linux-kernel - Re: [PATCH v5 0/5] cgroup: eliminate atomic rstat flushing

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Message-ID: <CAJD7tkZKBGzBjkTGiVtwA=0VJ_L+UdGXjMWL7RcurQoooHTAUg@mail.gmail.com>
Date:   Fri, 21 Apr 2023 11:54:25 -0700
From:   Yosry Ahmed <yosryahmed@...gle.com>
To:     Alexander Viro <viro@...iv.linux.org.uk>,
        Christian Brauner <brauner@...nel.org>,
        Johannes Weiner <hannes@...xchg.org>,
        Michal Hocko <mhocko@...nel.org>,
        Roman Gushchin <roman.gushchin@...ux.dev>,
        Shakeel Butt <shakeelb@...gle.com>,
        Muchun Song <songmuchun@...edance.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Tejun Heo <tj@...nel.org>
Cc:     Jan Kara <jack@...e.cz>, Jens Axboe <axboe@...nel.dk>,
        Michal Koutný <mkoutny@...e.com>,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        cgroups@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH v5 0/5] cgroup: eliminate atomic rstat flushing

On Fri, Apr 21, 2023 at 10:40 AM Yosry Ahmed <yosryahmed@...gle.com> wrote:
>
> A previous patch series ([1] currently in mm-stable) changed most
> atomic rstat flushing contexts to become non-atomic. This was done to
> avoid an expensive operation that scales with # cgroups and # cpus to
> happen with irqs disabled and scheduling not permitted. There were two
> remaining atomic flushing contexts after that series. This series tries
> to eliminate them as well, eliminating atomic rstat flushing completely.
>
> The two remaining atomic flushing contexts are:
> (a) wb_over_bg_thresh()->mem_cgroup_wb_stats()
> (b) mem_cgroup_threshold()->mem_cgroup_usage()
>
> For (a), flushing needs to be atomic as wb_writeback() calls
> wb_over_bg_thresh() with a spinlock held. However, it seems like the
> call to wb_over_bg_thresh() doesn't need to be protected by that
> spinlock, so this series proposes a refactoring that moves the call
> outside the lock criticial section and makes the stats flushing
> in mem_cgroup_wb_stats() non-atomic.
>
> For (b), flushing needs to be atomic as mem_cgroup_threshold() is called
> with irqs disabled. We only flush the stats when calculating the root
> usage, as it is approximated as the sum of some memcg stats (file, anon,
> and optionally swap) instead of the conventional page counter. This
> series proposes changing this calculation to use the global stats
> instead, eliminating the need for a memcg stat flush.
>
> After these 2 contexts are eliminated, we no longer need
> mem_cgroup_flush_stats_atomic() or cgroup_rstat_flush_atomic(). We can
> remove them and simplify the code.
>
> [1] https://lore.kernel.org/linux-mm/20230330191801.1967435-1-yosryahmed@google.com/
>
> RFC -> v1:
> - Collected R-b's and A-b's (Thanks everyone!).
> - Rebased onto mm-stable.
> - Cosmetic changes to commit logs.
>
> RFC: https://lore.kernel.org/linux-mm/20230403220337.443510-1-yosryahmed@google.com/

This is v1, not v5. I really suck at sending emails. Sorry.

>
> Yosry Ahmed (5):
>   writeback: move wb_over_bg_thresh() call outside lock section
>   memcg: flush stats non-atomically in mem_cgroup_wb_stats()
>   memcg: calculate root usage from global state
>   memcg: remove mem_cgroup_flush_stats_atomic()
>   cgroup: remove cgroup_rstat_flush_atomic()
>
>  fs/fs-writeback.c          | 16 +++++++----
>  include/linux/cgroup.h     |  1 -
>  include/linux/memcontrol.h |  5 ----
>  kernel/cgroup/rstat.c      | 26 ++++--------------
>  mm/memcontrol.c            | 54 ++++++++------------------------------
>  5 files changed, 27 insertions(+), 75 deletions(-)
>
> --
> 2.40.0.634.g4ca3ef3211-goog
>