On Thu, Sep 06, 2012 at 06:44:29PM +0800, Shaohua Li wrote:
> isolate_migratepages_range will take zone->lru_lock first and check if the lock
> is contented, if yes, it will release the lock. This isn't efficient. If the
> lock is truly contented, a lock/unlock pair will increase the lock contention.
> We'd better check if the lock is contended first. compact_trylock_irqsave
> perfectly meets the requirement.
>
> Signed-off-by: Shaohua Li <shli@xxxxxxxxxxxx>
> ---
> mm/compaction.c | 7 ++++---
> 1 file changed, 4 insertions(+), 3 deletions(-)
>
> Index: linux/mm/compaction.c
> ===================================================================
> --- linux.orig/mm/compaction.c 2012-09-06 14:46:13.923144263 +0800
> +++ linux/mm/compaction.c 2012-09-06 14:46:58.118588574 +0800
> @@ -295,9 +295,9 @@ isolate_migratepages_range(struct zone *
> }
>
> /* Time to isolate some pages for migration */
> - cond_resched();
Why did you remove the cond_resched()? I expect it's because
compact_checklock_irqsave() does a need_resched() check and if it is true
will either call cond_resched() or abort compaction. If it is aborting it
will not call cond_resched() but there is a reasonable expectation that
the caller will schedule soon. If this is the reasoning then it should be
included in the changelog. If it's an accident then leave the cond_resched()
where it is.
> - spin_lock_irqsave(&zone->lru_lock, flags);
> - locked = true;
> + locked = compact_trylock_irqsave(&zone->lru_lock, &flags, cc);
> + if (!locked)
> + goto skip;
There is no need for the goto. No useful work has taken place at this
point and there is no need to even trigger the tracepoint. Just return
0.
> for (; low_pfn < end_pfn; low_pfn++) {
> struct page *page;
>
> @@ -400,6 +400,7 @@ isolate_migratepages_range(struct zone *
> if (locked)
> spin_unlock_irqrestore(&zone->lru_lock, flags);
>
> +skip:
> trace_mm_compaction_isolate_migratepages(nr_scanned, nr_isolated);
>
> if (!nr_isolated)
--
Mel Gorman
SUSE Labs
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