Re: [PATCH 2/3] mm/util.c: make vm_memory_committed() more accurate

[Feng Tang]

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@xxxxxxxxx>
> > ---
> >  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