind > idea, and it is a workaound at most. Even the question isn't explained by > anyone in recent discussion. > > Also calling blk_mq_delay_run_hw_queue() inside .queue_rq() is a horrible > hack because it makes BLK_MQ_S_SCHED_RESTART not working, and degrades I/O > peformance a lot. > > Finally this patch makes sure that dm-rq returns BLK_STS_RESOURCE to blk-mq > only when underlying queue is out of resource, so we switch to return > DM_MAPIO_DELAY_REQUEU if either MPATHF_QUEUE_IO or MPATHF_PG_INIT_REQUIRED > is set in multipath_clone_and_map(). Sorry but in my opinion the above description shows that you don't understand the dm-mpath driver completely. > diff --git a/drivers/md/dm-mpath.c b/drivers/md/dm-mpath.c > index c8faa2b85842..8fe3f45407ce 100644 > --- a/drivers/md/dm-mpath.c > +++ b/drivers/md/dm-mpath.c > @@ -484,9 +484,7 @@ static int multipath_clone_and_map(struct dm_target *ti, struct request *rq, > return DM_MAPIO_KILL; > } else if (test_bit(MPATHF_QUEUE_IO, &m->flags) || > test_bit(MPATHF_PG_INIT_REQUIRED, &m->flags)) { > - if (pg_init_all_paths(m)) > - return DM_MAPIO_DELAY_REQUEUE; > - return DM_MAPIO_REQUEUE; > + return DM_MAPIO_DELAY_REQUEUE; > } This patch removes a pg_init_all_paths() call but you don't explain why you think it is allowed to remove that call. Did you perhaps remove that call by mistake? Bart. -- dm-devel mailing list dm-devel@redhat.com https://www.redhat.com/mailman/listinfo/dm-devel
On Mon, Nov 27, 2017 at 05:14:46PM +0000, Bart Van Assche wrote: > On Mon, 2017-11-27 at 13:07 +0800, Ming Lei wrote: > > If .queue_rq() returns BLK_STS_RESOURCE, blk-mq will rerun the queue in > > the three situations: > > > > 1) if BLK_MQ_S_SCHED_RESTART is set > > - queue is rerun after one rq is completed, see blk_mq_sched_restart() > > which is run from blk_mq_free_request() > > > > 2) run out of driver tag > > - queue is rerun after one tag is freed > > > > 3) otherwise > > - queue is run immediately in blk_mq_dispatch_rq_list() > > > > This random dealy of running hw queue is introduced by commit 6077c2d706097c0 > > (dm rq: Avoid that request processing stalls sporadically), which claimed > > one request processing stalling is fixed, but never explained the behind > > idea, and it is a workaound at most. Even the question isn't explained by > > anyone in recent discussion. > > > > Also calling blk_mq_delay_run_hw_queue() inside .queue_rq() is a horrible > > hack because it makes BLK_MQ_S_SCHED_RESTART not working, and degrades I/O > > peformance a lot. > > > > Finally this patch makes sure that dm-rq returns BLK_STS_RESOURCE to blk-mq > > only when underlying queue is out of resource, so we switch to return > > DM_MAPIO_DELAY_REQUEU if either MPATHF_QUEUE_IO or MPATHF_PG_INIT_REQUIRED > > is set in multipath_clone_and_map(). > > Sorry but in my opinion the above description shows that you don't understand > the dm-mpath driver completely. I have to treat your above comment as a noop since you never provide a explanation. Also I don't think it is wrong to deal with MPATHF_QUEUE_IO/MPATHF_PG_INIT_REQUIRED via DM_MAPIO_DELAY_REQUEUE, since both can seldom happen, and the delay won't cause performance issue. The idea behind this change is that this patchset switches to return BLK_STS_RESOURCE to blk-mq only when we run out of resource, but the above two(MPATHF_QUEUE_IO and MPATHF_PG_INIT_REQUIRED) don't belong to 'run out of resource'. > > > diff --git a/drivers/md/dm-mpath.c b/drivers/md/dm-mpath.c > > index c8faa2b85842..8fe3f45407ce 100644 > > --- a/drivers/md/dm-mpath.c > > +++ b/drivers/md/dm-mpath.c > > @@ -484,9 +484,7 @@ static int multipath_clone_and_map(struct dm_target *ti, struct request *rq, > > return DM_MAPIO_KILL; > > } else if (test_bit(MPATHF_QUEUE_IO, &m->flags) || > > test_bit(MPATHF_PG_INIT_REQUIRED, &m->flags)) { > > - if (pg_init_all_paths(m)) > > - return DM_MAPIO_DELAY_REQUEUE; > > - return DM_MAPIO_REQUEUE; > > + return DM_MAPIO_DELAY_REQUEUE; > > } > > This patch removes a pg_init_all_paths() call but you don't explain why you > think it is allowed to remove that call. Did you perhaps remove that call by > mistake? OK, that is a problem, will fix it in V2.
