Re: [PATCH] block: check more requests for multiple_queues in blk_attempt_plug_merge
From: Song Liu <song@kernel.org>
Date: 2022-03-10 22:10:45
Also in:
linux-block, stable
On Wed, Mar 9, 2022 at 11:23 PM Song Liu [off-list ref] wrote:
On Wed, Mar 9, 2022 at 10:48 PM Christoph Hellwig [off-list ref] wrote:quoted
On Tue, Mar 08, 2022 at 10:42:09PM -0800, Song Liu wrote:quoted
RAID arrays check/repair operations benefit a lot from merging requests. If we only check the previous entry for merge attempt, many merge will be missed. As a result, significant regression is observed for RAID check and repair. Fix this by checking more than just the previous entry when plug->multiple_queues == true.But this also means really significant CPU overhead for all other workloads.Would the following check help with these workloads? if (!plug->multiple_queues) break;quoted
quoted
This improves the check/repair speed of a 20-HDD raid6 from 19 MB/s to 103 MB/s.What driver uses multiple queues for HDDs? Can you explain the workload submitted by a md a bit better? I wonder if we can easily do the right thing straight in the md driver.It is the md sync_thread doing check and repair. Basically, the md thread reads all the disks and computes parity from data. Maybe we should add a new flag to struct blk_plug for this special case?
I meant something like:
diff --git c/block/blk-core.c w/block/blk-core.c
index 1039515c99d6..4fb09243e908 100644
--- c/block/blk-core.c
+++ w/block/blk-core.c@@ -1303,6 +1303,12 @@ void blk_finish_plug(struct blk_plug *plug) } EXPORT_SYMBOL(blk_finish_plug); +void blk_plug_merge_aggressively(struct blk_plug *plug) +{ + plug->aggresive_merge = true; +} +EXPORT_SYMBOL(blk_plug_merge_aggressively); + void blk_io_schedule(void) { /* Prevent hang_check timer from firing at us during very long I/O */
diff --git c/block/blk-merge.c w/block/blk-merge.c
index 4de34a332c9f..8b673288bc5f 100644
--- c/block/blk-merge.c
+++ w/block/blk-merge.c@@ -1089,12 +1089,14 @@ bool blk_attempt_plug_merge(structrequest_queue *q, struct bio *bio,
if (!plug || rq_list_empty(plug->mq_list))
return false;
- /* check the previously added entry for a quick merge attempt */
- rq = rq_list_peek(&plug->mq_list);
- if (rq->q == q) {
- if (blk_attempt_bio_merge(q, rq, bio, nr_segs, false) ==
- BIO_MERGE_OK)
- return true;
+ rq_list_for_each(&plug->mq_list, rq) {
+ if (rq->q == q) {
+ if (blk_attempt_bio_merge(q, rq, bio, nr_segs, false) ==
+ BIO_MERGE_OK)
+ return true;
+ }
+ if (!plug->aggresive_merge)
+ break;
}
return false;
}diff --git c/drivers/md/md.c w/drivers/md/md.c
index 4d38bd7dadd6..6be56632a412 100644
--- c/drivers/md/md.c
+++ w/drivers/md/md.c@@ -8901,6 +8901,7 @@ void md_do_sync(struct md_thread *thread) update_time = jiffies; blk_start_plug(&plug); + blk_plug_merge_aggressively(&plug); while (j < max_sectors) { sector_t sectors;
diff --git c/drivers/md/raid1.c w/drivers/md/raid1.c
index e2d8acb1e988..501d15532170 100644
--- c/drivers/md/raid1.c
+++ w/drivers/md/raid1.c@@ -838,6 +838,7 @@ static void flush_pending_writes(struct r1conf *conf) */ __set_current_state(TASK_RUNNING); blk_start_plug(&plug); + blk_plug_merge_aggressively(&plug); flush_bio_list(conf, bio); blk_finish_plug(&plug); } else
@@ -2591,6 +2592,7 @@ static void raid1d(struct md_thread *thread) } blk_start_plug(&plug); + blk_plug_merge_aggressively(&plug); for (;;) { flush_pending_writes(conf);
diff --git c/drivers/md/raid10.c w/drivers/md/raid10.c
index 2b969f70a31f..0a594613a075 100644
--- c/drivers/md/raid10.c
+++ w/drivers/md/raid10.c@@ -876,6 +876,7 @@ static void flush_pending_writes(struct r10conf *conf) __set_current_state(TASK_RUNNING); blk_start_plug(&plug); + blk_plug_merge_aggressively(&plug); /* flush any pending bitmap writes to disk * before proceeding w/ I/O */ md_bitmap_unplug(conf->mddev->bitmap);
@@ -3088,6 +3089,7 @@ static void raid10d(struct md_thread *thread) } blk_start_plug(&plug); + blk_plug_merge_aggressively(&plug); for (;;) { flush_pending_writes(conf);
diff --git c/drivers/md/raid5.c w/drivers/md/raid5.c
index ffe720c73b0a..a96884ca5f08 100644
--- c/drivers/md/raid5.c
+++ w/drivers/md/raid5.c@@ -6447,6 +6447,7 @@ static void raid5_do_work(struct work_struct *work) pr_debug("+++ raid5worker active\n"); blk_start_plug(&plug); + blk_plug_merge_aggressively(&plug); handled = 0; spin_lock_irq(&conf->device_lock); while (1) {
@@ -6497,6 +6498,7 @@ static void raid5d(struct md_thread *thread) md_check_recovery(mddev); blk_start_plug(&plug); + blk_plug_merge_aggressively(&plug); handled = 0; spin_lock_irq(&conf->device_lock); while (1) {
diff --git c/include/linux/blkdev.h w/include/linux/blkdev.h
index 16b47035e4b0..45b0da416302 100644
--- c/include/linux/blkdev.h
+++ w/include/linux/blkdev.h@@ -775,6 +775,7 @@ struct blk_plug { bool multiple_queues; bool has_elevator; bool nowait; + bool aggresive_merge; struct list_head cb_list; /* md requires an unplug callback */ };
@@ -791,6 +792,7 @@ extern struct blk_plug_cb*blk_check_plugged(blk_plug_cb_fn unplug, extern void blk_start_plug(struct blk_plug *); extern void blk_start_plug_nr_ios(struct blk_plug *, unsigned short); extern void blk_finish_plug(struct blk_plug *); +void blk_plug_merge_aggressively(struct blk_plug *plug); void blk_flush_plug(struct blk_plug *plug, bool from_schedule);