Date:   Fri, 27 Jan 2023 14:03:59 +0100
From:   Frederic Weisbecker <frederic@kernel.org>
To:     Leonardo =?iso-8859-1?Q?Br=E1s?= <leobras@redhat.com>
Cc:     Michal Hocko <mhocko@suse.com>,
        Roman Gushchin <roman.gushchin@linux.dev>,
        Marcelo Tosatti <mtosatti@redhat.com>,
        Johannes Weiner <hannes@cmpxchg.org>,
        Shakeel Butt <shakeelb@google.com>,
        Muchun Song <muchun.song@linux.dev>,
        Andrew Morton <akpm@linux-foundation.org>,
        cgroups@vger.kernel.org, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org,
        Frederic Weisbecker <fweisbecker@suse.de>
Subject: Re: [PATCH v2 0/5] Introduce memcg_stock_pcp remote draining
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On Fri, Jan 27, 2023 at 05:12:13AM -0300, Leonardo Br?s wrote:
> On Fri, 2023-01-27 at 04:22 -0300, Leonardo Br?s wrote:
> > > Hmm, OK, I have misunderstood your proposal. Yes, the overal pcp charges
> > > potentially left behind should be small and that shouldn't really be a
> > > concern for memcg oom situations (unless the limit is very small and
> > > workloads on isolated cpus using small hard limits is way beyond my
> > > imagination).
> > > 
> > > My first thought was that those charges could be left behind without any
> > > upper bound but in reality sooner or later something should be running
> > > on those cpus and if the memcg is gone the pcp cache would get refilled
> > > and old charges gone.
> > > 
> > > So yes, this is actually a better and even simpler solution. All we need
> > > is something like this
> > > diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> > > index ab457f0394ab..13b84bbd70ba 100644
> > > --- a/mm/memcontrol.c
> > > +++ b/mm/memcontrol.c
> > > @@ -2344,6 +2344,9 @@ static void drain_all_stock(struct mem_cgroup *root_memcg)
> > >  		struct mem_cgroup *memcg;
> > >  		bool flush = false;
> > >  
> > > +		if (cpu_is_isolated(cpu))
> > > +			continue;
> > > +
> > >  		rcu_read_lock();
> > >  		memcg = stock->cached;
> > >  		if (memcg && stock->nr_pages &&
> > > 
> > > There is no such cpu_is_isolated() AFAICS so we would need a help from
> > > NOHZ and cpuisol people to create one for us. Frederic, would such an
> > > abstraction make any sense from your POV?
> > 
> > 
> > IIUC, 'if (cpu_is_isolated())' would be instead:
> > 
> > if (!housekeeping_cpu(smp_processor_id(), HK_TYPE_DOMAIN) ||
> > !housekeeping_cpu(smp_processor_id(), HK_TYPE_WQ)
> 
> oh, sorry 's/smp_processor_id()/cpu/' here:
> 
> if(!housekeeping_cpu(cpu, HK_TYPE_DOMAIN) || !housekeeping_cpu(cpu, HK_TYPE_WQ))

Do you also need to handle cpuset.sched_load_balance=0 (aka. cpuset v2 "isolated"
partition type). It has the same effect as isolcpus=, but it can be changed at
runtime.

And then on_null_domain() look like what you need. You'd have to make that API
more generally available though, and rename it to something like
"bool cpu_has_null_domain(int cpu)".

But then you also need to handle concurrent cpuset changes. If you can tolerate
it to be racy, then RCU alone is fine.

Thanks.