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

From: KOSAKI Motohiro
Date: Tue May 17 2011 - 20:46:23 EST

(2011/05/18 1:15), Mel Gorman 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

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
in -mm.

[mgorman@xxxxxxx: Preserve call to cond_resched after each call into shrinker]
From: Minchan Kim<>
Signed-off-by: Mel Gorman<mgorman@xxxxxxx>

Looks good to me.
Reviewed-by: KOSAKI Motohiro <kosaki.motohiro@xxxxxxxxxxxxxx>

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