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

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Message-ID: <20180730162653.GM2494@hirez.programming.kicks-ass.net>
Date:   Mon, 30 Jul 2018 18:26:53 +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 Mon, Jul 30, 2018 at 10:30:11AM -0400, Rik van Riel wrote:

> > 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 don't think I saw that. I only saw your email from
> July 20th with this fragment of code, which does not
> appear to avoid the grab/drop cycles, and still fiddles
> with active_mm:

Yeah, that's it. Note how it doesn't do a grab+drop for kernel->kernel,
where the current could would have.

And also note that it only fiddles with active_mm if it does the
grab+drop thing (the below should have s/ifdef/ifndef/ to make more
sense maybe).

So for ARCH_NO_ACTIVE_MM we never touch ->active_mm and therefore
->active_mm == ->mm.

> +       /*
> +        * kernel -> kernel   lazy + transfer active
> +        *   user -> kernel   lazy + mmgrab() active
> +        *
> +        * kernel ->   user   switch + mmdrop() active
> +        *   user ->   user   switch
> +        */
> +       if (!next->mm) {                                // to kernel
> +               enter_lazy_tlb(prev->active_mm, next);
> +
#ifndef ARCH_NO_ACTIVE_MM
> +               next->active_mm = prev->active_mm;
> +               if (prev->mm)                           // from user
> +                       mmgrab(prev->active_mm);
		else
			prev->active_mm = NULL;
> +#endif
> +       } else {                                        // to user
> +               switch_mm_irqs_off(prev->active_mm, next->mm, next);
> +
#ifndef ARCH_NO_ACTIVE_MM
> +               if (!prev->mm) {                        // from kernel
> +                       /* will mmdrop() in finish_task_switch(). */
> +                       rq->prev_mm = prev->active_mm;
> +                       prev->active_mm = NULL;
> +               }
> +#endif