Re: [PATCH v2 3/5] mm: enlarge NUMA counters threshold size

[Michal Hocko]

On Tue 19-12-17 14:39:24, Kemi Wang wrote:
> We have seen significant overhead in cache bouncing caused by NUMA counters
> update in multi-threaded page allocation. See 'commit 1d90ca897cb0 ("mm:
> update NUMA counter threshold size")' for more details.
> 
> This patch updates NUMA counters to a fixed size of (MAX_S16 - 2) and deals
> with global counter update using different threshold size for node page
> stats.

Again, no numbers. To be honest I do not really like the special casing
here. Why are numa counters any different from PGALLOC which is
incremented for _every_ single page allocation?

> ---
>  mm/vmstat.c | 13 +++++++++++--
>  1 file changed, 11 insertions(+), 2 deletions(-)
> 
> diff --git a/mm/vmstat.c b/mm/vmstat.c
> index 9c681cc..64e08ae 100644
> --- a/mm/vmstat.c
> +++ b/mm/vmstat.c
> @@ -30,6 +30,8 @@
>  
>  #include "internal.h"
>  
> +#define VM_NUMA_STAT_THRESHOLD (S16_MAX - 2)
> +
>  #ifdef CONFIG_NUMA
>  int sysctl_vm_numa_stat = ENABLE_NUMA_STAT;
>  
> @@ -394,7 +396,11 @@ void __inc_node_state(struct pglist_data *pgdat, enum node_stat_item item)
>  	s16 v, t;
>  
>  	v = __this_cpu_inc_return(*p);
> -	t = __this_cpu_read(pcp->stat_threshold);
> +	if (item >= NR_VM_NUMA_STAT_ITEMS)
> +		t = __this_cpu_read(pcp->stat_threshold);
> +	else
> +		t = VM_NUMA_STAT_THRESHOLD;
> +
>  	if (unlikely(v > t)) {
>  		s16 overstep = t >> 1;
>  
> @@ -549,7 +555,10 @@ static inline void mod_node_state(struct pglist_data *pgdat,
>  		 * Most of the time the thresholds are the same anyways
>  		 * for all cpus in a node.
>  		 */
> -		t = this_cpu_read(pcp->stat_threshold);
> +		if (item >= NR_VM_NUMA_STAT_ITEMS)
> +			t = this_cpu_read(pcp->stat_threshold);
> +		else
> +			t = VM_NUMA_STAT_THRESHOLD;
>  
>  		o = this_cpu_read(*p);
>  		n = delta + o;
> -- 
> 2.7.4
> 

-- 
Michal Hocko
SUSE Labs