Re: [memcg] 45208c9105: aim7.jobs-per-min -14.0% regression

[Feng Tang]

On Thu, Sep 09, 2021 at 05:43:40PM -0700, Shakeel Butt wrote:
> On Mon, Sep 6, 2021 at 8:30 PM Feng Tang <feng.tang@xxxxxxxxx> wrote:
> >
> > Hi Shakeel,
> >
> > On Sun, Sep 05, 2021 at 03:15:46PM -0700, Shakeel Butt wrote:
> > > On Sun, Sep 5, 2021 at 5:27 AM kernel test robot <oliver.sang@xxxxxxxxx> wrote:
> > [...]
> > > > =========================================================================================
> > > > compiler/cpufreq_governor/disk/fs/kconfig/load/rootfs/tbox_group/test/testcase/ucode:
> > > >   gcc-9/performance/1BRD_48G/xfs/x86_64-rhel-8.3/3000/debian-10.4-x86_64-20200603.cgz/lkp-icl-2sp2/disk_rr/aim7/0xd000280
> > > >
> > > > commit:
> > > >   3c28c7680e ("memcg: switch lruvec stats to rstat")
> > > >   45208c9105 ("memcg: infrastructure to flush memcg stats")
> > >
> > > I am looking into this. I was hoping we have resolution for [1] as
> > > these patches touch similar data structures.
> > >
> > > [1] https://lore.kernel.org/all/20210811031734.GA5193@xsang-OptiPlex-9020/T/#u
> >
> > I tried 2 debug methods for that 36.4% vm-scalability regression:
> >
> > 1. Disable the HW cache prefetcher, no effect on this case
> > 2. relayout and add padding to 'struct cgroup_subsys_state', reduce
> >    the regression to 3.1%
> >
> 
> Thanks Feng but it seems like the issue for this commit is different.
> Rearranging the layout didn't help. Actually the cause of slowdown is
> the call to queue_work() inside __mod_memcg_lruvec_state().
> 
> At the moment, queue_work() is called after 32 updates. I changed it
> to 128 and the slowdown of will-it-scale:page_fault[1|2|3] halved
> (from around 10% to 5%). I am unable to run reaim or
> will-it-scale:fallocate2 as I was getting weird errors.
> 
> Feng, is it possible for you to run these benchmarks with the change
> (basically changing MEMCG_CHARGE_BATCH to 128 in the if condition
> before queue_work() inside __mod_memcg_lruvec_state())?

When I checked this, I tried different changes, including this batch
number change :), but it didn't recover the regression (the regression
is slightly reduced to about 12%)

Please check if my patch is what you want to test:

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 4d8c9af..a50a69a 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -682,7 +682,8 @@ void __mod_memcg_lruvec_state(struct lruvec *lruvec, enum node_stat_item idx,
 
 	/* Update lruvec */
 	__this_cpu_add(pn->lruvec_stats_percpu->state[idx], val);
-	if (!(__this_cpu_inc_return(stats_flush_threshold) % MEMCG_CHARGE_BATCH))
+//	if (!(__this_cpu_inc_return(stats_flush_threshold) % MEMCG_CHARGE_BATCH))
+	if (!(__this_cpu_inc_return(stats_flush_threshold) % 128))
 		queue_work(system_unbound_wq, &stats_flush_work);
 }

Thanks,
Feng

 

> For the formal patch/fix, I will write down a better explanation on
> what should be the batch size.
> 
> thanks,
> Shakeel