linux-kernel - Re: [PATCH 1/1] mm/backing-dev.c: Call del_timer_sync instead of del

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Date:	Mon, 5 Sep 2011 18:05:34 +0200
From:	Jan Kara <jack@...e.cz>
To:	"kautuk.c @samsung.com" <consul.kautuk@...il.com>
Cc:	Jan Kara <jack@...e.cz>, Andrew Morton <akpm@...ux-foundation.org>,
	Jens Axboe <jaxboe@...ionio.com>,
	Wu Fengguang <fengguang.wu@...el.com>,
	Dave Chinner <dchinner@...hat.com>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/1] mm/backing-dev.c: Call del_timer_sync instead of
 del_timer

  Hi,

On Mon 05-09-11 20:06:04, kautuk.c @samsung.com wrote:
> >  OK, I don't care much whether we have there del_timer() or
> > del_timer_sync(). Let me just say that the race you are afraid of is
> > probably not going to happen in practice so I'm not sure it's valid to be
> > afraid of CPU cycles being burned needlessly. The timer is armed when an
> > dirty inode is first attached to default bdi's dirty list. Then the default
> > bdi flusher thread would have to be woken up so that following happens:
> >        CPU1                            CPU2
> >  timer fires -> wakeup_timer_fn()
> >                                        bdi_forker_thread()
> >                                          del_timer(&me->wakeup_timer);
> >                                          wb_do_writeback(me, 0);
> >                                          ...
> >                                          set_current_state(TASK_INTERRUPTIBLE);
> >  wake_up_process(default_backing_dev_info.wb.task);
> >
> >  Especially wb_do_writeback() is going to take a long time so just that
> > single thing makes the race unlikely. Given del_timer_sync() is slightly
> > more costly than del_timer() even for unarmed timer, it is questionable
> > whether (chance race happens * CPU spent in extra loop) > (extra CPU spent
> > in del_timer_sync() * frequency that code is executed in
> > bdi_forker_thread())...
> >
> 
> Ok, so this means that we can compare the following 2 paths of code:
> i)   One extra iteration of the bdi_forker_thread loop, versus
> ii)  The amount of time it takes for the del_timer_sync to wait till the
> timer_fn on the other CPU finishes executing + schedule resulting in a
> guaranteed sleep.
  No, ii) is going to be as rare. But instead you should compare i) against:
iii) The amount of time it takes del_timer_sync() to check whether the
timer_fn is running on a different CPU (which is work del_timer() doesn't
do).

  We are going to spend time in iii) each and every time
if (wb_has_dirty_io(me) || !list_empty(&me->bdi->work_list))
  evaluates to true.

  Now frequency of i) and iii) happening is hard to evaluate so it's not
clear what's going to be better. Certainly I don't think such evaluation is
worth my time...

								Honza
-- 
Jan Kara <jack@...e.cz>
SUSE Labs, CR
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