Re: [PATCH v2] delayacct: Account blkio completion on the correct task

[Tejun Heo <tj@kernel.org>]

Hello,

On Mon, Dec 18, 2017 at 04:15:10PM +0000, Josh Snyder wrote:
> Before commit e33a9bba85a8 ("sched/core: move IO scheduling accounting from
> io_schedule_timeout() into scheduler"), delayacct_blkio_end was called after
> context-switching into the task which completed I/O. This resulted in double
> counting: the task would account a delay both waiting for I/O and for time
> spent in the runqueue.
> 
> With e33a9bba85a8, delayacct_blkio_end is called by try_to_wake_up. In
> ttwu, we have not yet context-switched. This is more correct, in that the
> delay accounting ends when the I/O is complete. But delayacct_blkio_end
> relies upon `get_current()`, and we have not yet context-switched into the
> task whose I/O completed. This results in the wrong task having its delay
> accounting statistics updated.
> 
> Instead of doing that, pass the task_struct being woken to
> delayacct_blkio_end, so that it can update the statistics of the correct
> task.
> 
> Fixes: e33a9bba85a8 ("sched/core: move IO scheduling accounting from io_schedule_timeout() into scheduler")
> Signed-off-by: Josh Snyder <joshs@netflix.com>

Acked-by: Tejun Heo <tj@kernel.org>

Thanks.

-- 
tejun