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 10:46:58 +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

Hi!

On Thu 27-08-20 16:28:37, 田 wrote:
> I understood what you said before:)

Good :)

> Totally agree with you, that we should fix the common path to make it to
> account IO wait time.  Currently kernel only has io_wait_event(),   which
> does not support timeout, maybe we need develop new interface like
> io_wait_event_hrtimeout(), then we can use it instead of
> wait_event_interruptible_hrtimeout()?

Yes, that's what I'd do.

								Honza

> 
> On 08/27/2020 15:55, Jan Kara wrote:
> 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
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR