Re: [patch 2/2 v2]compaction: check lock contention first before taking lock

[Andrew Morton <akpm@linux-foundation.org>]

On Mon, 10 Sep 2012 09:18:50 +0800
Shaohua Li <shli@kernel.org> 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.
> 
> V2:
> leave cond_resched() pointed out by Mel.
> 
> Signed-off-by: Shaohua Li <shli@fusionio.com>
> ---
>  mm/compaction.c |    5 +++--
>  1 file changed, 3 insertions(+), 2 deletions(-)
> 
> Index: linux/mm/compaction.c
> ===================================================================
> --- linux.orig/mm/compaction.c	2012-09-10 08:49:40.377869710 +0800
> +++ linux/mm/compaction.c	2012-09-10 08:53:10.295230575 +0800
> @@ -295,8 +295,9 @@ isolate_migratepages_range(struct zone *
>  
>  	/* Time to isolate some pages for migration */
>  	cond_resched();
> -	spin_lock_irqsave(&zone->lru_lock, flags);
> -	locked = true;
> +	locked = compact_trylock_irqsave(&zone->lru_lock, &flags, cc);
> +	if (!locked)
> +		return 0;
>  	for (; low_pfn < end_pfn; low_pfn++) {
>  		struct page *page;

Geeze that compact_checklock_irqsave stuff is naaaasty.

What happens if a process has need_resched set?  It cannot perform
compaction?  There is no relationship between the concepts "user
pressed ^C" and "this device driver or subsystem wants a high-order
allocation".

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