linux-kernel - Re: [PATCH v5 1/5] writeback: move wb_over_bg

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Date:   Fri, 5 May 2023 05:18:28 -1000
From:   Tejun Heo <tj@...nel.org>
To:     Yosry Ahmed <yosryahmed@...gle.com>
Cc:     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>,
        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 1/5] writeback: move wb_over_bg_thresh() call outside
 lock section

On Fri, Apr 21, 2023 at 05:40:16PM +0000, Yosry Ahmed 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@...gle.com>
> Reviewed-by: Michal Koutný <mkoutny@...e.com>
> Reviewed-by: Jan Kara <jack@...e.cz>

Acked-by: Tejun Heo <tj@...nel.org>

Thanks.

-- 
tejun