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).
Following is where the extra time is spent:
scsi_probe_and_add_lun
__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
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.
Fixes: 364b61818f65 ("blk-mq: clearing flush request reference in tags->rqs[]")
Signed-off-by: Yu Kuai <yukuai3@xxxxxxxxxx>
---
block/blk-mq-tag.c | 2 +-
block/blk-mq.c | 2 +-
2 files changed, 2 insertions(+), 2 deletions(-)
diff --git a/block/blk-mq-tag.c b/block/blk-mq-tag.c
index 2dcd738c6952..d002eefcdaf5 100644
--- a/block/blk-mq-tag.c
+++ b/block/blk-mq-tag.c
@@ -253,7 +253,7 @@ static struct request *blk_mq_find_and_get_req(struct blk_mq_tags *tags,
unsigned long flags;
spin_lock_irqsave(&tags->lock, flags);
- rq = tags->rqs[bitnr];
+ rq = READ_ONCE(tags->rqs[bitnr]);
if (!rq || rq->tag != bitnr || !req_ref_inc_not_zero(rq))
rq = NULL;
spin_unlock_irqrestore(&tags->lock, flags);
diff --git a/block/blk-mq.c b/block/blk-mq.c
index e9bf950983c7..21ace698d3be 100644
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -3421,7 +3421,7 @@ static void blk_mq_clear_flush_rq_mapping(struct blk_mq_tags *tags,
WARN_ON_ONCE(req_ref_read(flush_rq) != 0);
for (i = 0; i < queue_depth; i++)
- cmpxchg(&tags->rqs[i], flush_rq, NULL);
+ WRITE_ONCE(tags->rqs[i], NULL);
/*
* Wait until all pending iteration is done.
--
2.31.1
