Re: [PATCH 6.4 800/800] io_uring: Use io_schedule* in cqring wait

[Oleksandr Natalenko]

On neděle 23. července 2023 12:50:30 CEST Greg Kroah-Hartman wrote:
> On Sun, Jul 23, 2023 at 11:39:42AM +0200, Oleksandr Natalenko wrote:
> > Hello.
> > 
> > On neděle 16. července 2023 21:50:53 CEST Greg Kroah-Hartman wrote:
> > > From: Andres Freund <andres@xxxxxxxxxxx>
> > > 
> > > commit 8a796565cec3601071cbbd27d6304e202019d014 upstream.
> > > 
> > > I observed poor performance of io_uring compared to synchronous IO. That
> > > turns out to be caused by deeper CPU idle states entered with io_uring,
> > > due to io_uring using plain schedule(), whereas synchronous IO uses
> > > io_schedule().
> > > 
> > > The losses due to this are substantial. On my cascade lake workstation,
> > > t/io_uring from the fio repository e.g. yields regressions between 20%
> > > and 40% with the following command:
> > > ./t/io_uring -r 5 -X0 -d 1 -s 1 -c 1 -p 0 -S$use_sync -R 0 /mnt/t2/fio/write.0.0
> > > 
> > > This is repeatable with different filesystems, using raw block devices
> > > and using different block devices.
> > > 
> > > Use io_schedule_prepare() / io_schedule_finish() in
> > > io_cqring_wait_schedule() to address the difference.
> > > 
> > > After that using io_uring is on par or surpassing synchronous IO (using
> > > registered files etc makes it reliably win, but arguably is a less fair
> > > comparison).
> > > 
> > > There are other calls to schedule() in io_uring/, but none immediately
> > > jump out to be similarly situated, so I did not touch them. Similarly,
> > > it's possible that mutex_lock_io() should be used, but it's not clear if
> > > there are cases where that matters.
> > > 
> > > Cc: stable@xxxxxxxxxxxxxxx # 5.10+
> > > Cc: Pavel Begunkov <asml.silence@xxxxxxxxx>
> > > Cc: io-uring@xxxxxxxxxxxxxxx
> > > Cc: linux-kernel@xxxxxxxxxxxxxxx
> > > Signed-off-by: Andres Freund <andres@xxxxxxxxxxx>
> > > Link: https://lore.kernel.org/r/20230707162007.194068-1-andres@xxxxxxxxxxx
> > > [axboe: minor style fixup]
> > > Signed-off-by: Jens Axboe <axboe@xxxxxxxxx>
> > > Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
> > > ---
> > >  io_uring/io_uring.c |   15 +++++++++++++--
> > >  1 file changed, 13 insertions(+), 2 deletions(-)
> > > 
> > > --- a/io_uring/io_uring.c
> > > +++ b/io_uring/io_uring.c
> > > @@ -2575,6 +2575,8 @@ int io_run_task_work_sig(struct io_ring_
> > >  static inline int io_cqring_wait_schedule(struct io_ring_ctx *ctx,
> > >  					  struct io_wait_queue *iowq)
> > >  {
> > > +	int token, ret;
> > > +
> > >  	if (unlikely(READ_ONCE(ctx->check_cq)))
> > >  		return 1;
> > >  	if (unlikely(!llist_empty(&ctx->work_llist)))
> > > @@ -2585,11 +2587,20 @@ static inline int io_cqring_wait_schedul
> > >  		return -EINTR;
> > >  	if (unlikely(io_should_wake(iowq)))
> > >  		return 0;
> > > +
> > > +	/*
> > > +	 * Use io_schedule_prepare/finish, so cpufreq can take into account
> > > +	 * that the task is waiting for IO - turns out to be important for low
> > > +	 * QD IO.
> > > +	 */
> > > +	token = io_schedule_prepare();
> > > +	ret = 0;
> > >  	if (iowq->timeout == KTIME_MAX)
> > >  		schedule();
> > >  	else if (!schedule_hrtimeout(&iowq->timeout, HRTIMER_MODE_ABS))
> > > -		return -ETIME;
> > > -	return 0;
> > > +		ret = -ETIME;
> > > +	io_schedule_finish(token);
> > > +	return ret;
> > >  }
> > >  
> > >  /*
> > 
> > Reportedly, this caused a regression as reported in [1] [2] [3]. Not only v6.4.4 is affected, v6.1.39 is affected too.
> > 
> > Reverting this commit fixes the issue.
> > 
> > Please check.
> 
> Is this also an issue in 6.5-rc2?

As per [1], yes.

[1] https://bugzilla.kernel.org/show_bug.cgi?id=217699#c4

> 
> thanks,
> 
> greg k-h
> 


-- 
Oleksandr Natalenko (post-factum)

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