Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67;
Date:   Mon, 30 Jul 2018 18:26:53 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Rik van Riel <riel@surriel.com>
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>
Subject: Re: [PATCH v2 11/11] mm,sched: conditionally skip lazy TLB mm
 refcounting
Message-ID: <20180730162653.GM2494@hirez.programming.kicks-ass.net>
References: <20180728215357.3249-1-riel@surriel.com>
 <20180728215357.3249-11-riel@surriel.com>
 <CALCETrWS4pE5Tv3OErM3AD-E=CGG3U+2CBVecRF3N0c+g42vEg@mail.gmail.com>
 <20180729155452.37eddc11@imladris.surriel.com>
 <20180730095502.GG2494@hirez.programming.kicks-ass.net>
 <1532961011.28585.30.camel@surriel.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
Content-Transfer-Encoding: quoted-printable
In-Reply-To: <1532961011.28585.30.camel@surriel.com>
User-Agent: Mutt/1.10.0 (2018-05-17)
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

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.
>=20
> 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 =3D=3D ->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 =3D prev->active_mm;
> +               if (prev->mm)                           // from user
> +                       mmgrab(prev->active_mm);
		else
			prev->active_mm =3D 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 =3D prev->active_mm;
> +                       prev->active_mm =3D NULL;
> +               }
> +#endif