[Devel] Re: [PATCH v3 08/13] res_counter: return amount of charges after res_counter_uncharge

[Michal Hocko]

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 It could be out of date when you read it, right?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.

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.

> 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 at parallels.com>
> CC: Michal Hocko <mhocko at suse.cz>
> CC: Johannes Weiner <hannes at cmpxchg.org>
> CC: Suleiman Souhlal <suleiman at google.com>
> CC: Kamezawa Hiroyuki <kamezawa.hiroyu at jp.fujitsu.com>
> ---
>  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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-- 
Michal Hocko
SUSE Labs