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

[Ivan Babrou <ivan@cloudflare.com>]

On Fri, Mar 4, 2022 at 10:40 AM Shakeel Butt <shakeelb@google.com> 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.
>
> Link: https://lore.kernel.org/all/CA+wXwBSyO87ZX5PVwdHm-=dBjZYECGmfnydUicUyrQqndgX2MQ@mail.gmail.com [1]
> Fixes: 1f828223b799 ("memcg: flush lruvec stats in the refault")
> Signed-off-by: Shakeel Butt <shakeelb@google.com>
> Reported-by: Daniel Dao <dqminh@cloudflare.com>
> Cc: <stable@vger.kernel.org>
> ---

See my testing results here:
https://lore.kernel.org/linux-mm/CABWYdi2usrWOnOnmKYYvuFpE=yJmgtq4a7u6FiGJGJkskv+eVQ@mail.gmail.com/

Tested-by: Ivan Babrou <ivan@cloudflare.com>