Re: [PATCH v2 0/5] Introduce memcg_stock_pcp remote draining

[Michal Hocko]

On Fri 27-01-23 08:11:04, Michal Hocko wrote:
> [Cc Frederic]
> 
> On Thu 26-01-23 15:12:35, Roman Gushchin wrote:
> > On Thu, Jan 26, 2023 at 08:41:34AM +0100, Michal Hocko wrote:
> [...]
> > > > Essentially each cpu will try to grab the remains of the memory quota
> > > > and move it locally. I wonder in such circumstances if we need to disable the pcp-caching
> > > > on per-cgroup basis.
> > > 
> > > I think it would be more than sufficient to disable pcp charging on an
> > > isolated cpu.
> > 
> > It might have significant performance consequences.
> 
> Is it really significant?
> 
> > I'd rather opt out of stock draining for isolated cpus: it might slightly reduce
> > the accuracy of memory limits and slightly increase the memory footprint (all
> > those dying memcgs...), but the impact will be limited. Actually it is limited
> > by the number of cpus.
> 
> 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 &&

Btw. this would be over pessimistic. The following should make more
sense:
diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index ab457f0394ab..55e440e54504 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -2357,7 +2357,7 @@ static void drain_all_stock(struct mem_cgroup *root_memcg)
 		    !test_and_set_bit(FLUSHING_CACHED_CHARGE, &stock->flags)) {
 			if (cpu == curcpu)
 				drain_local_stock(&stock->work);
-			else
+			else if (!cpu_is_isolated(cpu))
 				schedule_work_on(cpu, &stock->work);
 		}
 	}
-- 
Michal Hocko
SUSE Labs