On Wed, Apr 19, 2023 at 4:38 AM Michal Koutný <mkoutny@xxxxxxxx> wrote:
>
> On Mon, Apr 03, 2023 at 10:03:33PM +0000, Yosry Ahmed <yosryahmed@xxxxxxxxxx> 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@xxxxxxxxxx>
> > ---
> > 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;
> > + }
> > +
>
> 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 Koutný <mkoutny@xxxxxxxx>
Thanks for taking a look!
>
