Re: [PATCH v2] mm, compaction: properly signal and act upon lock and need_sched() contention

[Michal Nazarewicz]

On Fri, May 16 2014, Vlastimil Babka wrote:
> Compaction uses compact_checklock_irqsave() function to periodically check for
> lock contention and need_resched() to either abort async compaction, or to
> free the lock, schedule and retake the lock. When aborting, cc->contended is
> set to signal the contended state to the caller. Two problems have been
> identified in this mechanism.
>
> First, compaction also calls directly cond_resched() in both scanners when no
> lock is yet taken. This call either does not abort async compaction, or set
> cc->contended appropriately. This patch introduces a new compact_should_abort()
> function to achieve both. In isolate_freepages(), the check frequency is
> reduced to once by SWAP_CLUSTER_MAX pageblocks to match what the migration
> scanner does in the preliminary page checks. In case a pageblock is found
> suitable for calling isolate_freepages_block(), the checks within there are
> done on higher frequency.
>
> Second, isolate_freepages() does not check if isolate_freepages_block()
> aborted due to contention, and advances to the next pageblock. This violates
> the principle of aborting on contention, and might result in pageblocks not
> being scanned completely, since the scanning cursor is advanced. This patch
> makes isolate_freepages_block() check the cc->contended flag and abort.
>
> In case isolate_freepages() has already isolated some pages before aborting
> due to contention, page migration will proceed, which is OK since we do not
> want to waste the work that has been done, and page migration has own checks
> for contention. However, we do not want another isolation attempt by either
> of the scanners, so cc->contended flag check is added also to
> compaction_alloc() and compact_finished() to make sure compaction is aborted
> right after the migration.
>
> Reported-by: Joonsoo Kim <iamjoonsoo.kim@xxxxxxx>
> Signed-off-by: Vlastimil Babka <vbabka@xxxxxxx>
> Reviewed-by: Naoya Horiguchi <n-horiguchi@xxxxxxxxxxxxx>
> Cc: Minchan Kim <minchan@xxxxxxxxxx>
> Cc: Mel Gorman <mgorman@xxxxxxx>
> Cc: Bartlomiej Zolnierkiewicz <b.zolnierkie@xxxxxxxxxxx>
> Cc: Michal Nazarewicz <mina86@xxxxxxxxxx>

Acked-by: Michal Nazarewicz <mina86@xxxxxxxxxx>

> Cc: Christoph Lameter <cl@xxxxxxxxx>
> Cc: Rik van Riel <riel@xxxxxxxxxx>
> ---
> v2: update struct compact_control comment (per Naoya Horiguchi)
>     rename to compact_should_abort() and add comments (per David Rientjes)
>     add cc->contended checks in compaction_alloc() and compact_finished()
>     (per Joonsoo Kim)
>     reduce frequency of checks in isolate_freepages() 
>
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