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From:   Yosry Ahmed <yosryahmed@google.com>
Date:   Thu, 20 Apr 2023 13:23:34 -0700
Message-ID: <CAJD7tkYxU7Bx7zAh=wRoGQeyu9UdR6noYiM3Q8Laaf2+13Jn=A@mail.gmail.com>
Subject: Re: [PATCH mm-unstable RFC 1/5] writeback: move wb_over_bg_thresh()
 call outside lock section
To:     =?UTF-8?Q?Michal_Koutn=C3=BD?= <mkoutny@suse.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>,
        Shakeel Butt <shakeelb@google.com>,
        Muchun Song <songmuchun@bytedance.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org,
        cgroups@vger.kernel.org, linux-mm@kvack.org
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On Wed, Apr 19, 2023 at 4:38=E2=80=AFAM Michal Koutn=C3=BD <mkoutny@suse.co=
m> wrote:
>
> On Mon, Apr 03, 2023 at 10:03:33PM +0000, 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 make the rstat flush atomically. On systems with a lot of
> > cpus/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) 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 the 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>
> > ---
> >  fs/fs-writeback.c | 16 +++++++++++-----
> >  1 file changed, 11 insertions(+), 5 deletions(-)
> >
> > diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c
> > index 195dc23e0d831..012357bc8daa3 100644
> > --- a/fs/fs-writeback.c
> > +++ b/fs/fs-writeback.c
> > @@ -2021,7 +2021,6 @@ static long wb_writeback(struct bdi_writeback *wb=
,
> >       struct blk_plug plug;
> >
> >       blk_start_plug(&plug);
> > -     spin_lock(&wb->list_lock);
> >       for (;;) {
> >               /*
> >                * Stop writeback when nr_pages has been consumed
> > @@ -2046,6 +2045,9 @@ static long wb_writeback(struct bdi_writeback *wb=
,
> >               if (work->for_background && !wb_over_bg_thresh(wb))
> >                       break;
> >
> > +
> > +             spin_lock(&wb->list_lock);
> > +
> >               /*
> >                * Kupdate and background works are special and we want t=
o
> >                * include all inodes that need writing. Livelock avoidan=
ce is
> > @@ -2075,13 +2077,19 @@ static long wb_writeback(struct bdi_writeback *=
wb,
> >                * mean the overall work is done. So we keep looping as l=
ong
> >                * as made some progress on cleaning pages or inodes.
> >                */
> > -             if (progress)
> > +             if (progress) {
> > +                     spin_unlock(&wb->list_lock);
> >                       continue;
> > +             }
> > +
>
> This would release wb->list_lock temporarily with progress but that's
> already not held continuously due to writeback_sb_inodes().
> Holding the lock could even be shortened by taking it later after
> trace_writeback_start().
>
> Altogether, the change looks OK,
> Reviewed-by: Michal Koutn=C3=BD <mkoutny@suse.com>

Thanks for taking a look!

>