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

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Message-ID: <1532978146.28585.32.camel@surriel.com>
Date:   Mon, 30 Jul 2018 15:15:46 -0400
From:   Rik van Riel <riel@...riel.com>
To:     Peter Zijlstra <peterz@...radead.org>
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, 2018-07-30 at 18:26 +0200, Peter Zijlstra wrote:
> 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).

I'll kick off a test with your variant. I don't think we
will see any performance difference on x86 (due to not
using a refcount at all any more), but unless Ingo is in
a hurry I guess there's no issue rewriting this part of
the patch series :)

Do the other patches look ok to you and Andy?


> 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
> 
> 
> 
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