linux-kernel - Re: [PATCH 2/2] vmscan: use atomic-long for shrinker batching

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Date:	Tue, 23 Aug 2011 10:56:51 +0400
From:	Konstantin Khlebnikov <khlebnikov@...nvz.org>
To:	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	Andrew Morton <akpm@...ux-foundation.org>
CC:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Dave Chinner <david@...morbit.com>
Subject: Re: [PATCH 2/2] vmscan: use atomic-long for shrinker batching

Konstantin Khlebnikov wrote:
>   		delta = (4 * nr_pages_scanned) / shrinker->seeks;
> @@ -329,12 +327,11 @@ unsigned long shrink_slab(struct shrink_control *shrink,
>   		 * manner that handles concurrent updates. If we exhausted the
>   		 * scan, there is no need to do an update.
>   		 */
> -		do {
> -			nr = shrinker->nr;
> -			new_nr = total_scan + nr;
> -			if (total_scan<= 0)
> -				break;
> -		} while (cmpxchg(&shrinker->nr, nr, new_nr) != nr);
> +		if (total_scan>  0)
> +			new_nr = atomic_long_add_return(total_scan,
> +					&shrinker->nr_in_batch);
> +		else
> +			new_nr = atomic_long_read(&shrinker->nr_in_batch);
>
>   		trace_mm_shrink_slab_end(shrinker, shrink_ret, nr, new_nr);

BTW, new_nr required only for tracing, maybe this will be better/faster,
because atomic accuracy there isn't required at all.

	if (total_scan > 0)
		atomic_long_add(total_scan, &shrinker->nr_in_batch);

	new_nr = atomic_long_read(&shrinker->nr_in_batch);
	trace_mm_shrink_slab_end(shrinker, shrink_ret, nr, new_nr);
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