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block: inherit request start time from bio for BLK_CGROUP

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Doing high IOPS testing with blk-cgroups enabled spends ~15-20% of the
time just doing ktime_get_ns() -> readtsc. We essentially read and
set the start time twice, one for the bio and then again when that bio
is mapped to a request.

Given that the time between the two is very short, inherit the bio
start time instead of reading it again. This cuts 1/3rd of the overhead
of the time keeping.

Acked-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
This commit is contained in:
Jens Axboe 2021-10-05 09:23:59 -06:00
parent a7b36ee6ba
commit 0006707723
1 changed files with 8 additions and 1 deletions
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9
block/blk-mq.c
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@ -725,7 +725,14 @@ void blk_mq_start_request(struct request *rq)
trace_block_rq_issue(rq);
if (test_bit(QUEUE_FLAG_STATS, &q->queue_flags)) {
rq->io_start_time_ns = ktime_get_ns();
u64 start_time;
#ifdef CONFIG_BLK_CGROUP
if (rq->bio)
start_time = bio_issue_time(&rq->bio->bi_issue);
else
#endif
start_time = ktime_get_ns();
rq->io_start_time_ns = start_time;
rq->stats_sectors = blk_rq_sectors(rq);
rq->rq_flags |= RQF_STATS;
rq_qos_issue(q, rq);