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

[Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp>]

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".

> @@ -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 ?


Thanks,
Daisuke Nishimura.

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