Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1756263Ab2K1X3a (ORCPT ); Wed, 28 Nov 2012 18:29:30 -0500 Received: from mail-qc0-f174.google.com ([209.85.216.174]:50256 "EHLO mail-qc0-f174.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753543Ab2K1X33 (ORCPT ); Wed, 28 Nov 2012 18:29:29 -0500 Date: Wed, 28 Nov 2012 15:29:30 -0800 (PST) From: Hugh Dickins X-X-Sender: hugh@eggly.anvils To: Michal Hocko cc: Andrew Morton , Ying Han , David Rientjes , Johannes Weiner , KAMEZAWA Hiroyuki , linux-kernel@vger.kernel.org, cgroups@vger.kernel.org, linux-mm@kvack.org Subject: Re: [patch] mm, memcg: avoid unnecessary function call when memcg is disabled In-Reply-To: <20121121083505.GA8761@dhcp22.suse.cz> Message-ID: References: <20121120134932.055bc192.akpm@linux-foundation.org> <20121121083505.GA8761@dhcp22.suse.cz> User-Agent: Alpine 2.00 (LNX 1167 2008-08-23) MIME-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2755 Lines: 65 On Wed, 21 Nov 2012, Michal Hocko wrote: > On Tue 20-11-12 13:49:32, Andrew Morton wrote: > > On Mon, 19 Nov 2012 17:44:34 -0800 (PST) > > David Rientjes 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. Here's a guess: as Ying's 456f998e patch started out in akpm's tree, shmem.c was calling mem_cgroup_count_vm_event(current->mm, PGMAJFAULT). Then I insisted that was inconsistent with how we usually account when one task touches another's address space, and rearranged it to work on vma->vm_mm instead. Done the original way, if the touching task were a kernel daemon (KSM's ksmd comes to my mind), then the current->mm could well have been NULL. I agree with you that it looks redundant now. Hugh -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/