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

[Michal Hocko]

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?

> 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