| Message ID | 20210106102428.551-3-jack@suse.cz (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
| Series | blk-mq: Improve performance of non-mq IO schedulers with multiple HW queues | expand |
On Wed, Jan 06, 2021 at 11:24:28AM +0100, Jan Kara wrote: > Currently when non-mq aware IO scheduler (BFQ, mq-deadline) is used for > a queue with multiple HW queues, the performance it rather bad. The > problem is that these IO schedulers use queue-wide locking and their > dispatch function does not respect the hctx it is passed in and returns > any request it finds appropriate. Thus locality of request access is > broken and dispatch from multiple CPUs just contends on IO scheduler > locks. For these IO schedulers there's little point in dispatching from > multiple CPUs. Instead dispatch always only from a single CPU to limit > contention. > > Below is a comparison of dbench runs on XFS filesystem where the storage > is a raid card with 64 HW queues and to it attached a single rotating > disk. BFQ is used as IO scheduler: > > clients MQ SQ MQ-Patched > Amean 1 39.12 (0.00%) 43.29 * -10.67%* 36.09 * 7.74%* > Amean 2 128.58 (0.00%) 101.30 * 21.22%* 96.14 * 25.23%* > Amean 4 577.42 (0.00%) 494.47 * 14.37%* 508.49 * 11.94%* > Amean 8 610.95 (0.00%) 363.86 * 40.44%* 362.12 * 40.73%* > Amean 16 391.78 (0.00%) 261.49 * 33.25%* 282.94 * 27.78%* > Amean 32 324.64 (0.00%) 267.71 * 17.54%* 233.00 * 28.23%* > Amean 64 295.04 (0.00%) 253.02 * 14.24%* 242.37 * 17.85%* > Amean 512 10281.61 (0.00%) 10211.16 * 0.69%* 10447.53 * -1.61%* > > Numbers are times so lower is better. MQ is stock 5.10-rc6 kernel. SQ is > the same kernel with megaraid_sas.host_tagset_enable=0 so that the card > advertises just a single HW queue. MQ-Patched is a kernel with this > patch applied. > > You can see multiple hardware queues heavily hurt performance in > combination with BFQ. The patch restores the performance. > > Signed-off-by: Jan Kara <jack@suse.cz> > --- > block/blk-mq.c | 62 ++++++++++++++++++++++++++++++++++------ > block/kyber-iosched.c | 1 + > include/linux/elevator.h | 2 ++ > 3 files changed, 56 insertions(+), 9 deletions(-) > > diff --git a/block/blk-mq.c b/block/blk-mq.c > index 57f3482b2c26..26e0f6e64a3a 100644 > --- a/block/blk-mq.c > +++ b/block/blk-mq.c > @@ -63,15 +63,20 @@ static int blk_mq_poll_stats_bkt(const struct request *rq) > return bucket; > } > > +/* Check if there are requests queued in hctx lists. */ > +static bool blk_mq_hctx_has_queued_rq(struct blk_mq_hw_ctx *hctx) > +{ > + return !list_empty_careful(&hctx->dispatch) || > + sbitmap_any_bit_set(&hctx->ctx_map); > +} > + blk_mq_hctx_mark_pending() is only called in case of none scheduler, so looks not necessary to check hctx->ctx_map in blk_mq_hctx_has_queued_rq() which is supposed to be used when real io scheduler is attached to MQ queue. Thanks, Ming
On Thu 07-01-21 14:19:18, Ming Lei wrote: > On Wed, Jan 06, 2021 at 11:24:28AM +0100, Jan Kara wrote: > > +/* Check if there are requests queued in hctx lists. */ > > +static bool blk_mq_hctx_has_queued_rq(struct blk_mq_hw_ctx *hctx) > > +{ > > + return !list_empty_careful(&hctx->dispatch) || > > + sbitmap_any_bit_set(&hctx->ctx_map); > > +} > > + > > blk_mq_hctx_mark_pending() is only called in case of none scheduler, so > looks not necessary to check hctx->ctx_map in blk_mq_hctx_has_queued_rq() > which is supposed to be used when real io scheduler is attached to MQ queue. Yes, I know. I just wanted to make the code less fragile... In particular I was somewhat uneasy that we'd rely on the implicit behavior that blk_mq_get_sqsched_hctx() can return non-NULL only if sbitmap_any_bit_set() is not needed. But maybe we could structure the code like: sq_hctx = NULL; if (blk_mq_has_sqsched(q)) sq_hctx = blk_mq_get_sq_hctx(q); queue_for_each_hw_ctx(q, hctx, i) { ... if (!sq_hctx || sq_hctx == hctx || !list_empty_careful(&hctx->dispatch)) ... run ... } Because then it is kind of obvious that sq_hctx is set only if there's IO scheduler for the queue and thus ctx_map is unused. What do you think? Honza
