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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Date:	Wed, 27 Apr 2011 09:36:20 +0100
From:	Mel Gorman <mgorman@...e.de>
To:	NeilBrown <neilb@...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, Apr 27, 2011 at 09:18:11AM +1000, NeilBrown wrote:
> On Tue, 26 Apr 2011 15:26:24 +0100 Mel Gorman <mgorman@...e.de> wrote:
> 
> > On Tue, Apr 26, 2011 at 10:30:59PM +1000, NeilBrown wrote:
> > > On Tue, 26 Apr 2011 08:36:53 +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);
> > > > +	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));
> > > 
> > 
> > Well, if a normal signal arrives, we do not necessarily want the
> > process to enter reclaim. For fatal signals, I allow it to continue
> > because it's not likely to be putting the system under more pressure
> > if it's exiting.
> 
> Yep, I understand that and it doesn't seem unreasonable.
> 
> However I don't think the code implements that correctly.
> 
> If you get a non-fatal signal, schedule will exit immediately (because of the
> TASK_INTERRUPTIBLE setting) and the 'while' clause will succeed because the
> signal is not fatal, so it will loop around and try to schedule again, which
> will again exit immediately - busy loop.
> 

Ah, I see. Once again, well spotted.

> > 
> > > (You also have an extraneous call to finish_wait)
> > > 
> > 
> > Which one? I'm not seeing a flow where finish_wait gets called twice
> > without a prepare_to_wait in between. 
> > 
> 
> You don't need to call finish_wait immediately before prepare_to_wait.
> 
> It really is best to just use the appropriate 'wait_event*' macro....
> 

wait_event_interruptible it is. Thanks

-- 
Mel Gorman
SUSE Labs
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