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From: Michal Hocko <mhocko@suse.cz>
To: Johannes Weiner <hannes@cmpxchg.org>
Cc: linux-mm@kvack.org, Tejun Heo <tj@kernel.org>,
	cgroups@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: Re: [patch v2] mm: memcontrol: convert reclaim iterator to simple css refcounting
Date: Thu, 25 Sep 2014 12:01:30 +0200	[thread overview]
Message-ID: <20140925100130.GC12090@dhcp22.suse.cz> (raw)
In-Reply-To: <20140924171653.GA10082@cmpxchg.org>

On Wed 24-09-14 13:16:53, Johannes Weiner wrote:
> On Wed, Sep 24, 2014 at 06:47:39PM +0200, Michal Hocko wrote:
> > On Fri 19-09-14 17:28:43, Johannes Weiner wrote:
[...]
> > > -		memcg = __mem_cgroup_iter_next(root, last_visited);
> > > +		do {
> > > +			pos = ACCESS_ONCE(mz->reclaim_iter[priority]);
> > > +		} while (pos && !css_tryget(&pos->css));
> > 
> > This is a bit confusing. AFAIU css_tryget fails only when the current
> > ref count is zero already. When do we keep cached memcg with zero count
> > behind? We always do css_get after cmpxchg.
> > 
> > Hmm, there is a small window between cmpxchg and css_get when we store
> > the current memcg into the reclaim_iter[priority]. If the current memcg
> > is root then we do not take any css reference before cmpxchg and so it
> > might drop down to zero in the mean time so other CPU might see zero I
> > guess. But I do not see how css_get after cmpxchg on such css works.
> > I guess I should go and check the css reference counting again.
> 
> It's not about root or the newly stored memcg, it's that you might
> read the position right before it's replaced and css_put(), at which

OK, got it

	CPU0					CPU1
pos = reclaim_iter[priority]
					cmpxchg(reclaim_iter[priority], pos, memcg)
					css_put(pos)	# -> 0
css_tryget(pos)

Thanks!
-- 
Michal Hocko
SUSE Labs

WARNING: multiple messages have this Message-ID (diff)
From: Michal Hocko <mhocko@suse.cz>
To: Johannes Weiner <hannes@cmpxchg.org>
Cc: linux-mm@kvack.org, Tejun Heo <tj@kernel.org>,
	cgroups@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: Re: [patch v2] mm: memcontrol: convert reclaim iterator to simple css refcounting
Date: Thu, 25 Sep 2014 12:01:30 +0200	[thread overview]
Message-ID: <20140925100130.GC12090@dhcp22.suse.cz> (raw)
In-Reply-To: <20140924171653.GA10082@cmpxchg.org>

On Wed 24-09-14 13:16:53, Johannes Weiner wrote:
> On Wed, Sep 24, 2014 at 06:47:39PM +0200, Michal Hocko wrote:
> > On Fri 19-09-14 17:28:43, Johannes Weiner wrote:
[...]
> > > -		memcg = __mem_cgroup_iter_next(root, last_visited);
> > > +		do {
> > > +			pos = ACCESS_ONCE(mz->reclaim_iter[priority]);
> > > +		} while (pos && !css_tryget(&pos->css));
> > 
> > This is a bit confusing. AFAIU css_tryget fails only when the current
> > ref count is zero already. When do we keep cached memcg with zero count
> > behind? We always do css_get after cmpxchg.
> > 
> > Hmm, there is a small window between cmpxchg and css_get when we store
> > the current memcg into the reclaim_iter[priority]. If the current memcg
> > is root then we do not take any css reference before cmpxchg and so it
> > might drop down to zero in the mean time so other CPU might see zero I
> > guess. But I do not see how css_get after cmpxchg on such css works.
> > I guess I should go and check the css reference counting again.
> 
> It's not about root or the newly stored memcg, it's that you might
> read the position right before it's replaced and css_put(), at which

OK, got it

	CPU0					CPU1
pos = reclaim_iter[priority]
					cmpxchg(reclaim_iter[priority], pos, memcg)
					css_put(pos)	# -> 0
css_tryget(pos)

Thanks!
-- 
Michal Hocko
SUSE Labs

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  parent reply	other threads:[~2014-09-25 10:01 UTC|newest]

Thread overview: 15+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2014-09-19 21:10 [patch] mm: memcontrol: convert reclaim iterator to simple css refcounting Johannes Weiner
2014-09-19 21:10 ` Johannes Weiner
2014-09-19 21:28 ` [patch v2] " Johannes Weiner
2014-09-19 21:28   ` Johannes Weiner
2014-09-22  9:53   ` Vladimir Davydov
2014-09-22  9:53     ` Vladimir Davydov
2014-09-22  9:53     ` Vladimir Davydov
2014-09-24 16:47   ` Michal Hocko
2014-09-24 16:47     ` Michal Hocko
2014-09-24 17:16     ` Johannes Weiner
2014-09-24 17:16       ` Johannes Weiner
2014-09-25  1:29       ` Tejun Heo
2014-09-25  1:29         ` Tejun Heo
2014-09-25 10:01       ` Michal Hocko [this message]
2014-09-25 10:01         ` Michal Hocko

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