[PATCH 1/1] [RFC] blk-mq: fix queue stalling on shared hctx restart
From: Roman Pen
Date: Wed Oct 18 2017 - 06:22:35 EST
Hi all,
the patch below fixes queue stalling when shared hctx marked for restart
(BLK_MQ_S_SCHED_RESTART bit) but q->shared_hctx_restart stays zero. The
root cause is that hctxs are shared between queues, but 'shared_hctx_restart'
belongs to the particular queue, which in fact may not need to be restarted,
thus we return from blk_mq_sched_restart() and leave shared hctx of another
queue never restarted.
The fix is to make shared_hctx_restart counter belong not to the queue, but
to tags, thereby counter will reflect real number of shared hctx needed to
be restarted.
During tests 1 hctx (set->nr_hw_queues) was used and all stalled requests
were noticed in dd->fifo_list of mq-deadline scheduler.
Seeming possible sequence of events:
1. Request A of queue A is inserted into dd->fifo_list of the scheduler.
2. Request B of queue A bypasses scheduler and goes directly to
hctx->dispatch.
3. Request C of queue B is inserted.
4. blk_mq_sched_dispatch_requests() is invoked, since hctx->dispatch is not
empty (request B is in the list) hctx is only marked for for next restart
and request A is left in a list (see comment "So it's best to leave them
there for as long as we can. Mark the hw queue as needing a restart in
that case." in blk-mq-sched.c)
5. Eventually request B is completed/freed and blk_mq_sched_restart() is
called, but by chance hctx from queue B is chosen for restart and request C
gets a chance to be dispatched.
6. Eventually request C is completed/freed and blk_mq_sched_restart() is
called, but shared_hctx_restart for queue B is zero and we return without
attempt to restart hctx from queue A, thus request A is stuck forever.
But stalling queue is not the only one problem with blk_mq_sched_restart().
My tests show that those loops thru all queues and hctxs can be very costly,
even with shared_hctx_restart counter, which aims to fix performance issue.
For my tests I create 128 devices with 64 hctx each, which share same tags
set.
The following is the fio and ftrace output for v4.14-rc4 kernel:
READ: io=5630.3MB, aggrb=573208KB/s, minb=573208KB/s, maxb=573208KB/s, mint=10058msec, maxt=10058msec
WRITE: io=5650.9MB, aggrb=575312KB/s, minb=575312KB/s, maxb=575312KB/s, mint=10058msec, maxt=10058msec
root@pserver16:~/roman# cat /sys/kernel/debug/tracing/trace_stat/* | grep blk_mq
Function Hit Time Avg s^2
-------- --- ---- --- ---
blk_mq_sched_restart 16347 9540759 us 583.639 us 8804801 us
blk_mq_sched_restart 7884 6073471 us 770.354 us 8780054 us
blk_mq_sched_restart 14176 7586794 us 535.185 us 2822731 us
blk_mq_sched_restart 7843 6205435 us 791.206 us 12424960 us
blk_mq_sched_restart 1490 4786107 us 3212.153 us 1949753 us
blk_mq_sched_restart 7892 6039311 us 765.244 us 2994627 us
blk_mq_sched_restart 15382 7511126 us 488.306 us 3090912 us
[cut]
And here are results with two patches reverted:
8e8320c9315c ("blk-mq: fix performance regression with shared tags")
6d8c6c0f97ad ("blk-mq: Restart a single queue if tag sets are shared")
READ: io=12884MB, aggrb=1284.3MB/s, minb=1284.3MB/s, maxb=1284.3MB/s, mint=10032msec, maxt=10032msec
WRITE: io=12987MB, aggrb=1294.6MB/s, minb=1294.6MB/s, maxb=1294.6MB/s, mint=10032msec, maxt=10032msec
root@pserver16:~/roman# cat /sys/kernel/debug/tracing/trace_stat/* | grep blk_mq
Function Hit Time Avg s^2
-------- --- ---- --- ---
blk_mq_sched_restart 50699 8802.349 us 0.173 us 121.771 us
blk_mq_sched_restart 50362 8740.470 us 0.173 us 161.494 us
blk_mq_sched_restart 50402 9066.337 us 0.179 us 113.009 us
blk_mq_sched_restart 50104 9366.197 us 0.186 us 188.645 us
blk_mq_sched_restart 50375 9317.727 us 0.184 us 54.218 us
blk_mq_sched_restart 50136 9311.657 us 0.185 us 446.790 us
blk_mq_sched_restart 50103 9179.625 us 0.183 us 114.472 us
[cut]
Timings and stdevs are terrible, which leads to significant difference:
570MB/s vs 1280MB/s.
This is RFC since current patch fixes queue stalling but performance issue
still remains and for me is not clear is it better to improve commit
6d8c6c0f97ad ("blk-mq: Restart a single queue if tag sets are shared")
making percpu restart lists (to avoid looping and to dequeue hctx immediately)
or revert it (frankly I did not notice any difference on small number of
devices and hctxs, when looping issue does not impact much).
