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From: Peter Zijlstra <peterz@infradead.org>
To: Thomas Gleixner <tglx@linutronix.de>
Cc: LKML <linux-kernel@vger.kernel.org>,
	x86@kernel.org, Linus Torvalds <torvalds@linuxfoundation.org>,
	Christoph Hellwig <hch@lst.de>,
	Matthew Wilcox <willy@infradead.org>,
	Daniel Vetter <daniel@ffwll.ch>,
	Andrew Morton <akpm@linux-foundation.org>,
	linux-mm@kvack.org,
	Linus Torvalds <torvalds@linux-foundation.org>,
	Ingo Molnar <mingo@kernel.org>,
	Juri Lelli <juri.lelli@redhat.com>,
	Vincent Guittot <vincent.guittot@linaro.org>,
	Dietmar Eggemann <dietmar.eggemann@arm.com>,
	Steven Rostedt <rostedt@goodmis.org>,
	Ben Segall <bsegall@google.com>, Mel Gorman <mgorman@suse.de>,
	Daniel Bristot de Oliveira <bristot@redhat.com>
Subject: Re: [patch V4 5/8] sched: highmem: Store local kmaps in task struct
Date: Thu, 19 Nov 2020 12:51:32 +0100	[thread overview]
Message-ID: <20201119115132.GN3121378@hirez.programming.kicks-ass.net> (raw)
In-Reply-To: <20201118204007.372935758@linutronix.de>

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?

  parent reply	other threads:[~2020-11-19 11:52 UTC|newest]

Thread overview: 48+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-11-18 19:48 [patch V4 0/8] mm/highmem: Preemptible variant of kmap_atomic & friends Thomas Gleixner
2020-11-18 19:48 ` [patch V4 1/8] mm/highmem: Provide and use CONFIG_DEBUG_KMAP_LOCAL Thomas Gleixner
2020-11-24 14:20   ` [tip: core/mm] " tip-bot2 for Thomas Gleixner
2020-11-18 19:48 ` [patch V4 2/8] mm/highmem: Provide CONFIG_DEBUG_KMAP_LOCAL_FORCE_MAP Thomas Gleixner
2020-11-18 21:13   ` Linus Torvalds
2020-11-19  8:46     ` Mel Gorman
2020-11-19 17:19       ` Linus Torvalds
2020-11-24 14:20   ` [tip: core/mm] " tip-bot2 for Thomas Gleixner
2020-11-18 19:48 ` [patch V4 3/8] x86: Support kmap_local() forced debugging Thomas Gleixner
2020-11-24 14:20   ` [tip: core/mm] " tip-bot2 for Thomas Gleixner
2021-01-06 23:01   ` [BUG] from " Steven Rostedt
2021-01-07  1:03     ` Linus Torvalds
2021-01-07  1:16       ` Steven Rostedt
2021-01-07  1:49       ` Steven Rostedt
2021-01-07  1:49       ` Jakub Kicinski
2021-01-07  2:11         ` Willem de Bruijn
2021-01-07  4:44           ` Willem de Bruijn
2021-01-07 19:47             ` Linus Torvalds
2021-01-07 20:52               ` Steven Rostedt
2021-01-07 21:07                 ` Willem de Bruijn
2020-11-18 19:48 ` [patch V4 4/8] sched: Make migrate_disable/enable() independent of RT Thomas Gleixner
2020-11-19  9:38   ` Mel Gorman
2020-11-19 11:14     ` Peter Zijlstra
2020-11-19 12:14       ` Mel Gorman
2020-11-19 14:17         ` Steven Rostedt
2020-11-19 17:23       ` Linus Torvalds
2020-11-19 18:28         ` Peter Zijlstra
2020-11-20  1:33           ` Thomas Gleixner
2020-11-20  9:29             ` Peter Zijlstra
2020-11-22 23:16               ` Andy Lutomirski
2020-11-23 21:15                 ` Thomas Gleixner
2020-11-23 21:25                   ` Thomas Gleixner
2020-11-23 22:07                     ` Andy Lutomirski
2020-11-23 23:10                       ` Thomas Gleixner
2020-11-24 10:29   ` [tip: sched/core] " tip-bot2 for Thomas Gleixner
2020-11-18 19:48 ` [patch V4 5/8] sched: highmem: Store local kmaps in task struct Thomas Gleixner
2020-11-19 11:33   ` Peter Zijlstra
2020-11-19 11:51   ` Peter Zijlstra [this message]
2020-11-19 12:12     ` Peter Zijlstra
2020-11-19 14:11       ` Thomas Gleixner
2020-11-24 14:20   ` [tip: core/mm] " tip-bot2 for Thomas Gleixner
2020-11-18 19:48 ` [patch V4 6/8] mm/highmem: Provide kmap_local* Thomas Gleixner
2020-11-24 14:20   ` [tip: core/mm] " tip-bot2 for Thomas Gleixner
2020-11-18 19:48 ` [patch V4 7/8] io-mapping: Provide iomap_local variant Thomas Gleixner
2020-11-24 14:20   ` [tip: core/mm] " tip-bot2 for Thomas Gleixner
2020-11-18 19:48 ` [patch V4 8/8] x86/crashdump/32: Simplify copy_oldmem_page() Thomas Gleixner
2020-11-24 14:20   ` [tip: core/mm] " tip-bot2 for Thomas Gleixner
2020-11-24  8:03 ` [patch V4 0/8] mm/highmem: Preemptible variant of kmap_atomic & friends Peter Zijlstra

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