linux-kernel - Re: [PATCH v4 0/4] memcg: non-unified flushing for userspace stats

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Message-ID: <b43508ea-c222-5e38-2486-a4a7e7263d61@redhat.com>
Date:   Thu, 31 Aug 2023 13:18:28 -0400
From:   Waiman Long <longman@...hat.com>
To:     Yosry Ahmed <yosryahmed@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>
Cc:     Johannes Weiner <hannes@...xchg.org>,
        Michal Hocko <mhocko@...nel.org>,
        Roman Gushchin <roman.gushchin@...ux.dev>,
        Shakeel Butt <shakeelb@...gle.com>,
        Muchun Song <muchun.song@...ux.dev>,
        Ivan Babrou <ivan@...udflare.com>, Tejun Heo <tj@...nel.org>,
        Michal Koutný <mkoutny@...e.com>,
        linux-mm@...ck.org, cgroups@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4 0/4] memcg: non-unified flushing for userspace stats

On 8/31/23 12:56, Yosry Ahmed wrote:
> Most memcg flushing contexts using "unified" flushing, where only one
> flusher is allowed at a time (others skip), and all flushers need to
> flush the entire tree. This works well with high concurrency, which
> mostly comes from in-kernel flushers (e.g. reclaim, refault, ..).
>
> For userspace reads, unified flushing leads to non-deterministic stats
> staleness and reading cost. This series clarifies and documents the
> differences between unified and non-unified flushing (patches 1 & 2),
> then opts userspace reads out of unified flushing (patch 3).
>
> This patch series is a follow up on the discussion in [1]. That was a
> patch that proposed that userspace reads wait for ongoing unified
> flushers to complete before returning. There were concerns about the
> latency that this introduces to userspace reads, especially with ongoing
> reports of expensive stat reads even with unified flushing. Hence, this
> series follows a different approach, by opting userspace reads out of
> unified flushing completely. The cost of userspace reads are now
> determinstic, and depend on the size of the subtree being read. This
> should fix both the *sometimes* expensive reads (due to flushing the
> entire tree) and occasional staless (due to skipping flushing).
>
> I attempted to remove unified flushing completely, but noticed that
> in-kernel flushers with high concurrency (e.g. hundreds of concurrent
> reclaimers). This sort of concurrency is not expected from userspace
> reads. More details about testing and some numbers in the last patch's
> changelog.
>
> v4 -> v5:
> - Fixed build error in the last patch with W=1 because of a missed
>    'static'.
>
> v4: https://lore.kernel.org/lkml/20230830175335.1536008-1-yosryahmed@google.com/
>
> Yosry Ahmed (4):
>    mm: memcg: properly name and document unified stats flushing
>    mm: memcg: add a helper for non-unified stats flushing
>    mm: memcg: let non-unified root stats flushes help unified flushes
>    mm: memcg: use non-unified stats flushing for userspace reads
>
>   include/linux/memcontrol.h |   8 +--
>   mm/memcontrol.c            | 106 +++++++++++++++++++++++++++----------
>   mm/vmscan.c                |   2 +-
>   mm/workingset.c            |   4 +-
>   4 files changed, 85 insertions(+), 35 deletions(-)
>
LGTM

Acked-by: Waiman Long <longman@...hat.com>