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:   Tue, 31 Jul 2018 17:12:41 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Rik van Riel <riel@surriel.com>
Cc:     Andy Lutomirski <luto@amacapital.net>,
        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: <20180731151241.GF2458@hirez.programming.kicks-ass.net>
References: <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>
 <20180730162653.GM2494@hirez.programming.kicks-ass.net>
 <1532999155.28585.56.camel@surriel.com>
 <20180731091248.GS2494@hirez.programming.kicks-ass.net>
 <C9664A02-AD53-4A6A-8A56-E729DC919E16@amacapital.net>
 <1533049383.28585.61.camel@surriel.com>
MIME-Version: 1.0
Content-Type: multipart/signed; micalg=pgp-sha512;
        protocol="application/pgp-signature"; boundary="GZVR6ND4mMseVXL/"
Content-Disposition: inline
In-Reply-To: <1533049383.28585.61.camel@surriel.com>
User-Agent: Mutt/1.10.0 (2018-05-17)
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk


--GZVR6ND4mMseVXL/
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
Content-Transfer-Encoding: quoted-printable

On Tue, Jul 31, 2018 at 11:03:03AM -0400, Rik van Riel wrote:
> On Tue, 2018-07-31 at 07:29 -0700, Andy Lutomirski wrote:
> > > On Jul 31, 2018, at 2:12 AM, Peter Zijlstra <peterz@infradead.org>
> > > wrote:
> > >=20
> > > > On Mon, Jul 30, 2018 at 09:05:55PM -0400, Rik van Riel wrote:
> > > > > 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.
> > > >=20
> > > > 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().
> > >=20
> > > I'm confused on the need for this. We mark the CPU lazy, why do we
> > > still
> > > care about this?
> >=20
> > I have considered renaming enter_lazy_tlb() to something like
> > lazy_switch_to_kernel_mm() (or an irqs_off variant) and making it
> > take no parameters or maybe just task pointer parameters.=20
>=20
> Of all the architectures, only Alpha uses the "mm"
> parameter to enter_lazy_tlb.
>=20
> It uses it to store the pgd address of the current
> active_mm context into the thread info of the next
> task.
>=20
> If it can get that from a per CPU variable, we can
> get rid of the mm parameter to enter_lazy_tlb.

We only really care for arches that select this new ARCH_NO_ACTIVE_MM
thingy. And x86 (the only one so far) really doesn't give a crap about
the argument.

We should just list/document that being one of the requirements for
selecting that symbol. All this will need semi decent documentation
anyway, otherwise the poor sod trying to understand this all in about 6
months time will go crazy :-)

--GZVR6ND4mMseVXL/
Content-Type: application/pgp-signature; name="signature.asc"

-----BEGIN PGP SIGNATURE-----
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=MnU6
-----END PGP SIGNATURE-----

--GZVR6ND4mMseVXL/--