linux-kernel - Re: [PATCHv4 17/17] writeback: lessen sync

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <20100527072240.GM22536@laptop>
Date:	Thu, 27 May 2010 17:22:40 +1000
From:	Nick Piggin <npiggin@...e.de>
To:	Al Viro <viro@...IV.linux.org.uk>
Cc:	Artem Bityutskiy <dedekind1@...il.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Jens Axboe <jens.axboe@...cle.com>,
	linux-fsdevel@...r.kernel.org
Subject: Re: [PATCHv4 17/17] writeback: lessen sync_supers wakeup count

On Thu, May 27, 2010 at 07:50:41AM +0100, Al Viro wrote:
> On Tue, May 25, 2010 at 04:49:12PM +0300, Artem Bityutskiy wrote:
> > From: Artem Bityutskiy <Artem.Bityutskiy@...ia.com>
> > 
> > The 'sync_supers' thread wakes up every 5 seconds (by default) and
> > writes back all super blocks. It keeps waking up even if there
> > are no dirty super-blocks. For many file-systems the superblock
> > becomes dirty very rarely, if ever, so 'sync_supers' does not do
> > anything most of the time.
> > 
> > This patch improves 'sync_supers' and makes sleep if all superblocks
> > are clean and there is nothing to do. This helps saving the power.
> > This optimization is important for small battery-powered devices.
> 
> > +void mark_sb_dirty(struct super_block *sb)
> > +{
> > +	sb->s_dirty = 1;
> > +
> > +	spin_lock(&supers_timer_lock);
> > +	if (!supers_timer_armed) {
> > +		bdi_arm_supers_timer();
> > +		supers_timer_armed = 1;
> > +	} else if (supers_timer_armed == -1)
> > +		supers_timer_armed = 1;
> > +	spin_unlock(&supers_timer_lock);
> > +}
> > +EXPORT_SYMBOL(mark_sb_dirty);
> 
> Ouch...   That turns a previously trivial operation into something
> much heavier; moreover, I'd rather see serious review of s_dirt
> uses.

Yeah, we definitely don't want to add global cacheline writes in the
common case. Also I don't know why you do the strange -1 value. I
couldn't seem to find where you defined bdi_arm_supers_timer();

But why doesn't this work?

  sb->s_dirty = 1;
  smp_mb(); /* corresponding MB is in test_and_clear_bit */
  if (unlikely(!supers_timer_armed)) {
    if (!test_and_set_bit(0, &supers_timer_armed))
        bdi_arm_supers_timer();
  }
    
  vs

  supers_timer_armed = 0;
again:
  sync_supers();
  if (test_and_clear_bit(0, &supers_timer_armed))
    goto again;
  
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/