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

["Leonardo Brás" <leobras@redhat.com>]

On Fri, 2023-01-27 at 08:11 +0100, 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 &&
> 
> 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)