linux-kernel - Re: [PATCH 6/7] sched/fair: Remove on_null

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Date: Fri, 5 Apr 2024 11:48:07 +0200
From: Pierre Gondois <pierre.gondois@....com>
To: Peter Zijlstra <peterz@...radead.org>
Cc: linux-kernel@...r.kernel.org, Aaron Lu <aaron.lu@...el.com>,
 Rui Zhang <rui.zhang@...el.com>,
 Anna-Maria Behnsen <anna-maria@...utronix.de>,
 Frederic Weisbecker <frederic@...nel.org>, Ingo Molnar <mingo@...nel.org>,
 Thomas Gleixner <tglx@...utronix.de>, Juri Lelli <juri.lelli@...hat.com>,
 Vincent Guittot <vincent.guittot@...aro.org>,
 Dietmar Eggemann <dietmar.eggemann@....com>,
 Steven Rostedt <rostedt@...dmis.org>, Ben Segall <bsegall@...gle.com>,
 Mel Gorman <mgorman@...e.de>, Daniel Bristot de Oliveira
 <bristot@...hat.com>, Valentin Schneider <vschneid@...hat.com>,
 Waiman Long <longman@...hat.com>, Andrew Morton <akpm@...ux-foundation.org>,
 Michal Hocko <mhocko@...e.com>
Subject: Re: [PATCH 6/7] sched/fair: Remove on_null_domain() and redundant
 checks

Hello Peter,

On 4/4/24 09:27, Peter Zijlstra wrote:
> On Wed, Apr 03, 2024 at 05:05:38PM +0200, Pierre Gondois wrote:
>> CPUs with a NULL sched domain are removed from the HKR_TYPE_SCHED
>> isolation mask. The two following checks are equialent:
>> - !housekeeping_runtime_test_cpu(cpu, HKR_TYPE_SCHED)
>> - on_null_domain(rq)
>>
>> Remove on_null_domain() and the redundant checks.
>>
>> Signed-off-by: Pierre Gondois <pierre.gondois@....com>
>> ---
>>   kernel/sched/fair.c | 18 ++++++------------
>>   1 file changed, 6 insertions(+), 12 deletions(-)
>>
>> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
>> index 3e0f2a0f153f..9657c8f2176b 100644
>> --- a/kernel/sched/fair.c
>> +++ b/kernel/sched/fair.c
>> @@ -11830,11 +11830,6 @@ static void sched_balance_domains(struct rq *rq, enum cpu_idle_type idle)
>>   
>>   }
>>   
>> -static inline int on_null_domain(struct rq *rq)
>> -{
>> -	return unlikely(!rcu_dereference_sched(rq->sd));
>> -}
>> -
>>   #ifdef CONFIG_NO_HZ_COMMON
>>   /*
>>    * NOHZ idle load balancing (ILB) details:
>> @@ -12040,7 +12035,7 @@ void nohz_balance_exit_idle(struct rq *rq)
>>   	SCHED_WARN_ON(rq != this_rq());
>>   
>>   	/* If we're a completely isolated CPU, we don't play: */
>> -	if (on_null_domain(rq))
>> +	if (!housekeeping_runtime_test_cpu(cpu_of(rq), HKR_TYPE_SCHED))
>>   		return;
>>   
>>   	if (likely(!rq->nohz_tick_stopped))
> 
> This seems broken, the whole null domain can happen with cpusets, but
> this housekeeping nonsense is predicated on CPU_ISOLATION and none of
> that is mandatory for CPUSETS.

ok right,
I will try to remove this implicit dependency,

Regards,
Pierre