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From:   Shakeel Butt <shakeelb@google.com>
Date:   Mon, 24 Apr 2023 10:16:34 -0700
Message-ID: <CALvZod5g9MYjYB5eY4Z9rX5goyh7xSU_b3nfCpJCoUrzmHpyAg@mail.gmail.com>
Subject: Re: [PATCH v5 1/5] writeback: move wb_over_bg_thresh() call outside
 lock section
To:     Yosry Ahmed <yosryahmed@google.com>
Cc:     Alexander Viro <viro@zeniv.linux.org.uk>,
        Christian Brauner <brauner@kernel.org>,
        Johannes Weiner <hannes@cmpxchg.org>,
        Michal Hocko <mhocko@kernel.org>,
        Roman Gushchin <roman.gushchin@linux.dev>,
        Muchun Song <songmuchun@bytedance.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Tejun Heo <tj@kernel.org>, Jan Kara <jack@suse.cz>,
        Jens Axboe <axboe@kernel.dk>,
        =?UTF-8?Q?Michal_Koutn=C3=BD?= <mkoutny@suse.com>,
        linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org,
        cgroups@vger.kernel.org, linux-mm@kvack.org
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On Fri, Apr 21, 2023 at 10:40=E2=80=AFAM Yosry Ahmed <yosryahmed@google.com=
> wrote:
>
> wb_over_bg_thresh() calls mem_cgroup_wb_stats() which invokes an rstat
> flush, which can be expensive on large systems. Currently,
> wb_writeback() calls wb_over_bg_thresh() within a lock section, so we
> have to do the rstat flush atomically. On systems with a lot of
> cpus and/or cgroups, this can cause us to disable irqs for a long time,
> potentially causing problems.
>
> Move the call to wb_over_bg_thresh() outside the lock section in
> preparation to make the rstat flush in mem_cgroup_wb_stats() non-atomic.
> The list_empty(&wb->work_list) check should be okay outside the lock
> section of wb->list_lock as it is protected by a separate lock
> (wb->work_lock), and wb_over_bg_thresh() doesn't seem like it is
> modifying any of wb->b_* lists the wb->list_lock is protecting.
> Also, the loop seems to be already releasing and reacquring the
> lock, so this refactoring looks safe.
>
> Signed-off-by: Yosry Ahmed <yosryahmed@google.com>
> Reviewed-by: Michal Koutn=C3=BD <mkoutny@suse.com>
> Reviewed-by: Jan Kara <jack@suse.cz>

Acked-by: Shakeel Butt <shakeelb@google.com>