在 2022/06/14 15:31, Ming Lei 写道:
On Tue, Jun 14, 2022 at 03:14:10PM +0800, Yu Kuai wrote:
We found that boot time is increased for about 8s after upgrading kernel
from v4.19 to v5.10(megaraid-sas is used in the environment).
But 'blk-mq: clearing flush request reference in tags->rqs[]' was merged
to v5.14, :-)
Hi,
Yes, but this patch is applied to 5.10 stable, thus we backport in our
v5.10. Sorry that I didn't mention that.
Following is where the extra time is spent:
__scsi_remove_device
blk_cleanup_queue
blk_mq_exit_queue
blk_mq_exit_hw_queues
blk_mq_exit_hctx
blk_mq_clear_flush_rq_mapping -> function latency is 0.1ms
So queue_depth looks pretty long, is it 4k?
No, in the environment, it's just 32, and nr_hw_queues is 128, which
means each blk_cleanup_queue() will cost about 10-20 ms.
But if it is 0.1ms, how can the 8sec delay be caused? That requires 80K hw queues
for making so long, so I guess there must be other delay added by the feature
of BLK_MQ_F_TAG_HCTX_SHARED.
Please see details in the reasons 2), scsi scan will call
__scsi_remove_device() a lot of times(each host, each channel, each
target).
cmpxchg
There are three reasons:
1) megaraid-sas is using multiple hctxs in v5.10, thus blk_mq_exit_hctx()
will be called much more times in v5.10 compared to v4.19.
2) scsi will scan for each target thus __scsi_remove_device() will be
called for many times.
3) blk_mq_clear_flush_rq_mapping() is introduced after v4.19, it will
call cmpxchg() for each request, and function latency is abount 0.1ms.
Since that blk_mq_clear_flush_rq_mapping() will only be called while the
queue is freezed already, which means there is no inflight request,
it's safe to set NULL for 'tags->rqs[]' directly instead of using
cmpxchg(). Tests show that with this change, function latency of
blk_mq_clear_flush_rq_mapping() is about 1us, and boot time is not
increased.
tags is shared among all LUNs attached to the host, so freezing single
request queue here means nothing, so your patch doesn't work.
You'are right, I forgot about that tags can be shared.
Please test the following patch, and see if it can improve boot delay for
your case.
diff --git a/block/blk-mq.c b/block/blk-mq.c
index e9bf950983c7..1463076a527c 100644
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -3443,8 +3443,9 @@ static void blk_mq_exit_hctx(struct request_queue *q,
if (blk_mq_hw_queue_mapped(hctx))
blk_mq_tag_idle(hctx);
- blk_mq_clear_flush_rq_mapping(set->tags[hctx_idx],
- set->queue_depth, flush_rq);
+ if (blk_queue_init_done(q))
+ blk_mq_clear_flush_rq_mapping(set->tags[hctx_idx],
+ set->queue_depth, flush_rq);
if (set->ops->exit_request)
set->ops->exit_request(set, flush_rq, hctx_idx);
Thanks for the patch, I test it and boot delay is fixed.
Kuai
Thanks,
Ming
.
