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

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Message-ID: <20201119115132.GN3121378@hirez.programming.kicks-ass.net>
Date:   Thu, 19 Nov 2020 12:51:32 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     LKML <linux-kernel@...r.kernel.org>, x86@...nel.org,
        Linus Torvalds <torvalds@...uxfoundation.org>,
        Christoph Hellwig <hch@....de>,
        Matthew Wilcox <willy@...radead.org>,
        Daniel Vetter <daniel@...ll.ch>,
        Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Ingo Molnar <mingo@...nel.org>,
        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>
Subject: Re: [patch V4 5/8] sched: highmem: Store local kmaps in task struct

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();
>  
>  	fire_sched_in_preempt_notifiers(current);
>  	/*

> +void __kmap_local_sched_out(void)
> +{
> +	struct task_struct *tsk = current;
> +	pte_t *kmap_pte = kmap_get_pte();
> +	int i;
> +
> +	/* Clear kmaps */
> +	for (i = 0; i < tsk->kmap_ctrl.idx; i++) {
> +	}
> +}
> +
> +void __kmap_local_sched_in(void)
> +{
> +	struct task_struct *tsk = current;
> +	pte_t *kmap_pte = kmap_get_pte();
> +	int i;
> +
> +	/* Restore kmaps */
> +	for (i = 0; i < tsk->kmap_ctrl.idx; i++) {
> +	}
> +}

So even in the optimal case, this adds an unconditional load of
tsk->kmap_ctrl.idx to schedule() (2 misses, one pre and one post).

Munging preempt-notifiers behind a static_branch, which in that same
optimal case, avoided touching curr->preempt_notifier, resulted in a
measurable performance improvement. See commit:

  1cde2930e154 ("sched/preempt: Add static_key() to preempt_notifiers")

Can we fudge some state in a cacheline we're already touching to avoid
this?