Re: [patch V4 5/8] sched: highmem: Store local kmaps in task struct

[Peter Zijlstra <peterz@infradead.org>]

On Wed, Nov 18, 2020 at 08:48:43PM +0100, Thomas Gleixner wrote:
> @@ -4073,6 +4089,7 @@ prepare_task_switch(struct rq *rq, struc
>  	perf_event_task_sched_out(prev, next);
>  	rseq_preempt(prev);
>  	fire_sched_out_preempt_notifiers(prev, next);
> +	kmap_local_sched_out();
>  	prepare_task(next);
>  	prepare_arch_switch(next);
>  }
> @@ -4139,6 +4156,7 @@ static struct rq *finish_task_switch(str
>  	finish_lock_switch(rq);
>  	finish_arch_post_lock_switch();
>  	kcov_finish_switch(current);
> +	kmap_local_sched_in();

This is asymmetric and deserves a comment. You do the sched_out with
IRQs disabled and rq->lock held, but do the sched_in with IRQs enabled
and rq->lock released.

I suppose doing it here reduces IRQ latency by however long it takes to
update and invalidate that handful of pages, is that worth the
asymmetry?

It mirrors preempt_notifiers I suppose, and they actually rely on this
asymmetry for something IIRC.

>  	fire_sched_in_preempt_notifiers(current);
>  	/*