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

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Date:   Mon, 30 Jul 2018 21:05:55 -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:
> 
> So for ARCH_NO_ACTIVE_MM we never touch ->active_mm and therefore
> ->active_mm == ->mm.

Close, but not true for kernel threads, which have a 
NULL ->mm, but a non-null ->active_mm that gets passed
to enter_lazy_tlb().

I stuck to the structure of your code, but ended up
removing all the ifdefs because the final mmdrop requires
that we actually keep track of the ->active_mm across
potentially several kernel->kernel context switches.

Ifdefs around the reference counting code are also not
needed, because grab_lazy_mm and drop_lazy_mm already
contain the equivalent of an ifdef themselves.

By morning I should know what the test results look
like. I expect they will be identical to my patches,
since the refcounting is disabled completely anyway :)

> > +       /*
> > +        * 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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