Re: [RFC PATCH] sched: Fix performance regression introduced by mm_cid

[Peter Zijlstra <peterz@infradead.org>]

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);