Re: kswapd0: excessive CPU usage

[Mel Gorman]

On Mon, Oct 15, 2012 at 11:54:13AM +0200, Jiri Slaby wrote:
> On 10/12/2012 03:57 PM, Mel Gorman wrote:
> > mm: vmscan: scale number of pages reclaimed by reclaim/compaction only in direct reclaim
> > 
> > Jiri Slaby reported the following:
> > 
> > 	(It's an effective revert of "mm: vmscan: scale number of pages
> > 	reclaimed by reclaim/compaction based on failures".)
> > 	Given kswapd had hours of runtime in ps/top output yesterday in the
> > 	morning and after the revert it's now 2 minutes in sum for the last 24h,
> > 	I would say, it's gone.
> > 
> > The intention of the patch in question was to compensate for the loss of
> > lumpy reclaim. Part of the reason lumpy reclaim worked is because it
> > aggressively reclaimed pages and this patch was meant to be a
> > sane compromise.
> > 
> > When compaction fails, it gets deferred and both compaction and
> > reclaim/compaction is deferred avoid excessive reclaim. However, since
> > commit c6543459 (mm: remove __GFP_NO_KSWAPD), kswapd is woken up each time
> > and continues reclaiming which was not taken into account when the patch
> > was developed.
> > 
> > As it is not taking deferred compaction into account in this path it scans
> > aggressively before falling out and making the compaction_deferred check in
> > compaction_ready. This patch avoids kswapd scaling pages for reclaim and
> > leaves the aggressive reclaim to the process attempting the THP
> > allocation.
> > 
> > Signed-off-by: Mel Gorman <mgorman@xxxxxxx>
> > ---
> >  mm/vmscan.c |   10 ++++++++--
> >  1 file changed, 8 insertions(+), 2 deletions(-)
> > 
> > diff --git a/mm/vmscan.c b/mm/vmscan.c
> > index 2624edc..2b7edfa 100644
> > --- a/mm/vmscan.c
> > +++ b/mm/vmscan.c
> > @@ -1763,14 +1763,20 @@ static bool in_reclaim_compaction(struct scan_control *sc)
> >  #ifdef CONFIG_COMPACTION
> >  /*
> >   * If compaction is deferred for sc->order then scale the number of pages
> > - * reclaimed based on the number of consecutive allocation failures
> > + * reclaimed based on the number of consecutive allocation failures. This
> > + * scaling only happens for direct reclaim as it is about to attempt
> > + * compaction. If compaction fails, future allocations will be deferred
> > + * and reclaim avoided. On the other hand, kswapd does not take compaction
> > + * deferral into account so if it scaled, it could scan excessively even
> > + * though allocations are temporarily not being attempted.
> >   */
> >  static unsigned long scale_for_compaction(unsigned long pages_for_compaction,
> >  			struct lruvec *lruvec, struct scan_control *sc)
> >  {
> >  	struct zone *zone = lruvec_zone(lruvec);
> >  
> > -	if (zone->compact_order_failed <= sc->order)
> > +	if (zone->compact_order_failed <= sc->order &&
> > +	    !current_is_kswapd())
> >  		pages_for_compaction <<= zone->compact_defer_shift;
> >  	return pages_for_compaction;
> >  }
> 
> Yes, applying this instead of the revert fixes the issue as well.
> 

Thanks Jiri.

-- 
Mel Gorman
SUSE Labs
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