Re: [PATCH] mm: vmscan: Correctly check if reclaimer should schedule during shrink_slab

[Colin Ian King <colin.king@canonical.com>]

On Thu, 2011-05-19 at 09:09 +0900, Minchan Kim wrote:
> Hi Colin.
> 
> Sorry for bothering you. :(

No problem at all, I've very happy to re-test.

> I hope this test is last.
> 
> We(Mel, KOSAKI and me) finalized opinion.
> 
> Could you test below patch with patch[1/4] of Mel's series(ie,
> !pgdat_balanced  of sleeping_prematurely)?
> If it is successful, we will try to merge this version instead of
> various cond_resched sprinkling version.

tested with the patch below + patch[1/4] of Mel's series.  300 cycles,
2.5 hrs of soak testing: works OK.

Colin
> 
> 
> On Wed, May 18, 2011 at 1:15 AM, Mel Gorman <mgorman@suse.de> wrote:
> > It has been reported on some laptops that kswapd is consuming large
> > amounts of CPU and not being scheduled when SLUB is enabled during
> > large amounts of file copying. It is expected that this is due to
> > kswapd missing every cond_resched() point because;
> >
> > shrink_page_list() calls cond_resched() if inactive pages were isolated
> >        which in turn may not happen if all_unreclaimable is set in
> >        shrink_zones(). If for whatver reason, all_unreclaimable is
> >        set on all zones, we can miss calling cond_resched().
> >
> > balance_pgdat() only calls cond_resched if the zones are not
> >        balanced. For a high-order allocation that is balanced, it
> >        checks order-0 again. During that window, order-0 might have
> >        become unbalanced so it loops again for order-0 and returns
> >        that it was reclaiming for order-0 to kswapd(). It can then
> >        find that a caller has rewoken kswapd for a high-order and
> >        re-enters balance_pgdat() without ever calling cond_resched().
> >
> > shrink_slab only calls cond_resched() if we are reclaiming slab
> >        pages. If there are a large number of direct reclaimers, the
> >        shrinker_rwsem can be contended and prevent kswapd calling
> >        cond_resched().
> >
> > This patch modifies the shrink_slab() case. If the semaphore is
> > contended, the caller will still check cond_resched(). After each
> > successful call into a shrinker, the check for cond_resched() is
> > still necessary in case one shrinker call is particularly slow.
> >
> > This patch replaces
> > mm-vmscan-if-kswapd-has-been-running-too-long-allow-it-to-sleep.patch
> > in -mm.
> >
> > [mgorman@suse.de: Preserve call to cond_resched after each call into shrinker]
> > From: Minchan Kim <minchan.kim@gmail.com>
> > Signed-off-by: Mel Gorman <mgorman@suse.de>
> > ---
> >  mm/vmscan.c |    9 +++++++--
> >  1 files changed, 7 insertions(+), 2 deletions(-)
> >
> > diff --git a/mm/vmscan.c b/mm/vmscan.c
> > index af24d1e..0bed248 100644
> > --- a/mm/vmscan.c
> > +++ b/mm/vmscan.c
> > @@ -230,8 +230,11 @@ unsigned long shrink_slab(unsigned long scanned, gfp_t gfp_mask,
> >        if (scanned == 0)
> >                scanned = SWAP_CLUSTER_MAX;
> >
> > -       if (!down_read_trylock(&shrinker_rwsem))
> > -               return 1;       /* Assume we'll be able to shrink next time */
> > +       if (!down_read_trylock(&shrinker_rwsem)) {
> > +               /* Assume we'll be able to shrink next time */
> > +               ret = 1;
> > +               goto out;
> > +       }
> >
> >        list_for_each_entry(shrinker, &shrinker_list, list) {
> >                unsigned long long delta;
> > @@ -282,6 +285,8 @@ unsigned long shrink_slab(unsigned long scanned, gfp_t gfp_mask,
> >                shrinker->nr += total_scan;
> >        }
> >        up_read(&shrinker_rwsem);
> > +out:
> > +       cond_resched();
> >        return ret;
> >  }
> >
> >
> 
> 
>