Re: [PATCH mm-unstable RFC 1/5] writeback: move wb_over_bg_thresh() call outside lock section

[Yosry Ahmed <yosryahmed@google.com>]

On Thu, Apr 20, 2023 at 11:53 AM Shakeel Butt <shakeelb@google.com> wrote:
>
> +Jens & Jan
>
> The patch looks good but it would be nice to pass this patch through
> the eyes of experts of this area.

Thanks for taking a look and CC'ing folks. I will make sure to include
them in the next rounds as well. FWIW, Jens & Jan did not show up when
I ran scripts/get_maintainers.ph if I remember correctly.

>
> On Mon, Apr 3, 2023 at 3:03 PM 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 to
> >                  * include all inodes that need writing. Livelock avoidance is
> > @@ -2075,13 +2077,19 @@ static long wb_writeback(struct bdi_writeback *wb,
> >                  * mean the overall work is done. So we keep looping as long
> >                  * as made some progress on cleaning pages or inodes.
> >                  */
> > -               if (progress)
> > +               if (progress) {
> > +                       spin_unlock(&wb->list_lock);
> >                         continue;
> > +               }
> > +
> >                 /*
> >                  * No more inodes for IO, bail
> >                  */
> > -               if (list_empty(&wb->b_more_io))
> > +               if (list_empty(&wb->b_more_io)) {
> > +                       spin_unlock(&wb->list_lock);
> >                         break;
> > +               }
> > +
> >                 /*
> >                  * Nothing written. Wait for some inode to
> >                  * become available for writeback. Otherwise
> > @@ -2093,9 +2101,7 @@ static long wb_writeback(struct bdi_writeback *wb,
> >                 spin_unlock(&wb->list_lock);
> >                 /* This function drops i_lock... */
> >                 inode_sleep_on_writeback(inode);
> > -               spin_lock(&wb->list_lock);
> >         }
> > -       spin_unlock(&wb->list_lock);
> >         blk_finish_plug(&plug);
> >
> >         return nr_pages - work->nr_pages;
> > --
> > 2.40.0.348.gf938b09366-goog
> >