Re: kswapd0: excessive CPU usage

[Zdenek Kabelac]

Dne 15.10.2012 13:09, Mel Gorman napsal(a):
> 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.


I've applied this patch on 3.7.0-rc3 kernel - and I still see excessive CPU 
usage - mainly  after  suspend/resume

Here is just simple  kswapd backtrace from running kernel:

kswapd0         R  running task        0    30      2 0x00000000
 ffff8801331ddae8 0000000000000082 ffff880135b8a340 0000000000000008
 ffff880135b8a340 ffff8801331ddfd8 ffff8801331ddfd8 ffff8801331ddfd8
 ffff880071db8000 ffff880135b8a340 0000000000000286 ffff8801331dc000
Call Trace:
 [<ffffffff81555cd2>] preempt_schedule+0x42/0x60
 [<ffffffff81557b75>] _raw_spin_unlock+0x55/0x60
 [<ffffffff811929d1>] put_super+0x31/0x40
 [<ffffffff81192aa2>] drop_super+0x22/0x30
 [<ffffffff81193be9>] prune_super+0x149/0x1b0
 [<ffffffff81141e2a>] shrink_slab+0xba/0x510
 [<ffffffff81185baa>] ? mem_cgroup_iter+0x17a/0x2e0
 [<ffffffff81185afa>] ? mem_cgroup_iter+0xca/0x2e0
 [<ffffffff811450f9>] balance_pgdat+0x629/0x7f0
 [<ffffffff81145434>] kswapd+0x174/0x620
 [<ffffffff8106fd20>] ? __init_waitqueue_head+0x60/0x60
 [<ffffffff811452c0>] ? balance_pgdat+0x7f0/0x7f0
 [<ffffffff8106f50b>] kthread+0xdb/0xe0
 [<ffffffff8106f430>] ? kthread_create_on_node+0x140/0x140
 [<ffffffff8155fb1c>] ret_from_fork+0x7c/0xb0
 [<ffffffff8106f430>] ? kthread_create_on_node+0x140/0x140


Zdenek


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