diff --git a/drivers/md/dm-mpath.c b/drivers/md/dm-mpath.c index c8faa2b85842..8fe3f45407ce 100644 --- a/drivers/md/dm-mpath.c +++ b/drivers/md/dm-mpath.c @@ -484,9 +484,7 @@ static int multipath_clone_and_map(struct dm_target *ti, struct request *rq, return DM_MAPIO_KILL; } else if (test_bit(MPATHF_QUEUE_IO, &m->flags) || test_bit(MPATHF_PG_INIT_REQUIRED, &m->flags)) { - if (pg_init_all_paths(m)) - return DM_MAPIO_DELAY_REQUEUE; - return DM_MAPIO_REQUEUE; + return DM_MAPIO_DELAY_REQUEUE; } memset(mpio, 0, sizeof(*mpio)); diff --git a/drivers/md/dm-rq.c b/drivers/md/dm-rq.c index 9d32f25489c2..cbe8a06ef8b0 100644 --- a/drivers/md/dm-rq.c +++ b/drivers/md/dm-rq.c @@ -758,7 +758,6 @@ static blk_status_t dm_mq_queue_rq(struct blk_mq_hw_ctx *hctx, /* Undo dm_start_request() before requeuing */ rq_end_stats(md, rq); rq_completed(md, rq_data_dir(rq), false); - blk_mq_delay_run_hw_queue(hctx, 100/*ms*/); return BLK_STS_RESOURCE; }
If .queue_rq() returns BLK_STS_RESOURCE, blk-mq will rerun the queue in the three situations: 1) if BLK_MQ_S_SCHED_RESTART is set - queue is rerun after one rq is completed, see blk_mq_sched_restart() which is run from blk_mq_free_request() 2) run out of driver tag - queue is rerun after one tag is freed 3) otherwise - queue is run immediately in blk_mq_dispatch_rq_list() This random dealy of running hw queue is introduced by commit 6077c2d706097c0 (dm rq: Avoid that request processing stalls sporadically), which claimed one request processing stalling is fixed, but never explained the behind idea, and it is a workaound at most. Even the question isn't explained by anyone in recent discussion. Also calling blk_mq_delay_run_hw_queue() inside .queue_rq() is a horrible hack because it makes BLK_MQ_S_SCHED_RESTART not working, and degrades I/O peformance a lot. Finally this patch makes sure that dm-rq returns BLK_STS_RESOURCE to blk-mq only when underlying queue is out of resource, so we switch to return DM_MAPIO_DELAY_REQUEU if either MPATHF_QUEUE_IO or MPATHF_PG_INIT_REQUIRED is set in multipath_clone_and_map(). Signed-off-by: Ming Lei <ming.lei@redhat.com> --- drivers/md/dm-mpath.c | 4 +--- drivers/md/dm-rq.c | 1 - 2 files changed, 1 insertion(+), 4 deletions(-)