Subject: Re: [PATCH 12/13] mm: Throttle direct reclaimers if PF_MEMALLOC reserves are low and swap is backed by network storage

From: NeilBrown <[email-masked]>
Date: Tue, 26 Apr 2011 22:30:59 +1000

On Tue, 26 Apr 2011 08:36:53 +0100 Mel Gorman <[email-masked]> wrote:

> +/*
> + * Throttle direct reclaimers if backing storage is backed by the network
> + * and the PFMEMALLOC reserve for the preferred node is getting dangerously
> + * depleted. kswapd will continue to make progress and wake the processes
> + * when the low watermark is reached
> + */
> +static void throttle_direct_reclaim(gfp_t gfp_mask, struct zonelist *zonelist,
> +					nodemask_t *nodemask)
> +{
> +	struct zone *zone;
> +	int high_zoneidx = gfp_zone(gfp_mask);
> +	DEFINE_WAIT(wait);
> +
> +	/* Check if the pfmemalloc reserves are ok */
> +	first_zones_zonelist(zonelist, high_zoneidx, NULL, &zone);
> +	prepare_to_wait(&zone->zone_pgdat->pfmemalloc_wait, &wait,
> +							TASK_INTERRUPTIBLE);
> +	if (pfmemalloc_watermark_ok(zone->zone_pgdat, high_zoneidx))
> +		goto out;
> +
> +	/* Throttle */
> +	do {
> +		schedule();
> +		finish_wait(&zone->zone_pgdat->pfmemalloc_wait, &wait);
> +		prepare_to_wait(&zone->zone_pgdat->pfmemalloc_wait, &wait,
> +							TASK_INTERRUPTIBLE);
> +	} while (!pfmemalloc_watermark_ok(zone->zone_pgdat, high_zoneidx) &&
> +			!fatal_signal_pending(current));
> +
> +out:
> +	finish_wait(&zone->zone_pgdat->pfmemalloc_wait, &wait);
> +}

You are doing an interruptible wait, but only checking for fatal signals.
So if a non-fatal signal arrives, you will busy-wait.

So I suspect you want TASK_KILLABLE, so just use:

    wait_event_killable(zone->zone_pgdat->pfmemalloc_wait,
                        pgmemalloc_watermark_ok(zone->zone_pgdata,
                                                high_zoneidx));

(You also have an extraneous call to finish_wait)

NeilBrown

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