Re: [PATCH] memcg: sync flush only if periodic flush is delayed

[Andrew Morton]

On Fri,  4 Mar 2022 18:40:40 +0000 Shakeel Butt <shakeelb@xxxxxxxxxx> wrote:

> Daniel Dao has reported [1] a regression on workloads that may trigger
> a lot of refaults (anon and file). The underlying issue is that flushing
> rstat is expensive. Although rstat flush are batched with (nr_cpus *
> MEMCG_BATCH) stat updates, it seems like there are workloads which
> genuinely do stat updates larger than batch value within short amount of
> time. Since the rstat flush can happen in the performance critical
> codepaths like page faults, such workload can suffer greatly.
> 
> This patch fixes this regression by making the rstat flushing
> conditional in the performance critical codepaths. More specifically,
> the kernel relies on the async periodic rstat flusher to flush the stats
> and only if the periodic flusher is delayed by more than twice the
> amount of its normal time window then the kernel allows rstat flushing
> from the performance critical codepaths.
> 
> Now the question: what are the side-effects of this change? The worst
> that can happen is the refault codepath will see 4sec old lruvec stats
> and may cause false (or missed) activations of the refaulted page which
> may under-or-overestimate the workingset size. Though that is not very
> concerning as the kernel can already miss or do false activations.
> 
> There are two more codepaths whose flushing behavior is not changed by
> this patch and we may need to come to them in future. One is the
> writeback stats used by dirty throttling and second is the deactivation
> heuristic in the reclaim. For now keeping an eye on them and if there is
> report of regression due to these codepaths, we will reevaluate then.
> 
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
>
> ...
>
> @@ -648,10 +652,16 @@ void mem_cgroup_flush_stats(void)
>  		__mem_cgroup_flush_stats();
>  }
>  
> +void mem_cgroup_flush_stats_delayed(void)
> +{
> +	if (rstat_flush_time && time_after64(jiffies_64, flush_next_time))

rstat_flush_time isn't defined for me and my googling indicates this is
the first time the symbol has been used in the history of the world. 
I'm stumped.

> +		mem_cgroup_flush_stats();
> +}
> +
>
> ...
>