On Thu, Jan 07, 2021 at 12:18:15PM +0100, Jan Kara wrote: > On Thu 07-01-21 14:19:18, Ming Lei wrote: > > On Wed, Jan 06, 2021 at 11:24:28AM +0100, Jan Kara wrote: > > > +/* Check if there are requests queued in hctx lists. */ > > > +static bool blk_mq_hctx_has_queued_rq(struct blk_mq_hw_ctx *hctx) > > > +{ > > > + return !list_empty_careful(&hctx->dispatch) || > > > + sbitmap_any_bit_set(&hctx->ctx_map); > > > +} > > > + > > > > blk_mq_hctx_mark_pending() is only called in case of none scheduler, so > > looks not necessary to check hctx->ctx_map in blk_mq_hctx_has_queued_rq() > > which is supposed to be used when real io scheduler is attached to MQ queue. > > Yes, I know. I just wanted to make the code less fragile... In particular I > was somewhat uneasy that we'd rely on the implicit behavior that > blk_mq_get_sqsched_hctx() can return non-NULL only if sbitmap_any_bit_set() > is not needed. But maybe we could structure the code like: BTW, I mentioned the point because sbitmap_any_bit_set(hctx->ctx_map) may take some CPU cycle in case that nr_cpu_ids is big. > > sq_hctx = NULL; > if (blk_mq_has_sqsched(q)) > sq_hctx = blk_mq_get_sq_hctx(q); > queue_for_each_hw_ctx(q, hctx, i) { > ... > if (!sq_hctx || sq_hctx == hctx || > !list_empty_careful(&hctx->dispatch)) > ... run ... > } > > Because then it is kind of obvious that sq_hctx is set only if there's IO > scheduler for the queue and thus ctx_map is unused. What do you think? IMO, the above is more readable and efficient. Thanks, Ming
diff --git a/block/blk-mq.c b/block/blk-mq.c index 57f3482b2c26..26e0f6e64a3a 100644 --- a/block/blk-mq.c +++ b/block/blk-mq.c @@ -63,15 +63,20 @@ static int blk_mq_poll_stats_bkt(const struct request *rq) return bucket; } +/* Check if there are requests queued in hctx lists. */ +static bool blk_mq_hctx_has_queued_rq(struct blk_mq_hw_ctx *hctx) +{ + return !list_empty_careful(&hctx->dispatch) || + sbitmap_any_bit_set(&hctx->ctx_map); +} + /* * Check if any of the ctx, dispatch list or elevator * have pending work in this hardware queue. */ static bool blk_mq_hctx_has_pending(struct blk_mq_hw_ctx *hctx) { - return !list_empty_careful(&hctx->dispatch) || - sbitmap_any_bit_set(&hctx->ctx_map) || - blk_mq_sched_has_work(hctx); + return blk_mq_hctx_has_queued_rq(hctx) || blk_mq_sched_has_work(hctx); } /* @@ -1646,6 +1651,31 @@ void blk_mq_run_hw_queue(struct blk_mq_hw_ctx *hctx, bool async) } EXPORT_SYMBOL(blk_mq_run_hw_queue); +static struct blk_mq_hw_ctx *blk_mq_get_sqsched_hctx(struct request_queue *q) +{ + struct elevator_queue *e = q->elevator; + struct blk_mq_hw_ctx *hctx; + + /* + * The queue has multiple hardware queues but uses IO scheduler that + * does not respect hardware queues when dispatching? This is not a + * great setup but it can be sensible when we have a single rotational + * disk behind a raid card. Just don't bother with multiple HW queues + * and dispatch from hctx for the current CPU since running multiple + * queues just causes lock contention inside the scheduler and + * pointless cache bouncing because the hctx is not respected by the IO + * scheduler's dispatch function anyway. + */ + if (q->nr_hw_queues > 1 && e && e->type->ops.dispatch_request && + !