Re: [RFC][-mm] Memory controller hierarchy support (v1)

[yamamoto@valinux.co.jp (YAMAMOTO Takashi)]

> -int res_counter_charge(struct res_counter *counter, unsigned long val)
> +int res_counter_charge(struct res_counter *counter, unsigned long val,
> +			struct res_counter **limit_exceeded_at)
>  {
>  	int ret;
>  	unsigned long flags;
> +	struct res_counter *c, *unroll_c;
>  
> -	spin_lock_irqsave(&counter->lock, flags);
> -	ret = res_counter_charge_locked(counter, val);
> -	spin_unlock_irqrestore(&counter->lock, flags);
> +	*limit_exceeded_at = NULL;
> +	local_irq_save(flags);
> +	for (c = counter; c != NULL; c = c->parent) {
> +		spin_lock(&c->lock);
> +		ret = res_counter_charge_locked(c, val);
> +		spin_unlock(&c->lock);
> +		if (ret < 0) {
> +			*limit_exceeded_at = c;
> +			goto unroll;
> +		}
> +	}
> +	local_irq_restore(flags);
> +	return 0;
> +
> +unroll:
> +	for (unroll_c = counter; unroll_c != c; unroll_c = unroll_c->parent) {
> +		spin_lock(&unroll_c->lock);
> +		res_counter_uncharge_locked(unroll_c, val);
> +		spin_unlock(&unroll_c->lock);
> +	}
> +	local_irq_restore(flags);
>  	return ret;
>  }

i wonder how much performance impacts this involves.

it increases the number of atomic ops per charge/uncharge and
makes the common case (success) of every charge/uncharge in a system
touch a global (ie. root cgroup's) cachelines.

> +		/*
> +		 * Ideally we need to hold cgroup_mutex here
> +		 */
> +		list_for_each_entry_safe_from(cgroup, cgrp,
> +				&curr_cgroup->children, sibling) {
> +			struct mem_cgroup *mem_child;
> +
> +			mem_child = mem_cgroup_from_cont(cgroup);
> +			ret = try_to_free_mem_cgroup_pages(mem_child,
> +								gfp_mask);
> +			mem->last_scanned_child = mem_child;
> +			if (ret == 0)
> +				break;
> +		}

if i read it correctly, it makes us hit the last child again and again.

i think you want to reclaim from all cgroups under the curr_cgroup
including eg. children's children.

YAMAMOTO Takashi