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Date:	Fri, 25 Jul 2014 13:32:37 +0100
From:	Mel Gorman <>
To:	Vlastimil Babka <>
Cc:, Andrew Morton <>,
	David Rientjes <>,,
	Michal Nazarewicz <>,
	Naoya Horiguchi <>,
	Christoph Lameter <>,
	Rik van Riel <>,
	Joonsoo Kim <>,
	Minchan Kim <>,
	Zhang Yanfei <>
Subject: Re: [PATCH V4 08/15] mm, compaction: periodically drop lock and
 restore IRQs in scanners

On Wed, Jul 16, 2014 at 03:48:16PM +0200, Vlastimil Babka wrote:
> Compaction scanners regularly check for lock contention and need_resched()
> through the compact_checklock_irqsave() function. However, if there is no
> contention, the lock can be held and IRQ disabled for potentially long time.
> This has been addressed by commit b2eef8c0d0 ("mm: compaction: minimise the
> time IRQs are disabled while isolating pages for migration") for the migration
> scanner. However, the refactoring done by commit 2a1402aa04 ("mm: compaction:
> acquire the zone->lru_lock as late as possible") has changed the conditions so
> that the lock is dropped only when there's contention on the lock or
> need_resched() is true. Also, need_resched() is checked only when the lock is
> already held. The comment "give a chance to irqs before checking need_resched"
> is therefore misleading, as IRQs remain disabled when the check is done.
> This patch restores the behavior intended by commit b2eef8c0d0 and also tries
> to better balance and make more deterministic the time spent by checking for
> contention vs the time the scanners might run between the checks. It also
> avoids situations where checking has not been done often enough before. The
> result should be avoiding both too frequent and too infrequent contention
> checking, and especially the potentially long-running scans with IRQs disabled
> and no checking of need_resched() or for fatal signal pending, which can happen
> when many consecutive pages or pageblocks fail the preliminary tests and do not
> reach the later call site to compact_checklock_irqsave(), as explained below.
> Before the patch:
> In the migration scanner, compact_checklock_irqsave() was called each loop, if
> reached. If not reached, some lower-frequency checking could still be done if
> the lock was already held, but this would not result in aborting contended
> async compaction until reaching compact_checklock_irqsave() or end of
> pageblock. In the free scanner, it was similar but completely without the
> periodical checking, so lock can be potentially held until reaching the end of
> pageblock.
> After the patch, in both scanners:
> The periodical check is done as the first thing in the loop on each
> SWAP_CLUSTER_MAX aligned pfn, using the new compact_unlock_should_abort()
> function, which always unlocks the lock (if locked) and aborts async compaction
> if scheduling is needed. It also aborts any type of compaction when a fatal
> signal is pending.
> The compact_checklock_irqsave() function is replaced with a slightly different
> compact_trylock_irqsave(). The biggest difference is that the function is not
> called at all if the lock is already held. The periodical need_resched()
> checking is left solely to compact_unlock_should_abort(). The lock contention
> avoidance for async compaction is achieved by the periodical unlock by
> compact_unlock_should_abort() and by using trylock in compact_trylock_irqsave()
> and aborting when trylock fails. Sync compaction does not use trylock.
> Signed-off-by: Vlastimil Babka <>
> Reviewed-by: Zhang Yanfei <>
> Acked-by: Minchan Kim <>
> Cc: Mel Gorman <>
> Cc: Michal Nazarewicz <>
> Cc: Naoya Horiguchi <>
> Cc: Christoph Lameter <>
> Cc: Rik van Riel <>
> Cc: David Rientjes <>

Acked-by: Mel Gorman <>

Mel Gorman
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