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

[Balbir Singh]

YAMAMOTO Takashi wrote:
>> -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.
> 

Yes, it does. I'll run some tests to see what the overhead looks like. The
multi-hierarchy feature is very useful though and one of the TODOs is to make
the feature user selectable (possibly at run-time)

>> +		/*
>> +		 * 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.
> 

Hmm.. it should probably be set at the beginining of the loop. I'll retest


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

Yes, good point, I should break out the function, so that we can work around the
recursion problem. Charging can cause further recursion, since we check for
last_counter.

> YAMAMOTO Takashi


-- 
	Warm Regards,
	Balbir Singh
	Linux Technology Center
	IBM, ISTL
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