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Message-ID: <1532999155.28585.56.camel@surriel.com>
Subject: Re: [PATCH v2 11/11] mm,sched: conditionally skip lazy TLB mm
 refcounting
From:   Rik van Riel <riel@surriel.com>
To:     Peter Zijlstra <peterz@infradead.org>
Cc:     Andy Lutomirski <luto@kernel.org>,
        LKML <linux-kernel@vger.kernel.org>,
        kernel-team <kernel-team@fb.com>, X86 ML <x86@kernel.org>,
        Vitaly Kuznetsov <vkuznets@redhat.com>,
        Ingo Molnar <mingo@kernel.org>, Mike Galbraith <efault@gmx.de>,
        Dave Hansen <dave.hansen@intel.com>,
        Catalin Marinas <catalin.marinas@arm.com>,
        Benjamin Herrenschmidt <benh@kernel.crashing.org>
Date:   Mon, 30 Jul 2018 21:05:55 -0400
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         <20180730095502.GG2494@hirez.programming.kicks-ass.net>
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On Mon, 2018-07-30 at 18:26 +0200, Peter Zijlstra wrote:
>=20
> So for ARCH_NO_ACTIVE_MM we never touch ->active_mm and therefore
> ->active_mm =3D=3D ->mm.

Close, but not true for kernel threads, which have a=20
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);
> > +
>=20
> #ifndef ARCH_NO_ACTIVE_MM
> > +               next->active_mm =3D prev->active_mm;
> > +               if (prev->mm)                           // from
> > user
> > +                       mmgrab(prev->active_mm);
>=20
> 		else
> 			prev->active_mm =3D NULL;
> > +#endif
> > +       } else {                                        // to user
> > +               switch_mm_irqs_off(prev->active_mm, next->mm,
> > next);
> > +
>=20
> #ifndef ARCH_NO_ACTIVE_MM
> > +               if (!prev->mm) {                        // from
> > kernel
> > +                       /* will mmdrop() in finish_task_switch().
> > */
> > +                       rq->prev_mm =3D prev->active_mm;
> > +                       prev->active_mm =3D NULL;
> > +               }
> > +#endif
>=20
>=20
--=20
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