On Wed, Apr 27, 2011 at 09:18:11AM +1000, NeilBrown wrote:
> On Tue, 26 Apr 2011 15:26:24 +0100 Mel Gorman <mgorman@xxxxxxx> 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@xxxxxxx> 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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