linux-kernel - Re: [patch 1/3] mm: memcontrol: lockless page counters

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Message-ID: <20141008123134.GA14361@cmpxchg.org>
Date:	Wed, 8 Oct 2014 08:31:34 -0400
From:	Johannes Weiner <hannes@...xchg.org>
To:	Michal Hocko <mhocko@...e.cz>
Cc:	linux-mm@...ck.org, Vladimir Davydov <vdavydov@...allels.com>,
	Greg Thelen <gthelen@...gle.com>, Dave Hansen <dave@...1.net>,
	cgroups@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [patch 1/3] mm: memcontrol: lockless page counters

On Tue, Oct 07, 2014 at 05:15:43PM +0200, Michal Hocko wrote:
> On Wed 24-09-14 11:43:08, Johannes Weiner wrote:
> > @@ -1490,12 +1495,23 @@ int mem_cgroup_inactive_anon_is_low(struct lruvec *lruvec)
> >   */
> >  static unsigned long mem_cgroup_margin(struct mem_cgroup *memcg)
> >  {
> > -	unsigned long long margin;
> > +	unsigned long margin = 0;
> > +	unsigned long count;
> > +	unsigned long limit;
> >  
> > -	margin = res_counter_margin(&memcg->res);
> > -	if (do_swap_account)
> > -		margin = min(margin, res_counter_margin(&memcg->memsw));
> > -	return margin >> PAGE_SHIFT;
> > +	count = page_counter_read(&memcg->memory);
> > +	limit = ACCESS_ONCE(memcg->memory.limit);
> > +	if (count < limit)
> > +		margin = limit - count;
> > +
> > +	if (do_swap_account) {
> > +		count = page_counter_read(&memcg->memsw);
> > +		limit = ACCESS_ONCE(memcg->memsw.limit);
> > +		if (count < limit)
> 
> I guess you wanted (count <= limit) here?

Yes.  Fixed it up, thanks.

> > @@ -2293,33 +2295,31 @@ static DEFINE_MUTEX(percpu_charge_mutex);
> >  static bool consume_stock(struct mem_cgroup *memcg, unsigned int nr_pages)
> >  {
> >  	struct memcg_stock_pcp *stock;
> > -	bool ret = true;
> > +	bool ret = false;
> >  
> >  	if (nr_pages > CHARGE_BATCH)
> > -		return false;
> > +		return ret;
> >  
> >  	stock = &get_cpu_var(memcg_stock);
> > -	if (memcg == stock->cached && stock->nr_pages >= nr_pages)
> > +	if (memcg == stock->cached && stock->nr_pages >= nr_pages) {
> >  		stock->nr_pages -= nr_pages;
> > -	else /* need to call res_counter_charge */
> > -		ret = false;
> > +		ret = true;
> > +	}
> >  	put_cpu_var(memcg_stock);
> >  	return ret;
> 
> This change is not really needed but at least it woke me up after some
> monotonic and mechanical changes...

This hunk started with removing the res_counter_charge comment.  IIRC,
Andrew advocated minor cleanups in the area in the past, so I figured
I make the thing a bit more readable while I'm there anyway.
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