Re: [PATCH 3/3] block: skip blkcg walk in blk_cgroup_congested() when nothing throttled

[Usama Arif <usama.arif@linux.dev>]

On 10/08/2026 19:56, Tejun Heo wrote:
> Hello,
> 
> On Thu, Aug 06, 2026 at 11:37:20AM -0700, Usama Arif wrote:
>> @@ -53,6 +53,9 @@ EXPORT_SYMBOL_GPL(blkcg_root);
>>  struct cgroup_subsys_state * const blkcg_root_css = &blkcg_root.css;
>>  EXPORT_SYMBOL_GPL(blkcg_root_css);
>>  
>> +/* number of blkcgs with a non-zero congestion_count */
>> +atomic_t blkcg_congested_blkcgs __read_mostly = ATOMIC_INIT(0);
> 
> Maybe name it to clarify that this is a count. nr_ or _cnt or whatever.
> 
>> @@ -1350,6 +1353,16 @@ static void blkcg_css_free(struct cgroup_subsys_state *css)
>>  	struct blkcg *blkcg = css_to_blkcg(css);
>>  	int i;
>>  
>> +	/*
>> +	 * Every blkg holds a reference on this css and drops any delay it
>> +	 * still has from pd_free_fn(), so this is expected to be zero.  Should
>> +	 * a policy ever leave one behind, drop it here rather than let it pin
>> +	 * blkcg_congested_blkcgs and disable the fast path for the rest of the
>> +	 * boot.  Nothing can race with us at this point.
>> +	 */
>> +	if (atomic_xchg(&blkcg->congestion_count, 0) > 0)
>> +		atomic_dec(&blkcg_congested_blkcgs);
> 
> Maybe put WARN_ON_ONCE() on it?
> 
>> +static inline void blkcg_dec_congestion_count(struct blkcg *blkcg)
>> +{
>> +	if (atomic_dec_return(&blkcg->congestion_count) == 0)
>> +		atomic_dec(&blkcg_congested_blkcgs);
> 
> atomic_dec_and_test() is minutely more efficient.
> 
> Thanks.
> 

Thanks! Ack on all 3. I will wait a day incase there are more comments
and send a new revision tomorrow with all 3 addressed.