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Subject: Re: [PATCH v2 11/11] mm,sched: conditionally skip lazy TLB mm refcounting
From:   Andy Lutomirski <luto@amacapital.net>
In-Reply-To: <20180731091248.GS2494@hirez.programming.kicks-ass.net>
Date:   Tue, 31 Jul 2018 07:29:26 -0700
Cc:     Rik van Riel <riel@surriel.com>, 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>
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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> <20180730162653.GM2494@hirez.programming.kicks-ass.net> <1532999155.28585.56.camel@surriel.com> <20180731091248.GS2494@hirez.programming.kicks-ass.net>
To:     Peter Zijlstra <peterz@infradead.org>
Sender: linux-kernel-owner@vger.kernel.org
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> 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?

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 ma=
ybe just task pointer parameters. M=