--
Roman
Signed-off-by: Roman Pen <roman.penyaev@xxxxxxxxxxxxxxxx>
Cc: linux-kernel@xxxxxxxxxxxxxxx
Cc: linux-block@xxxxxxxxxxxxxxx
Cc: Bart Van Assche <bart.vanassche@xxxxxxxxxxx>
Cc: Christoph Hellwig <hch@xxxxxx>
Cc: Hannes Reinecke <hare@xxxxxxxx>
Cc: Jens Axboe <axboe@xxxxxx>
---
block/blk-mq-sched.c | 10 +++++-----
block/blk-mq-tag.c | 1 +
block/blk-mq-tag.h | 1 +
block/blk-mq.c | 4 ++--
include/linux/blkdev.h | 2 --
5 files changed, 9 insertions(+), 9 deletions(-)
diff --git a/block/blk-mq-sched.c b/block/blk-mq-sched.c
index 4ab69435708c..a19a7f275173 100644
--- a/block/blk-mq-sched.c
+++ b/block/blk-mq-sched.c
@@ -60,10 +60,10 @@ static void blk_mq_sched_mark_restart_hctx(struct blk_mq_hw_ctx *hctx)
return;
if (hctx->flags & BLK_MQ_F_TAG_SHARED) {
- struct request_queue *q = hctx->queue;
+ struct blk_mq_tags *tags = hctx->tags;
if (!test_and_set_bit(BLK_MQ_S_SCHED_RESTART, &hctx->state))
- atomic_inc(&q->shared_hctx_restart);
+ atomic_inc(&tags->shared_hctx_restart);
} else
set_bit(BLK_MQ_S_SCHED_RESTART, &hctx->state);
}
@@ -74,10 +74,10 @@ static bool blk_mq_sched_restart_hctx(struct blk_mq_hw_ctx *hctx)
return false;
if (hctx->flags & BLK_MQ_F_TAG_SHARED) {
- struct request_queue *q = hctx->queue;
+ struct blk_mq_tags *tags = hctx->tags;
if (test_and_clear_bit(BLK_MQ_S_SCHED_RESTART, &hctx->state))
- atomic_dec(&q->shared_hctx_restart);
+ atomic_dec(&tags->shared_hctx_restart);
} else
clear_bit(BLK_MQ_S_SCHED_RESTART, &hctx->state);
@@ -312,7 +312,7 @@ void blk_mq_sched_restart(struct blk_mq_hw_ctx *const hctx)
* If this is 0, then we know that no hardware queues
* have RESTART marked. We're done.
*/
- if (!atomic_read(&queue->shared_hctx_restart))
+ if (!atomic_read(&tags->shared_hctx_restart))
return;
rcu_read_lock();
diff --git a/block/blk-mq-tag.c b/block/blk-mq-tag.c
index d0be72ccb091..598f8e0095ff 100644
--- a/block/blk-mq-tag.c
+++ b/block/blk-mq-tag.c
@@ -385,6 +385,7 @@ struct blk_mq_tags *blk_mq_init_tags(unsigned int total_tags,
tags->nr_tags = total_tags;
tags->nr_reserved_tags = reserved_tags;
+ atomic_set(&tags->shared_hctx_restart, 0);
return blk_mq_init_bitmap_tags(tags, node, alloc_policy);
}
diff --git a/block/blk-mq-tag.h b/block/blk-mq-tag.h
index 5cb51e53cc03..adf05c8811cd 100644
--- a/block/blk-mq-tag.h
+++ b/block/blk-mq-tag.h
@@ -11,6 +11,7 @@ struct blk_mq_tags {
unsigned int nr_reserved_tags;
atomic_t active_queues;
+ atomic_t shared_hctx_restart;
struct sbitmap_queue bitmap_tags;
struct sbitmap_queue breserved_tags;
diff --git a/block/blk-mq.c b/block/blk-mq.c
index 4603b115e234..7639f978ea2c 100644
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -2120,11 +2120,11 @@ static void queue_set_hctx_shared(struct request_queue *q, bool shared)
queue_for_each_hw_ctx(q, hctx, i) {
if (shared) {
if (test_bit(BLK_MQ_S_SCHED_RESTART, &hctx->state))
- atomic_inc(&q->shared_hctx_restart);
+ atomic_inc(&hctx->tags->shared_hctx_restart);
hctx->flags |= BLK_MQ_F_TAG_SHARED;
} else {
if (test_bit(BLK_MQ_S_SCHED_RESTART, &hctx->state))
- atomic_dec(&q->shared_hctx_restart);
+ atomic_dec(&hctx->tags->shared_hctx_restart);
hctx->flags &= ~BLK_MQ_F_TAG_SHARED;
}
}
diff --git a/include/linux/blkdev.h b/include/linux/blkdev.h
index 2a5d52fa90f5..3852a9ea87d0 100644
--- a/include/linux/blkdev.h
+++ b/include/linux/blkdev.h
@@ -393,8 +393,6 @@ struct request_queue {
int nr_rqs[2]; /* # allocated [a]sync rqs */
int nr_rqs_elvpriv; /* # allocated rqs w/ elvpriv */
- atomic_t shared_hctx_restart;
-
struct blk_queue_stats *stats;
struct rq_wb *rq_wb;
--
2.13.1