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

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Date:   Thu, 19 Nov 2020 15:11:29 +0100
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Peter Zijlstra <peterz@...radead.org>
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 Thu, Nov 19 2020 at 13:12, Peter Zijlstra wrote:
> On Thu, Nov 19, 2020 at 12:51:32PM +0100, Peter Zijlstra wrote:
>> > +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?
>
> The only state we seem to consistently look at after schedule() is
> need_resched()'s TIF_NEED_RESCHED.
>
> But adding a TIF_flag to all archs and setting/clearing it from
> kmap_local might be a bit daft.. :/

Yes, and we all agreed that pushing the burden on CONFIG_HIGHMEM is
fine. It's slow, so making it a tad slower is not the end of the
world. Remember we want to get rid of it :)

Thanks,

        tglx