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

From: Minchan Kim
Date: Wed May 18 2011 - 20:09:43 EST

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@xxxxxxx> 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
> Â Â Â Âwhich in turn may not happen if all_unreclaimable is set in
> Â Â Â Âshrink_zones(). If for whatver reason, all_unreclaimable is
> Â Â Â Âset on all zones, we can miss calling cond_resched().
> balance_pgdat() only calls cond_resched if the zones are not
> Â Â Â Âbalanced. For a high-order allocation that is balanced, it
> Â Â Â Âchecks order-0 again. During that window, order-0 might have
> Â Â Â Âbecome unbalanced so it loops again for order-0 and returns
> Â Â Â Âthat it was reclaiming for order-0 to kswapd(). It can then
> Â Â Â Âfind that a caller has rewoken kswapd for a high-order and
> Â Â Â Âre-enters balance_pgdat() without ever calling cond_resched().
> shrink_slab only calls cond_resched() if we are reclaiming slab
> Â Â Â Âpages. If there are a large number of direct reclaimers, the
> Â Â Â Âshrinker_rwsem can be contended and prevent kswapd calling
> Â Â Â Âcond_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@xxxxxxx: Preserve call to cond_resched after each call into shrinker]
> From: Minchan Kim <>
> Signed-off-by: Mel Gorman <mgorman@xxxxxxx>
> ---
> Âmm/vmscan.c | Â Â9 +++++++--
> Â1 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,
> Â Â Â Âif (scanned == 0)
> Â Â Â Â Â Â Â Âscanned = SWAP_CLUSTER_MAX;
> - Â Â Â if (!down_read_trylock(&shrinker_rwsem))
> - Â Â Â Â Â Â Â return 1; Â Â Â /* Assume we'll be able to shrink next time */
> + Â Â Â if (!down_read_trylock(&shrinker_rwsem)) {
> + Â Â Â Â Â Â Â /* Assume we'll be able to shrink next time */
> + Â Â Â Â Â Â Â ret = 1;
> + Â Â Â Â Â Â Â goto out;
> + Â Â Â }
> Â Â Â Âlist_for_each_entry(shrinker, &shrinker_list, list) {
> Â Â Â Â Â Â Â Âunsigned long long delta;
> @@ -282,6 +285,8 @@ unsigned long shrink_slab(unsigned long scanned, gfp_t gfp_mask,
> Â Â Â Â Â Â Â Âshrinker->nr += total_scan;
> Â Â Â Â}
> Â Â Â Âup_read(&shrinker_rwsem);
> +out:
> + Â Â Â cond_resched();
> Â Â Â Âreturn ret;
> Â}

Kind regards,
Minchan Kim
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