Re: [PATCH -tip] cpuacct: per-cgroup utime/stime statistics - v4

[Bharata B Rao]

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@xxxxxxxxxxxxxxxxxx> 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@xxxxxxxxxxxxxx>
> ---
> 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().

Regards,
Bharata.
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