Re: [PATCH] mm/page-writeback: Fix performance when BDI's share of ratio is 0.

[chi wu <wuchi.zero@gmail.com>]

Andrew Morton <akpm@linux-foundation.org> 于2021年5月10日周一 上午7:36写道：
>
> On Thu, 29 Apr 2021 06:50:46 +0800 Chi Wu <wuchi.zero@gmail.com> wrote:
>
> > Fix performance when BDI's share of ratio is 0.
> >
> > The issue is similar to commit 74d369443325 ("writeback: Fix
> > performance regression in wb_over_bg_thresh()").
> >
> > Balance_dirty_pages and the writeback worker will also disagree on
> > whether writeback when a BDI uses BDI_CAP_STRICTLIMIT and BDI's share
> > of the thresh ratio is zero.
> >
> > For example, A thread on cpu0 writes 32 pages and then
> > balance_dirty_pages, it will wake up background writeback and pauses
> > because wb_dirty > wb->wb_thresh = 0 (share of thresh ratio is zero).
> > A thread may runs on cpu0 again because scheduler prefers pre_cpu.
> > Then writeback worker may runs on other cpus(1,2..) which causes the
> > value of wb_stat(wb, WB_RECLAIMABLE) in wb_over_bg_thresh is 0 and does
> > not writeback and returns.
> >
> > Thus, balance_dirty_pages keeps looping, sleeping and then waking up the
> > worker who will do nothing. It remains stuck in this state until the
> > writeback worker hit the right dirty cpu or the dirty pages expire.
> >
> > The fix that we should get the wb_stat_sum radically when thresh is low.
>
> (optimistically Cc's various people who might remember how this code works)

Thanks, I'll correct it in the future.

>
> > Signed-off-by: Chi Wu <wuchi.zero@gmail.com>
>
> Thanks.  I'll add it for some testing and hopefully someone will find
> the time to review this.
>
> > ---
> >  mm/page-writeback.c | 20 ++++++++++++++++----
> >  1 file changed, 16 insertions(+), 4 deletions(-)
> >
> > diff --git a/mm/page-writeback.c b/mm/page-writeback.c
> > index 0062d5c57d41..bd7052295246 100644
> > --- a/mm/page-writeback.c
> > +++ b/mm/page-writeback.c
> > @@ -1945,6 +1945,8 @@ bool wb_over_bg_thresh(struct bdi_writeback *wb)
> >       struct dirty_throttle_control * const gdtc = &gdtc_stor;
> >       struct dirty_throttle_control * const mdtc = mdtc_valid(&mdtc_stor) ?
> >                                                    &mdtc_stor : NULL;
> > +     unsigned long reclaimable;
> > +     unsigned long thresh;
> >
> >       /*
> >        * Similar to balance_dirty_pages() but ignores pages being written
> > @@ -1957,8 +1959,13 @@ bool wb_over_bg_thresh(struct bdi_writeback *wb)
> >       if (gdtc->dirty > gdtc->bg_thresh)
> >               return true;
> >
> > -     if (wb_stat(wb, WB_RECLAIMABLE) >
> > -         wb_calc_thresh(gdtc->wb, gdtc->bg_thresh))
> > +     thresh = wb_calc_thresh(gdtc->wb, gdtc->bg_thresh);
> > +     if (thresh < 2 * wb_stat_error())
> > +             reclaimable = wb_stat_sum(wb, WB_RECLAIMABLE);
> > +     else
> > +             reclaimable = wb_stat(wb, WB_RECLAIMABLE);
> > +
> > +     if (reclaimable > thresh)
> >               return true;
> >
> >       if (mdtc) {
> > @@ -1972,8 +1979,13 @@ bool wb_over_bg_thresh(struct bdi_writeback *wb)
> >               if (mdtc->dirty > mdtc->bg_thresh)
> >                       return true;
> >
> > -             if (wb_stat(wb, WB_RECLAIMABLE) >
> > -                 wb_calc_thresh(mdtc->wb, mdtc->bg_thresh))
> > +             thresh = wb_calc_thresh(mdtc->wb, mdtc->bg_thresh);
> > +             if (thresh < 2 * wb_stat_error())
> > +                     reclaimable = wb_stat_sum(wb, WB_RECLAIMABLE);
> > +             else
> > +                     reclaimable = wb_stat(wb, WB_RECLAIMABLE);
> > +
> > +             if (reclaimable > thresh)
> >                       return true;
> >       }
> >
> > --
> > 2.17.1