Re: [PATCH v3 08/13] res_counter: return amount of charges afterres_counter_uncharge

[Glauber Costa]

On 10/01/2012 02:00 PM, Michal Hocko wrote:
> On Tue 18-09-12 18:04:05, Glauber Costa wrote:
>> It is useful to know how many charges are still left after a call to
>> res_counter_uncharge. 
> 
>> While it is possible to issue a res_counter_read
>> after uncharge, this is racy. It would be better if uncharge itself
>> would tell us what the current status is.
> 
> Well I am not sure how less racy it would be if you return the old
> value. It could be out of date when you read it, right? (this is even
> more visible with res_counter_uncharge_until)
> res_counter_read_u64 uses locks only for 32b when your change could help
> to reduce lock contention. Other than that it is just res_counter_member
> which is one cmp and a dereference. Sure you safe something but it is
> barely noticable I guess.
> 

Sure it can. But this is the same semantics of the atomic updates, which
were always considered to be good enough for cases like this.


> I am not saying I do not like this change I just think that the
> above part of the changelog doesn't fit. So it would be much better if
> you tell us why this is needed for your patchset because the usage is
> not part of the patch.
> 

I can update the changelog, for sure. But for the record, this has the
goal of taking the get/put pair out of the charge/uncharge path.

If I have 8k of data, and two threads decrement 4k each, a update + read
may return 0 for both. With this patch, only one of them will see 0, and
will proceed with the reference drop.

Again, this is the same semantics as all of the atomic variables in the
kernel.


>> Since the current return value is void, we don't need to worry about
>> anything breaking due to this change: nobody relied on that, and only
>> users appearing from now on will be checking this value.
>>
>> Signed-off-by: Glauber Costa <glommer@xxxxxxxxxxxxx>
>> CC: Michal Hocko <mhocko@xxxxxxx>
>> CC: Johannes Weiner <hannes@xxxxxxxxxxx>
>> CC: Suleiman Souhlal <suleiman@xxxxxxxxxx>
>> CC: Kamezawa Hiroyuki <kamezawa.hiroyu@xxxxxxxxxxxxxx>
>> ---
>>  Documentation/cgroups/resource_counter.txt |  7 ++++---
>>  include/linux/res_counter.h                | 12 +++++++-----
>>  kernel/res_counter.c                       | 20 +++++++++++++-------
>>  3 files changed, 24 insertions(+), 15 deletions(-)
>>
>> diff --git a/Documentation/cgroups/resource_counter.txt b/Documentation/cgroups/resource_counter.txt
>> index 0c4a344..c4d99ed 100644
>> --- a/Documentation/cgroups/resource_counter.txt
>> +++ b/Documentation/cgroups/resource_counter.txt
>> @@ -83,16 +83,17 @@ to work with it.
>>  	res_counter->lock internally (it must be called with res_counter->lock
>>  	held). The force parameter indicates whether we can bypass the limit.
>>  
>> - e. void res_counter_uncharge[_locked]
>> + e. u64 res_counter_uncharge[_locked]
>>  			(struct res_counter *rc, unsigned long val)
>>  
>>  	When a resource is released (freed) it should be de-accounted
>>  	from the resource counter it was accounted to.  This is called
>> -	"uncharging".
>> +	"uncharging". The return value of this function indicate the amount
>> +	of charges still present in the counter.
>>  
>>  	The _locked routines imply that the res_counter->lock is taken.
>>  
>> - f. void res_counter_uncharge_until
>> + f. u64 res_counter_uncharge_until
>>  		(struct res_counter *rc, struct res_counter *top,
>>  		 unsinged long val)
>>  
>> diff --git a/include/linux/res_counter.h b/include/linux/res_counter.h
>> index 7d7fbe2..4b173b6 100644
>> --- a/include/linux/res_counter.h
>> +++ b/include/linux/res_counter.h
>> @@ -130,14 +130,16 @@ int res_counter_charge_nofail(struct res_counter *counter,
>>   *
>>   * these calls check for usage underflow and show a warning on the console
>>   * _locked call expects the counter->lock to be taken
>> + *
>> + * returns the total charges still present in @counter.
>>   */
>>  
>> -void res_counter_uncharge_locked(struct res_counter *counter, unsigned long val);
>> -void res_counter_uncharge(struct res_counter *counter, unsigned long val);
>> +u64 res_counter_uncharge_locked(struct res_counter *counter, unsigned long val);
>> +u64 res_counter_uncharge(struct res_counter *counter, unsigned long val);
>>  
>> -void res_counter_uncharge_until(struct res_counter *counter,
>> -				struct res_counter *top,
>> -				unsigned long val);
>> +u64 res_counter_uncharge_until(struct res_counter *counter,
>> +			       struct res_counter *top,
>> +			       unsigned long val);
>>  /**
>>   * res_counter_margin - calculate chargeable space of a counter
>>   * @cnt: the counter
>> diff --git a/kernel/res_counter.c b/kernel/res_counter.c
>> index ad581aa..7b3d6dc 100644
>> --- a/kernel/res_counter.c
>> +++ b/kernel/res_counter.c
>> @@ -86,33 +86,39 @@ int res_counter_charge_nofail(struct res_counter *counter, unsigned long val,
>>  	return __res_counter_charge(counter, val, limit_fail_at, true);
>>  }
>>  
>> -void res_counter_uncharge_locked(struct res_counter *counter, unsigned long val)
>> +u64 res_counter_uncharge_locked(struct res_counter *counter, unsigned long val)
>>  {
>>  	if (WARN_ON(counter->usage < val))
>>  		val = counter->usage;
>>  
>>  	counter->usage -= val;
>> +	return counter->usage;
>>  }
>>  
>> -void res_counter_uncharge_until(struct res_counter *counter,
>> -				struct res_counter *top,
>> -				unsigned long val)
>> +u64 res_counter_uncharge_until(struct res_counter *counter,
>> +			       struct res_counter *top,
>> +			       unsigned long val)
>>  {
>>  	unsigned long flags;
>>  	struct res_counter *c;
>> +	u64 ret = 0;
>>  
>>  	local_irq_save(flags);
>>  	for (c = counter; c != top; c = c->parent) {
>> +		u64 r;
>>  		spin_lock(&c->lock);
>> -		res_counter_uncharge_locked(c, val);
>> +		r = res_counter_uncharge_locked(c, val);
>> +		if (c == counter)
>> +			ret = r;
>>  		spin_unlock(&c->lock);
>>  	}
>>  	local_irq_restore(flags);
>> +	return ret;
>>  }
>>  
>> -void res_counter_uncharge(struct res_counter *counter, unsigned long val)
>> +u64 res_counter_uncharge(struct res_counter *counter, unsigned long val)
>>  {
>> -	res_counter_uncharge_until(counter, NULL, val);
>> +	return res_counter_uncharge_until(counter, NULL, val);
>>  }
>>  
>>  static inline unsigned long long *
>> -- 
>> 1.7.11.4
>>
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