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

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Message-Id: <C9664A02-AD53-4A6A-8A56-E729DC919E16@amacapital.net>
Date:   Tue, 31 Jul 2018 07:29:26 -0700
From:   Andy Lutomirski <luto@...capital.net>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Rik van Riel <riel@...riel.com>, 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 Jul 31, 2018, at 2:12 AM, Peter Zijlstra <peterz@...radead.org> wrote:
> 
>> 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:
>>> 
>>> 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'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 maybe just task pointer parameters. M