linux-kernel - Re: [PATCH] mm: per-cgroup memory reclaim stats

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Date:   Fri, 12 May 2017 12:42:06 -0400
From:   Johannes Weiner <hannes@...xchg.org>
To:     Balbir Singh <bsingharora@...il.com>
Cc:     Roman Gushchin <guro@...com>, Tejun Heo <tj@...nel.org>,
        Li Zefan <lizefan@...wei.com>,
        Michal Hocko <mhocko@...nel.org>,
        Vladimir Davydov <vdavydov.dev@...il.com>,
        cgroups@...r.kernel.org, linux-doc@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH] mm: per-cgroup memory reclaim stats

On Fri, May 12, 2017 at 12:25:22PM +1000, Balbir Singh wrote:
> On Thu, 2017-05-11 at 20:16 +0100, Roman Gushchin wrote:
> > The meaning of each value is the same as for global counters,
> > available using /proc/vmstat.
> > 
> > Also, for consistency, rename mem_cgroup_count_vm_event() to
> > count_memcg_event_mm().
> > 
> 
> I still prefer the mem_cgroup_count_vm_event() name, or memcg_count_vm_event(),
> the namespace upfront makes it easier to parse where to look for the the
> implementation and also grep. In any case the rename should be independent
> patch, but I don't like the name you've proposed.

The memory controller is no longer a tacked-on feature to the VM - the
entire reclaim path is designed around cgroups at this point. The
namespacing is just cumbersome and doesn't add add any value, IMO.

This name is also more consistent with the stats interface, where we
use nodes, zones, memcgs all equally to describe scopes/containers:

inc_node_state(), inc_zone_state(), inc_memcg_state()

> > @@ -357,6 +357,17 @@ static inline unsigned short mem_cgroup_id(struct mem_cgroup *memcg)
> >  }
> >  struct mem_cgroup *mem_cgroup_from_id(unsigned short id);
> >  
> > +static inline struct mem_cgroup *lruvec_memcg(struct lruvec *lruvec)
> 
> mem_cgroup_from_lruvec()?

This name is consistent with other lruvec accessors such as
lruvec_pgdat() and lruvec_lru_size() etc.

> > @@ -1741,11 +1748,16 @@ shrink_inactive_list(unsigned long nr_to_scan, struct lruvec *lruvec,
> >  
> >  	spin_lock_irq(&pgdat->lru_lock);
> >  
> > -	if (global_reclaim(sc)) {
> > -		if (current_is_kswapd())
> > +	if (current_is_kswapd()) {
> > +		if (global_reclaim(sc))
> >  			__count_vm_events(PGSTEAL_KSWAPD, nr_reclaimed);
> > -		else
> > +		count_memcg_events(lruvec_memcg(lruvec), PGSTEAL_KSWAPD,
> > +				   nr_reclaimed);
> 
> Has the else gone missing? What happens if it's global_reclaim(), do
> we still account the count in memcg?
> 
> > +	} else {
> > +		if (global_reclaim(sc))
> >  			__count_vm_events(PGSTEAL_DIRECT, nr_reclaimed);
> > +		count_memcg_events(lruvec_memcg(lruvec), PGSTEAL_DIRECT,
> > +				   nr_reclaimed);
> 
> It sounds like memcg accumlates both global and memcg reclaim driver
> counts -- is this what we want?

Yes.

Consider a fully containerized system that is using only memory.low
and thus exclusively global reclaim to enforce the partitioning, NOT
artificial limits and limit reclaim. In this case, we still want to
know how much reclaim activity each group is experiencing.