(e->type->elevator_features & ELEVATOR_F_MQ_AWARE)) { + hctx = blk_mq_map_queue_type(q, HCTX_TYPE_DEFAULT, + raw_smp_processor_id()); + if (!blk_mq_hctx_stopped(hctx)) + return hctx; + } + return NULL; +} + /** * blk_mq_run_hw_queues - Run all hardware queues in a request queue. * @q: Pointer to the request queue to run. @@ -1653,14 +1683,21 @@ EXPORT_SYMBOL(blk_mq_run_hw_queue); */ void blk_mq_run_hw_queues(struct request_queue *q, bool async) { - struct blk_mq_hw_ctx *hctx; + struct blk_mq_hw_ctx *hctx, *sq_hctx; int i; + sq_hctx = blk_mq_get_sqsched_hctx(q); queue_for_each_hw_ctx(q, hctx, i) { if (blk_mq_hctx_stopped(hctx)) continue; - - blk_mq_run_hw_queue(hctx, async); + /* + * Dispatch from this hctx either if there's no hctx preferred + * by IO scheduler or if it has requests that bypass the + * scheduler. + */ + if (!sq_hctx || blk_mq_hctx_has_queued_rq(hctx) || + sq_hctx == hctx) + blk_mq_run_hw_queue(hctx, async); } } EXPORT_SYMBOL(blk_mq_run_hw_queues); @@ -1672,14 +1709,21 @@ EXPORT_SYMBOL(blk_mq_run_hw_queues); */ void blk_mq_delay_run_hw_queues(struct request_queue *q, unsigned long msecs) { - struct blk_mq_hw_ctx *hctx; + struct blk_mq_hw_ctx *hctx, *sq_hctx; int i; + sq_hctx = blk_mq_get_sqsched_hctx(q); queue_for_each_hw_ctx(q, hctx, i) { if (blk_mq_hctx_stopped(hctx)) continue; - - blk_mq_delay_run_hw_queue(hctx, msecs); + /* + * Dispatch from this hctx either if there's no hctx preferred + * by IO scheduler or if it has requests that bypass the + * scheduler. + */ + if (!sq_hctx || blk_mq_hctx_has_queued_rq(hctx) || + sq_hctx == hctx) + blk_mq_delay_run_hw_queue(hctx, msecs); } } EXPORT_SYMBOL(blk_mq_delay_run_hw_queues); diff --git a/block/kyber-iosched.c b/block/kyber-iosched.c index dc89199bc8c6..c25c41d0d061 100644 --- a/block/kyber-iosched.c +++ b/block/kyber-iosched.c @@ -1029,6 +1029,7 @@ static struct elevator_type kyber_sched = { #endif .elevator_attrs = kyber_sched_attrs, .elevator_name = "kyber", + .elevator_features = ELEVATOR_F_MQ_AWARE, .elevator_owner = THIS_MODULE, }; diff --git a/include/linux/elevator.h b/include/linux/elevator.h index bacc40a0bdf3..1fe8e105b83b 100644 --- a/include/linux/elevator.h +++ b/include/linux/elevator.h @@ -172,6 +172,8 @@ extern struct request *elv_rb_find(struct rb_root *, sector_t); /* Supports zoned block devices sequential write constraint */ #define ELEVATOR_F_ZBD_SEQ_WRITE (1U << 0) +/* Supports scheduling on multiple hardware queues */ +#define ELEVATOR_F_MQ_AWARE (1U << 1) #endif /* CONFIG_BLOCK */ #endif
Currently when non-mq aware IO scheduler (BFQ, mq-deadline) is used for a queue with multiple HW queues, the performance it rather bad. The problem is that these IO schedulers use queue-wide locking and their dispatch function does not respect the hctx it is passed in and returns any request it finds appropriate. Thus locality of request access is broken and dispatch from multiple CPUs just contends on IO scheduler locks. For these IO schedulers there's little point in dispatching from multiple CPUs. Instead dispatch always only from a single CPU to limit contention. Below is a comparison of dbench runs on XFS filesystem where the storage is a raid card with 64 HW queues and to it attached a single rotating disk. BFQ is used as IO scheduler: clients MQ SQ MQ-Patched Amean 1 39.12 (0.00%) 43.29 * -10.67%* 36.09 * 7.74%* Amean 2 128.58 (0.00%) 101.30 * 21.22%* 96.14 * 25.23%* Amean 4 577.42 (0.00%) 494.47 * 14.37%* 508.49 * 11.94%* Amean 8 610.95 (0.00%) 363.86 * 40.44%* 362.12 * 40.73%* Amean 16 391.78 (0.00%) 261.49 * 33.25%* 282.94 * 27.78%* Amean 32 324.64 (0.00%) 267.71 * 17.54%* 233.00 * 28.23%* Amean 64 295.04 (0.00%) 253.02 * 14.24%* 242.37 * 17.85%* Amean 512 10281.61 (0.00%) 10211.16 * 0.69%* 10447.53 * -1.61%* Numbers are times so lower is better. MQ is stock 5.10-rc6 kernel. SQ is the same kernel with megaraid_sas.host_tagset_enable=0 so that the card advertises just a single HW queue. MQ-Patched is a kernel with this patch applied. You can see multiple hardware queues heavily hurt performance in combination with BFQ. The patch restores the performance. Signed-off-by: Jan Kara <jack@suse.cz> --- block/blk-mq.c | 62 ++++++++++++++++++++++++++++++++++------ block/kyber-iosched.c | 1 + include/linux/elevator.h | 2 ++ 3 files changed, 56 insertions(+), 9 deletions(-)