mm: vmscan: correctly check if reclaimer should schedule during shrink_slab

mm: vmscan: correctly check if reclaimer should schedule during shrink_slab

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() remains in
case one shrinker is particularly slow.

[[email protected]: preserve call to cond_resched after each call into shrinker]
Signed-off-by: Mel Gorman <[email protected]>
Signed-off-by: Minchan Kim <[email protected]>
Cc: Rik van Riel <[email protected]>
Cc: Johannes Weiner <[email protected]>
Cc: Wu Fengguang <[email protected]>
Cc: James Bottomley <[email protected]>
Tested-by: Colin King <[email protected]>
Cc: Raghavendra D Prabhu <[email protected]>
Cc: Jan Kara <[email protected]>
Cc: Chris Mason <[email protected]>
Cc: Christoph Lameter <[email protected]>
Cc: Pekka Enberg <[email protected]>
Cc: Rik van Riel <[email protected]>
Cc: <[email protected]>		[2.6.38+]
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>