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

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Message-ID: <20180731091248.GS2494@hirez.programming.kicks-ass.net>
Date:   Tue, 31 Jul 2018 11:12:48 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Rik van Riel <riel@...riel.com>
Cc:     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 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?

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