Re: [PATCH 2/5] memcg: use array and ID for quick look up

[KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>]

On Mon, 23 Aug 2010 12:35:33 +0900
Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp> wrote:

> Hi,
> 
> > +/* 0 is unused */
> > +static atomic_t mem_cgroup_num;
> > +#define NR_MEMCG_GROUPS (CONFIG_MEM_CGROUP_MAX_GROUPS + 1)
> > +static struct mem_cgroup *mem_cgroups[NR_MEMCG_GROUPS] __read_mostly;
> > +
> > +/* Must be called under rcu_read_lock */
> > +static struct mem_cgroup *id_to_memcg(unsigned short id)
> > +{
> > +	struct mem_cgroup *ret;
> > +	/* see mem_cgroup_free() */
> > +	ret = rcu_dereference_check(mem_cgroups[id], rch_read_lock_held());
> > +	if (likely(ret && ret->valid))
> > +		return ret;
> > +	return NULL;
> > +}
> > +
> I prefer "mem" to "ret".
> 
Hmm, ok.


> > @@ -2231,7 +2244,7 @@ __mem_cgroup_commit_charge_swapin(struct
> >  
> >  		id = swap_cgroup_record(ent, 0);
> >  		rcu_read_lock();
> > -		memcg = mem_cgroup_lookup(id);
> > +		memcg = id_to_memcg(id);
> >  		if (memcg) {
> >  			/*
> >  			 * This recorded memcg can be obsolete one. So, avoid
> > @@ -2240,9 +2253,10 @@ __mem_cgroup_commit_charge_swapin(struct
> >  			if (!mem_cgroup_is_root(memcg))
> >  				res_counter_uncharge(&memcg->memsw, PAGE_SIZE);
> >  			mem_cgroup_swap_statistics(memcg, false);
> > +			rcu_read_unlock();
> >  			mem_cgroup_put(memcg);
> > -		}
> > -		rcu_read_unlock();
> > +		} else
> > +			rcu_read_unlock();
> >  	}
> >  	/*
> >  	 * At swapin, we may charge account against cgroup which has no tasks.
> > @@ -2495,7 +2509,7 @@ void mem_cgroup_uncharge_swap(swp_entry_
> >  
> >  	id = swap_cgroup_record(ent, 0);
> >  	rcu_read_lock();
> > -	memcg = mem_cgroup_lookup(id);
> > +	memcg = id_to_memcg(id);
> >  	if (memcg) {
> >  		/*
> >  		 * We uncharge this because swap is freed.
> > @@ -2504,9 +2518,10 @@ void mem_cgroup_uncharge_swap(swp_entry_
> >  		if (!mem_cgroup_is_root(memcg))
> >  			res_counter_uncharge(&memcg->memsw, PAGE_SIZE);
> >  		mem_cgroup_swap_statistics(memcg, false);
> > +		rcu_read_unlock();
> >  		mem_cgroup_put(memcg);
> > -	}
> > -	rcu_read_unlock();
> > +	} else
> > +		rcu_read_unlock();
> >  }
> >  
> >  /**
> Could you explain why we need rcu_read_unlock() before mem_cgroup_put() ?
> I suspect that it's because mem_cgroup_put() can free the memcg, but do we
> need mem->valid then ?
> 
mem_cgroup_put() may call synchronize_rcu(). So, we have to unlock before it.

Thanks,
-Kame



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