linux-kernel - Re: [PATCH -tip] cpuacct: per-cgroup utime/stime statistics

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Date:	Mon, 23 Mar 2009 16:01:12 +0900
From:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
To:	bharata@...ux.vnet.ibm.com
Cc:	linux-kernel@...r.kernel.org, Balaji Rao <balajirrao@...il.com>,
	Dhaval Giani <dhaval@...ux.vnet.ibm.com>,
	Balbir Singh <balbir@...ux.vnet.ibm.com>,
	Li Zefan <lizf@...fujitsu.com>,
	Paul Menage <menage@...gle.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Ingo Molnar <mingo@...e.hu>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Subject: Re: [PATCH -tip] cpuacct: per-cgroup utime/stime statistics - v4

On Mon, 23 Mar 2009 12:06:03 +0530
Bharata B Rao <bharata@...ux.vnet.ibm.com> wrote:

> On Mon, Mar 23, 2009 at 01:55:28PM +0900, KAMEZAWA Hiroyuki wrote:
> > On Mon, 23 Mar 2009 10:05:38 +0530
> > Bharata B Rao <bharata@...ux.vnet.ibm.com> wrote:
> > 
> > > Hi Ingo,
> > > 
> > > Here is the v4 of the cpuacct statistics patch to obtain per-cgroup
> > > system and user times  with appropriate tags and complete changelog.
> > > This applies against the latest -tip plus the cpuacct hierarchy fix v2
> > > which I posted just now. Could you please include this in -tip ?
> > > 
> > > Changelog:
> > > 
> > > v4
> > > - Remove comments in cpuacct_update_stats() which explained why rcu_read_lock()
> > >   was needed (as per Peter Zijlstra's review comments).
> > > - Don't say that percpu_counter_read() is broken in Documentation/cpuacct.txt
> > >   as per KAMEZAWA Hiroyuki's review comments.
> > > 
> > Broken -> isn't safe ? no difference ;)
> 
> From your comment last time, I thought you meant calling it broken would
> be harsh.
> 
> > Can't this help you ?
> > 
> > 
> > Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
> > ---
> > Index: mmotm-2.6.29-Mar21/include/linux/percpu_counter.h
> > ===================================================================
> > --- mmotm-2.6.29-Mar21.orig/include/linux/percpu_counter.h
> > +++ mmotm-2.6.29-Mar21/include/linux/percpu_counter.h
> > @@ -62,6 +62,16 @@ static inline s64 percpu_counter_read(st
> >  	return fbc->count;
> >  }
> > 
> > +static inline s64 percpu_counter_read_safe(struct percpu_counter *fbc)
> > +{
> > +	s64 ret;
> > +
> > +	spin_lock(&fbc->lock);
> > +	ret = fbc->count;
> > +	spin_unlock(&fbc->lock);
> > +	return ret;
> > +}
> > +
> >  /*
> >   * It is possible for the percpu_counter_read() to return a small negative
> >   * number for some counter which should never be negative.
> > @@ -114,6 +124,11 @@ static inline s64 percpu_counter_read(st
> >  	return fbc->count;
> >  }
> > 
> > +static inline s64 percpu_counter_read_safe(struct percpu_counter *fbc)
> > +{
> > +	return fbc->count;
> > +}
> > +
> >  static inline s64 percpu_counter_read_positive(struct percpu_counter *fbc)
> >  {
> >  	return fbc->count;
> 
> Yes it helps, but for the records, let me note that it makes the readside
> ~20% slower.
> 
> [I measured time spent by cpuacct_stats_show(), which is part of
> the read routine for cpuacct.stat by using kretprobe and for 100 reads
> (or 100 executions of cpuacct_stats_show()), I get the following numbers:
> percpu_counter_read() - 3166367 ns
> percpu_counter_read_safe() - 3793546 ns which is ~20% slower. ]
> 
> I would prefer that this patch should be included in its current form
> and we could separately fix percpu_counter_read() given that there
> has been an attempt in the past to fix this (http://lkml.org/lkml/2008/8/27/303)
> 
> Moreover, I don't know how much acceptable it is to work around the problem
> in percpu_counter_read() by introducing another variant
> percpu_counter_read_safe().
> 

Hmm, ok, how about this way ?

Removet this.

+- It is possible to see slightly outdated values for stime and utime
+  due to the batch processing nature of percpu_counter.

Anyway, we get overhead of vfs_read(), interrupts, at el ;)

put into todo list.

+TODO:
+- It is theoretically possible to see wrong values for stime and utime.
+  This is because percpu_counter_read() on 32bit systems isn't safe
+  against concurrent writes. needs to be fixed.


Thanks,
-Kame

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