block: inherit request start time from bio for BLK_CGROUP
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>
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