linux-kernel - Re: [PATCH] aio: use wait_for_completion_io() when waiting for completion of io

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Date:   Thu, 27 Aug 2020 09:55:37 +0200
From:   Jan Kara <jack@...e.cz>
To:     田 <xianting_tian@....com>
Cc:     Jan Kara <jack@...e.cz>, "bcrl@...ck.org" <bcrl@...ck.org>,
        "viro@...iv.linux.org.uk" <viro@...iv.linux.org.uk>,
        "linux-aio@...ck.org" <linux-aio@...ck.org>,
        "linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] aio: use wait_for_completion_io() when waiting for
 completion of io

Hello!

On Wed 26-08-20 23:44:11, 田 wrote:
> thanks for your kindly reply,
> the normal wait path read_events()->wait_event_interruptible_hrtimeout(),
> which will call schedule(), it does not account IO wait time.

Not sure if there isn't some misunderstanding so I'll repeat what I've
said: Yes, above path will not account as IO wait time and IMO that is much
more common path which should be accounted as IO wait time. So I think that
without fixing that path, fixing cornercases like you did in your patch is
rather pointless.

								Honza

> On 08/26/2020 21:23, Jan Kara wrote:
> On Wed 05-08-20 09:35:51, Xianting Tian wrote:
> > When waiting for the completion of io, we need account iowait time. As
> > wait_for_completion() calls schedule_timeout(), which doesn't account
> > iowait time. While wait_for_completion_io() calls io_schedule_timeout(),
> > which will account iowait time.
> >
> > So using wait_for_completion_io() instead of wait_for_completion()
> > when waiting for completion of io before exit_aio and io_destroy.
> >
> > Signed-off-by: Xianting Tian <xianting_tian@....com>
> 
> Thanks for the patch! It looks good to me but IMO this is just scratching
> the surface.  E.g. for AIO we are mostly going to wait in read_events() by
> wait_event_interruptible_hrtimeout() and *that* doesn't account as IO wait
> either? Which is IMO far bigger misaccounting... The two case you fix seem
> to be just rare cornercases so what they do isn't a big deal either way.
> 
> So I agree it may be worth it to properly account waiting for AIO but if
> you want to do that, then please handle mainly the common cases in AIO
> code.
> 
>                                         Honza
> 
> > ---
> >  fs/aio.c | 4 ++--
> >  1 file changed, 2 insertions(+), 2 deletions(-)
> >
> > diff --git a/fs/aio.c b/fs/aio.c
> > index 91e7cc4..498b8a0 100644
> > --- a/fs/aio.c
> > +++ b/fs/aio.c
> > @@ -892,7 +892,7 @@ void exit_aio(struct mm_struct *mm)
> >  
> >       if (!atomic_sub_and_test(skipped, &wait.count)) {
> >            /* Wait until all IO for the context are done. */
> > -          wait_for_completion(&wait.comp);
> > +          wait_for_completion_io(&wait.comp);
> >       }
> >  
> >       RCU_INIT_POINTER(mm->ioctx_table, NULL);
> > @@ -1400,7 +1400,7 @@ static long read_events(struct kioctx *ctx, long min_nr, long nr,
> >             * is destroyed.
> >             */
> >            if (!ret)
> > -               wait_for_completion(&wait.comp);
> > +               wait_for_completion_io(&wait.comp);
> >  
> >            return ret;
> >       }
> > --
> > 1.8.3.1
> >
> --
> Jan Kara <jack@...e.com>
> SUSE Labs, CR
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR