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

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Message-ID: <20110428102244.6e1113e9@notabene.brown>
Date:	Thu, 28 Apr 2011 10:22:44 +1000
From:	NeilBrown <neilb@...e.de>
To:	Mel Gorman <mgorman@...e.de>
Cc:	Linux-MM <linux-mm@...ck.org>,
	Linux-Netdev <netdev@...r.kernel.org>,
	LKML <linux-kernel@...r.kernel.org>,
	David Miller <davem@...emloft.net>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Subject: Re: [PATCH 12/13] mm: Throttle direct reclaimers if PF_MEMALLOC
 reserves are low and swap is backed by network storage

On Wed, 27 Apr 2011 17:08:10 +0100 Mel Gorman <mgorman@...e.de> 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);
> +	if (pfmemalloc_watermark_ok(zone->zone_pgdat, high_zoneidx))
> +		return;

As the first thing that 'wait_event_interruptible" does is test the condition
and return if it is true, this "if () return;" is pointless.
 
> +
> +	/* Throttle */
> +	wait_event_interruptible(zone->zone_pgdat->pfmemalloc_wait,
> +		pfmemalloc_watermark_ok(zone->zone_pgdat, high_zoneidx));
> +}

I was surprised that you chose wait_event_interruptible as your previous code
was almost exactly "wait_event_killable".

Is there some justification for not throttling processes which happen to have
a (non-fatal) signal pending?

Thanks,
NeilBrown



> +
>  unsigned long try_to_free_pages(struct zonelist *zonelist, int order,
>  				gfp_t gfp_mask, nodemask_t *nodemask)
>  {
> @@ -2133,6 +2172,15 @@ unsigned long try_to_free_pages(struct zonelist *zonelist, int order,
>  		.nodemask = nodemask,
>  	};
>  
> +	throttle_direct_reclaim(gfp_mask, zonelist, nodemask);
> +
> +	/*
> +	 * Do not enter reclaim if fatal signal is pending. 1 is returned so
> +	 * that the page allocator does not consider triggering OOM
> +	 */
> +	if (fatal_signal_pending(current))
> +		return 1;
> +
>  	trace_mm_vmscan_direct_reclaim_begin(order,
>  				sc.may_writepage,
>  				gfp_mask);
> @@ -2488,6 +2536,12 @@ loop_again:
>  			}
>  
>  		}
> +
> +		/* Wake throttled direct reclaimers if low watermark is met */
> +		if (waitqueue_active(&pgdat->pfmemalloc_wait) &&
> +				pfmemalloc_watermark_ok(pgdat, MAX_NR_ZONES - 1))
> +			wake_up_interruptible(&pgdat->pfmemalloc_wait);
> +
>  		if (all_zones_ok || (order && pgdat_balanced(pgdat, balanced, *classzone_idx)))
>  			break;		/* kswapd: all done */
>  		/*

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