Re: [PATCH] delayacct: Record blkio start on the correct task

[Jie Du]

Hi,

Please disregard this patch. As Christian pointed out in the report
thread, 5d010dd27be9 ("sched: Account I/O waits independently of delayed
dequeue") in peterz/queue sched/urgent already fixes this, and goes
further: it takes the blkio timestamp in try_to_block_task(), while the
task is still current, and covers nr_iowait too. I missed it before
sending, sorry for the noise.

I gave 5d010dd27be9 a try on top of v7.4-rc1 with the reproducer from
my patch, under QEMU (TCG, 2 vCPUs), 4 rounds of 60s each. Threads whose
blkio delay is larger than their age:

                      v7.4-rc1           v7.4-rc1 + 5d010dd27be9
  DELAY_DEQUEUE       20 of 14718        0 of 19122
  NO_DELAY_DEQUEUE     0 of 15766        0 of 20713

I also booted it with x86_debug.config and ran the reproducer for
2 rounds of 30s with no warnings in the log, and getdelays on a thread
doing 1000 O_DIRECT reads shows 1000 IO delays, one per read, averaging
0.715ms (min 0.54ms, max 1.59ms), so normal accounting looks fine.

Tested-by: Jie Du <806144828@xxxxxx>

Thanks,
Jie