Re: [net] 4890b686f4: netperf.Throughput_Mbps -69.4% regression

Eric Dumazet
2022-06-27T17:05:33Z

On Mon, Jun 27, 2022 at 6:48 PM Shakeel Butt <shakeelb@google.com> wrote:
>
> On Mon, Jun 27, 2022 at 9:26 AM Eric Dumazet <edumazet@google.com> wrote:
> >
> [...]
> > >
> >
> > I simply did the following and got much better results.
> >
> > But I am not sure if updates to ->usage are really needed that often...
>
> I suspect we need to improve the per-cpu memcg stock usage here. Were
> the updates mostly from uncharge path or charge path or that's
> irrelevant?

I wonder if the cache is always used...

stock = this_cpu_ptr(&memcg_stock);
if (memcg == stock->cached && stock->nr_pages >= nr_pages) {

Apparently the per-cpu cache is only used for one memcg at a time ?

Not sure how this would scale to hosts with dozens of memcgs.

Maybe we could add some metrics to have an idea of the cache hit/miss ratio :/

>
> I think doing full drain (i.e. drain_stock()) within __refill_stock()
> when the local cache is larger than MEMCG_CHARGE_BATCH is not best.
> Rather we should always keep at least MEMCG_CHARGE_BATCH for such
> scenarios.
>
> >
> >
> > diff --git a/include/linux/page_counter.h b/include/linux/page_counter.h
> > index 679591301994d316062f92b275efa2459a8349c9..e267be4ba849760117d9fd041e22c2a44658ab36
> > 100644
> > --- a/include/linux/page_counter.h
> > +++ b/include/linux/page_counter.h
> > @@ -3,12 +3,15 @@
> > #define _LINUX_PAGE_COUNTER_H
> >
> > #include <linux/atomic.h>
> > +#include <linux/cache.h>
> > #include <linux/kernel.h>
> > #include <asm/page.h>
> >
> > struct page_counter {
> > - atomic_long_t usage;
> > - unsigned long min;
> > + /* contended cache line. */
> > + atomic_long_t usage ____cacheline_aligned_in_smp;
> > +
> > + unsigned long min ____cacheline_aligned_in_smp;
>
> Do we need to align 'min' too?

Probably if there is a hierarchy ...

propagate_protected_usage() seems to have potential high cost.

>
> > unsigned long low;
> > unsigned long high;
> > unsigned long max;
> > @@ -27,12 +30,6 @@ struct page_counter {
> > unsigned long watermark;
> > unsigned long failcnt;
> >
> > - /*
> > - * 'parent' is placed here to be far from 'usage' to reduce
> > - * cache false sharing, as 'usage' is written mostly while
> > - * parent is frequently read for cgroup's hierarchical
> > - * counting nature.
> > - */
> > struct page_counter *parent;
> > };
> >
> >
> >