Re: [PATCH v2 1/2] memcg: flush stats only if updated

["Michal Koutný" <mkoutny@suse.com>]

On Thu, Oct 14, 2021 at 09:31:46AM -0700, Shakeel Butt <shakeelb@google.com> wrote:
> Thanks for your review. This forces me to think more on this because each
> update does not necessarily be a single page sized update e.g. adding a hugepage
> to an LRU.

Aha, hugepages... (I also noticed that on the opposite size scale are
NR_SLAB_{UN,}RECLAIMABLE_B, the complementary problem to too big error
would be too frequent flushes.)

> Though I think the error is time bounded by 2 seconds but in those 2 seconds
> mathematically the error can be large.

Honestly, I can't tell how much the (transient) errors in various
node_stat_item entries will or won't affect MM behavior. But having some
guards on it sounds practical when some problems to troubleshoot arise.

> What do you think of the following change? It will bound the error
> better within the 2 seconds window.
> [...]
> -static inline void memcg_rstat_updated(struct mem_cgroup *memcg)
> +static inline void memcg_rstat_updated(struct mem_cgroup *memcg, int val)
>  {
> +	unsigned int x;
> +
>  	cgroup_rstat_updated(memcg->css.cgroup, smp_processor_id());
> -	if (!(__this_cpu_inc_return(stats_updates) % MEMCG_CHARGE_BATCH))
> -		atomic_inc(&stats_flush_threshold);
> +
> +	x = abs(__this_cpu_add_return(stats_diff, val));
> +	if (x > MEMCG_CHARGE_BATCH) {
> +		atomic_add(x / MEMCG_CHARGE_BATCH, &stats_flush_threshold);
> +		__this_cpu_write(stats_diff, 0);
> +	}
>  }

Looks better wrt meaningful error calculation (and hopefully still
doesn't add too much to the hot path).

> @@ -807,7 +813,7 @@ void __count_memcg_events(struct mem_cgroup *memcg, enum vm_event_item idx,
>  		return;
>  
>  	__this_cpu_add(memcg->vmstats_percpu->events[idx], count);
> -	memcg_rstat_updated(memcg);
> +	memcg_rstat_updated(memcg, val);

s/val/count/

(Just thinking loudly.) At one moment I thought, it could
effectively be even s/val/0/ since events aren't(?) inputs for reclaim
calculations. But with the introduced stats_diff it may happen
stats_diff flickers (its abs value) within the MEMCG_CHARGE_BATCH and
stats_flush_threshold would never be incremented. Basically disabling
the periodic flush. Therefore the events must also increment the
stats_diff.