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

From: Mel Gorman <[email-masked]>
Date: Tue, 26 Apr 2011 15:26:24 +0100

On Tue, Apr 26, 2011 at 10:30:59PM +1000, NeilBrown wrote:
> 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));
> 

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.

> (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. 

-- 
Mel Gorman
SUSE Labs