linux-kernel - Re: [PATCH v2 11/11] mm,sched: conditionally skip lazy TLB mm refcounting

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Date:   Mon, 30 Jul 2018 11:55:02 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Rik van Riel <riel@...riel.com>
Cc:     Andy Lutomirski <luto@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        kernel-team <kernel-team@...com>, X86 ML <x86@...nel.org>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Ingo Molnar <mingo@...nel.org>, Mike Galbraith <efault@....de>,
        Dave Hansen <dave.hansen@...el.com>,
        Catalin Marinas <catalin.marinas@....com>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>
Subject: Re: [PATCH v2 11/11] mm,sched: conditionally skip lazy TLB mm
 refcounting

On Sun, Jul 29, 2018 at 03:54:52PM -0400, Rik van Riel wrote:
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index c45de46fdf10..11724c9e88b0 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -2691,7 +2691,7 @@ static struct rq *finish_task_switch(struct task_struct *prev)
>  	 */
>  	if (mm) {
>  		membarrier_mm_sync_core_before_usermode(mm);
> -		mmdrop(mm);
> +		drop_lazy_mm(mm);
>  	}
>  	if (unlikely(prev_state == TASK_DEAD)) {
>  		if (prev->sched_class->task_dead)
> @@ -2805,7 +2805,7 @@ context_switch(struct rq *rq, struct task_struct *prev,
>  	 */
>  	if (!mm) {
>  		next->active_mm = oldmm;
> -		mmgrab(oldmm);
> +		grab_lazy_mm(oldmm);
>  		enter_lazy_tlb(oldmm, next);
>  	} else
>  		switch_mm_irqs_off(oldmm, mm, next);

What happened to the rework I did there? That not only avoided fiddling
with active_mm, but also avoids grab/drop cycles for the other
architectures when doing task->kthread->kthread->task things.

I agree with Andy that if you avoid the refcount fiddling, then you
should also not muck with active_mm.

That is, if you keep active_mm for now (which seems a reasonable first
step) then at least ensure you keep ->mm == ->active_mm at all times.