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

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Message-ID: <20180731151241.GF2458@hirez.programming.kicks-ass.net>
Date:   Tue, 31 Jul 2018 17:12:41 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Rik van Riel <riel@...riel.com>
Cc:     Andy Lutomirski <luto@...capital.net>,
        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 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@...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. 
> 
> Of all the architectures, only Alpha uses the "mm"
> parameter to enter_lazy_tlb.
> 
> It uses it to store the pgd address of the current
> active_mm context into the thread info of the next
> task.
> 
> 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 :-)

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