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Subject[PATCH] blk-mq: add cond_resched() in __blk_mq_alloc_rq_maps()
Date
We found it takes more time of blk_mq_alloc_rq_maps() in kernel space when
testing nvme hot-plugging. The test and anlysis as below.

Debug code,
1, blk_mq_alloc_rq_maps():
u64 start, end;
depth = set->queue_depth;
start = ktime_get_ns();
pr_err("[%d:%s switch:%ld,%ld] queue depth %d, nr_hw_queues %d\n",
current->pid, current->comm, current->nvcsw, current->nivcsw,
set->queue_depth, set->nr_hw_queues);
do {
err = __blk_mq_alloc_rq_maps(set);
if (!err)
break;

set->queue_depth >>= 1;
if (set->queue_depth < set->reserved_tags + BLK_MQ_TAG_MIN) {
err = -ENOMEM;
break;
}
} while (set->queue_depth);
end = ktime_get_ns();
pr_err("[%d:%s switch:%ld,%ld] all hw queues init cost time %lld ns\n",
current->pid, current->comm,
current->nvcsw, current->nivcsw, end - start);

2, __blk_mq_alloc_rq_maps():
u64 start, end;
for (i = 0; i < set->nr_hw_queues; i++) {
start = ktime_get_ns();
if (!__blk_mq_alloc_rq_map(set, i))
goto out_unwind;
end = ktime_get_ns();
pr_err("hw queue %d init cost time %lld\n", i, end - start);
}

Test nvme hot-plugging with above debug code, we found it totally cost more
than 3ms in kernel space without being scheduled out when alloc rqs for all
hw queues, each hw queue cost about 140-250us. The time cost will be
increased with hw queue number increasing. And if __blk_mq_alloc_rq_maps()
returns -ENOMEM, it will try "queue_depth >>= 1", it will cost more time.
[ 428.428771] nvme nvme0: pci function 10000:01:00.0
[ 428.428798] nvme 10000:01:00.0: enabling device (0000 -> 0002)
[ 428.428806] pcieport 10000:00:00.0: can't derive routing for PCI INT A
[ 428.428809] nvme 10000:01:00.0: PCI INT A: no GSI
[ 432.593374] [4688:kworker/u33:8 switch:663,2] queue depth 30, nr_hw_queues 1
[ 432.593404] hw queue 0 init cost time 22883 ns
[ 432.593408] [4688:kworker/u33:8 switch:663,2] all hw queues init cost time 35960 ns
[ 432.595953] nvme nvme0: 16/0/0 default/read/poll queues
[ 432.595958] [4688:kworker/u33:8 switch:700,2] queue depth 1023, nr_hw_queues 16
[ 432.596203] hw queue 0 init cost time 242630 ns
[ 432.596441] hw queue 1 init cost time 235913 ns
[ 432.596659] hw queue 2 init cost time 216461 ns
[ 432.596877] hw queue 3 init cost time 215851 ns
[ 432.597107] hw queue 4 init cost time 228406 ns
[ 432.597336] hw queue 5 init cost time 227298 ns
[ 432.597564] hw queue 6 init cost time 224633 ns
[ 432.597785] hw queue 7 init cost time 219954 ns
[ 432.597937] hw queue 8 init cost time 150930 ns
[ 432.598082] hw queue 9 init cost time 143496 ns
[ 432.598231] hw queue 10 init cost time 147261 ns
[ 432.598397] hw queue 11 init cost time 164522 ns
[ 432.598542] hw queue 12 init cost time 143401 ns
[ 432.598692] hw queue 13 init cost time 148934 ns
[ 432.598841] hw queue 14 init cost time 147194 ns
[ 432.598991] hw queue 15 init cost time 148942 ns
[ 432.598993] [4688:kworker/u33:8 switch:700,2] all hw queues init cost time 3035099 ns
[ 432.602611] nvme0n1: p1

So use this patch to trigger schedule between each hw queue init, to avoid
stuck of other tasks.

Signed-off-by: Xianting Tian <tian.xianting@h3c.com>
---
block/blk-mq.c | 8 +++++++-
1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/block/blk-mq.c b/block/blk-mq.c
index b3d2785ee..12418ecf5 100644
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -3256,10 +3256,13 @@ static int __blk_mq_alloc_rq_maps(struct blk_mq_tag_set *set)
{
int i;

- for (i = 0; i < set->nr_hw_queues; i++)
+ for (i = 0; i < set->nr_hw_queues; i++) {
if (!__blk_mq_alloc_map_and_request(set, i))
goto out_unwind;

+ cond_resched();
+ }
+
return 0;

out_unwind:
@@ -3292,6 +3295,9 @@ static int blk_mq_alloc_map_and_requests(struct blk_mq_tag_set *set)
}
} while (set->queue_depth);

+
+
+
if (!set->queue_depth || err) {
pr_err("blk-mq: failed to allocate request map\n");
return -ENOMEM;
--
2.17.1
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