linux-kernel - Re: [RFC PATCH] sched: Fix performance regression introduced by mm

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Date:   Tue, 4 Apr 2023 09:24:54 -0400
From:   Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     linux-kernel@...r.kernel.org, Aaron Lu <aaron.lu@...el.com>
Subject: Re: [RFC PATCH] sched: Fix performance regression introduced by
 mm_cid

On 2023-04-04 05:05, Peter Zijlstra wrote:
> On Mon, Apr 03, 2023 at 02:13:42PM -0400, Mathieu Desnoyers wrote:
>>   void sched_mm_cid_exit_signals(struct task_struct *t)
>>   {
>>   	struct mm_struct *mm = t->mm;
>> -	unsigned long flags;
>> +	struct rq_flags rf;
>> +	struct rq *rq;
>>   
>>   	if (!mm)
>>   		return;
>> -	local_irq_save(flags);
>> +
>> +	preempt_disable();
>> +	rq = this_rq();
>> +	rq_lock_irqsave(rq, &rf);
>>   	mm_cid_put(mm, t->mm_cid);
>> -	t->mm_cid = -1;
>> +	t->last_mm_cid = t->mm_cid = -1;
>>   	t->mm_cid_active = 0;
>> -	local_irq_restore(flags);
>> +	rq_unlock_irqrestore(rq, &rf);
>> +	preempt_enable();
>>   }
> 
> FWIW a *slightly* cheaper form is:
> 
> 	preempt_disable();
> 	rq = this_rq();
> 	rq_lock_irqsave(rq, &rf);
> 	preempt_enable_noresched(); /* holding spinlock */
> 	...
> 	rq_unlock_irqrestore(rq, &rf);

Good point, updated for next round.

Thanks,

Mathieu



-- 
Mathieu Desnoyers
EfficiOS Inc.
https://www.efficios.com