linux-kernel - Re: [patch] mm, memcg: avoid unnecessary function call when memcg is disabled

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Date:	Wed, 21 Nov 2012 09:35:20 +0100
From:	Michal Hocko <mhocko@...e.cz>
To:	Andrew Morton <akpm@...ux-foundation.org>,
	Ying Han <yinghan@...gle.com>
Cc:	David Rientjes <rientjes@...gle.com>,
	Johannes Weiner <hannes@...xchg.org>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	Hugh Dickins <hughd@...gle.com>, linux-kernel@...r.kernel.org,
	cgroups@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [patch] mm, memcg: avoid unnecessary function call when memcg is
 disabled

On Tue 20-11-12 13:49:32, Andrew Morton wrote:
> On Mon, 19 Nov 2012 17:44:34 -0800 (PST)
> David Rientjes <rientjes@...gle.com> wrote:
> 
> > While profiling numa/core v16 with cgroup_disable=memory on the command 
> > line, I noticed mem_cgroup_count_vm_event() still showed up as high as 
> > 0.60% in perftop.
> > 
> > This occurs because the function is called extremely often even when memcg 
> > is disabled.
> > 
> > To fix this, inline the check for mem_cgroup_disabled() so we avoid the 
> > unnecessary function call if memcg is disabled.
> > 
> > ...
> >
> > diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
> > --- a/include/linux/memcontrol.h
> > +++ b/include/linux/memcontrol.h
> > @@ -181,7 +181,14 @@ unsigned long mem_cgroup_soft_limit_reclaim(struct zone *zone, int order,
> >  						gfp_t gfp_mask,
> >  						unsigned long *total_scanned);
> >  
> > -void mem_cgroup_count_vm_event(struct mm_struct *mm, enum vm_event_item idx);
> > +void __mem_cgroup_count_vm_event(struct mm_struct *mm, enum vm_event_item idx);
> > +static inline void mem_cgroup_count_vm_event(struct mm_struct *mm,
> > +					     enum vm_event_item idx)
> > +{
> > +	if (mem_cgroup_disabled() || !mm)
> > +		return;
> > +	__mem_cgroup_count_vm_event(mm, idx);
> > +}
> 
> Does the !mm case occur frequently enough to justify inlining it, or
> should that test remain out-of-line?

Now that you've asked about it I started looking around and I cannot see
how mm can ever be NULL. The condition is there since the very beginning
(456f998e memcg: add the pagefault count into memcg stats) but all the
callers are page fault handlers and those shouldn't have mm==NULL.
Or is there anything obvious I am missing?

Ying, the whole thread starts https://lkml.org/lkml/2012/11/19/545 but
the primary question is why we need !mm test for mem_cgroup_count_vm_event
at all.

Thanks!
-- 
Michal Hocko
SUSE Labs
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