Re: [patch 1/2 v2]compaction: abort compaction loop if lock is contended or run too long

[Andrea Arcangeli <aarcange@redhat.com>]

On Thu, Sep 13, 2012 at 10:38:26AM +0100, Mel Gorman wrote:
> I agree with Minchan. Andrea's patch ignores the fact that free page
> isolation might have aborted due to lock contention. It's not necessarily
> going to be isolating the pages it needs for migration.

Actually I thought of calling putback_lru_pages first, but then I
thought it was better to just complete the current slice.

Note that putback_lru_pages can take the lru_lock immediately too when
the pagevec gets full which won't work any better than if the
cc->contended was set by the freepages isolation and we do
migrate_pages.

There's no way to abort lockless from that point, so I think it's
better to take the last locks to finish the current slice of work and
then abort if it's still contended (which confirms we're really
trashing).

Skipping isolated pages without rewinding low_pfn would also reduce
compaction reliability so that should be evaluated as well. And
rewinding with the putback_lru_pages would risk livelocks.

I agree Minchan's patch would fix the problem too, and this should be
a fairly uncommon path so either ways shouldn't make a noticeable
difference.

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