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

[Minchan Kim <minchan.kim@gmail.com>]

Hi Colin.

Sorry for bothering you. :(
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.


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;
>  }
>
>



-- 
Kind regards,
Minchan Kim