linux-kernel - Re: [PATCH 2/3] mm/util.c: make vm_memory

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Message-ID: <20200515081109.GD69177@shbuild999.sh.intel.com>
Date:   Fri, 15 May 2020 16:11:09 +0800
From:   Feng Tang <feng.tang@...el.com>
To:     Michal Hocko <mhocko@...nel.org>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Johannes Weiner <hannes@...xchg.org>,
        Matthew Wilcox <willy@...radead.org>,
        Mel Gorman <mgorman@...e.de>,
        Kees Cook <keescook@...omium.org>,
        Luis Chamberlain <mcgrof@...nel.org>,
        Iurii Zaikin <yzaikin@...gle.com>,
        "Kleen, Andi" <andi.kleen@...el.com>,
        "Chen, Tim C" <tim.c.chen@...el.com>,
        "Hansen, Dave" <dave.hansen@...el.com>,
        "Huang, Ying" <ying.huang@...el.com>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 2/3] mm/util.c: make vm_memory_committed() more accurate

On Fri, May 15, 2020 at 03:23:07PM +0800, Michal Hocko wrote:
> On Fri 08-05-20 15:25:16, Feng Tang wrote:
> > percpu_counter_sum_positive() will provide more accurate info.
> 
> Why do we need that?

This is a preparation for patch 3/3, which will enlarge the batch size
of percpu-counter 'vm_committed_as'. Andi Kleen concerned that may
hurt the accuracy for those readers (currently only /proc/meminfo
and hyperV balloon drivers for status check)

As in worst case, the deviation could be 'batch * nr_cpus', and
with patch 3/3, that could be totalram_pages/16.

Thanks,
Feng

> > Its time cost is about 800 nanoseconds on a 2C/4T platform and
> > 2~3 microseconds on a 2S/36C/72T server in normal case, and in
> > worst case where vm_committed_as's spinlock is under severe
> > contention, it costs 30~40 microseconds for the 2S/36C/72T sever,
> > which should be fine for its two users: /proc/meminfo and HV ballon
> > driver's status trace per second.
> 
> OK, this explains that the additional overhead is not terrible but there
> is no actual information on why the imprecision is a problem.
> 
> > Signed-off-by: Feng Tang <feng.tang@...el.com>
> > ---
> >  mm/util.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> > 
> > diff --git a/mm/util.c b/mm/util.c
> > index 988d11e..3de78e9 100644
> > --- a/mm/util.c
> > +++ b/mm/util.c
> > @@ -774,7 +774,7 @@ struct percpu_counter vm_committed_as ____cacheline_aligned_in_smp;
> >   */
> >  unsigned long vm_memory_committed(void)
> >  {
> > -	return percpu_counter_read_positive(&vm_committed_as);
> > +	return percpu_counter_sum_positive(&vm_committed_as);
> >  }
> >  EXPORT_SYMBOL_GPL(vm_memory_committed);
> >  
> > -- 
> > 2.7.4
> 
> -- 
> Michal Hocko
> SUSE Labs