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

[Michal Hocko]

On Fri 27-01-23 05:12:13, Leonardo Brás wrote:
> On Fri, 2023-01-27 at 04:22 -0300, Leonardo Brás wrote:
> > 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)
> 
> oh, sorry 's/smp_processor_id()/cpu/' here:
> 
> if(!housekeeping_cpu(cpu, HK_TYPE_DOMAIN) || !housekeeping_cpu(cpu, HK_TYPE_WQ))

Or maybe we can get a nice abstract API so that we do not have to really
care about those low level details. I do not really know what those
really mean and hopefully I shouldn't really need to know.

-- 
Michal Hocko
SUSE Labs