linux-kernel - Re: [PATCH 1/4] sched/fair: Fix cookie check on __select_idle

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Message-ID: <6e771747-ba73-4a6a-9982-05fea69136a4@linux.ibm.com>
Date: Thu, 25 Sep 2025 12:05:45 +0530
From: Madadi Vineeth Reddy <vineethr@...ux.ibm.com>
To: K Prateek Nayak <kprateek.nayak@....com>,
        Fernand Sieber <sieberf@...zon.com>
Cc: mingo@...hat.com, peterz@...radead.org, linux-kernel@...r.kernel.org,
        juri.lelli@...hat.com, vincent.guittot@...aro.org,
        dietmar.eggemann@....com, rostedt@...dmis.org, bsegall@...gle.com,
        mgorman@...e.de, bristot@...hat.com, vschneid@...hat.com,
        dwmw@...zon.co.uk, jschoenh@...zon.de, liuyuxua@...zon.com,
        Madadi Vineeth Reddy <vineethr@...ux.ibm.com>
Subject: Re: [PATCH 1/4] sched/fair: Fix cookie check on __select_idle_cpu()

Hi Prateek,

On 23/09/25 14:12, K Prateek Nayak wrote:
> Hello Fernand,
> 
> On 9/22/2025 6:09 PM, Fernand Sieber wrote:
>> @@ -7447,7 +7447,7 @@ static inline int sched_balance_find_dst_cpu(struct sched_domain *sd, struct tas
>>  static inline int __select_idle_cpu(int cpu, struct task_struct *p)
>>  {
>>  	if ((available_idle_cpu(cpu) || sched_idle_cpu(cpu)) &&
>> -	    sched_cpu_cookie_match(cpu_rq(cpu), p))
>> +	    sched_core_cookie_match(cpu_rq(cpu), p))
> 
> __select_idle_cpu() is only called when "has_idle_core" is false which
> means it is highly unlikely we'll find an idle core. In such cases, just
> matching the cookie should be sufficient right?

Agreed. The only code path I could find where __select_idle_cpu() is called
with has_idle_core == true is in the non-CONFIG_SCHED_SMT case, which is not
relevant to core scheduling.

Thanks,
Madadi Vineeth Reddy

> 
> Do you have any benchmark numbers which shows a large difference with
> these changes?
> 
>>  		return cpu;
>>  
>>  	return -1;
>> @@ -7546,6 +7546,9 @@ static int select_idle_smt(struct task_struct *p, struct sched_domain *sd, int t
>>  {
>>  	int cpu;
>>  
>> +	if (!sched_core_cookie_match(cpu_rq(target), p))
>> +		return -1;
>> +
> 
> select_idle_smt() is again called when "has_idle_core" is false and
> sched_cpu_cookie_match() should be sufficient for most part here too.
> 
>>  	for_each_cpu_and(cpu, cpu_smt_mask(target), p->cpus_ptr) {
>>  		if (cpu == target)
>>  			continue;
>