linux-kernel - Re: [RFC v2 2/2] [MOCKUP] sched/mm: Lightweight lazy mm refcounting

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Message-Id: <D9715BFE-744E-49B4-A10B-32735123BE6D@amacapital.net>
Date:   Fri, 4 Dec 2020 06:37:04 -0800
From:   Andy Lutomirski <luto@...capital.net>
To:     Nicholas Piggin <npiggin@...il.com>
Cc:     Andy Lutomirski <luto@...nel.org>,
        Anton Blanchard <anton@...abs.org>,
        Arnd Bergmann <arnd@...db.de>,
        Catalin Marinas <catalin.marinas@....com>,
        Dave Hansen <dave.hansen@...el.com>,
        Jann Horn <jannh@...gle.com>,
        linux-arch <linux-arch@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Linux-MM <linux-mm@...ck.org>,
        linuxppc-dev <linuxppc-dev@...ts.ozlabs.org>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Nadav Amit <nadav.amit@...il.com>,
        Rik van Riel <riel@...riel.com>,
        Will Deacon <will@...nel.org>, X86 ML <x86@...nel.org>
Subject: Re: [RFC v2 2/2] [MOCKUP] sched/mm: Lightweight lazy mm refcounting



> On Dec 3, 2020, at 11:54 PM, Nicholas Piggin <npiggin@...il.com> wrote:
> 
> Excerpts from Andy Lutomirski's message of December 4, 2020 3:26 pm:
>> This is a mockup.  It's designed to illustrate the algorithm and how the
>> code might be structured.  There are several things blatantly wrong with
>> it:
>> 
>> The coding stype is not up to kernel standards.  I have prototypes in the
>> wrong places and other hacks.
>> 
>> There's a problem with mm_cpumask() not being reliable.
> 
> Interesting, this might be a way to reduce those IPIs with fairly 
> minimal fast path cost. Would be interesting to see how much performance 
> advantage it has over my dumb simple shoot-lazies.

My real motivation isn’t really performance per se. I think there’s considerable value in keeping the core algorithms the same across all architectures, and I think my approach can manage that with only a single hint from the architecture as to which CPUs to scan.

With shoot-lazies, in contrast, enabling it everywhere would either malfunction or have very poor performance or even DoS issues on arches like arm64 and s390x that don’t track mm_cpumask at all.  I’m sure we could come up with some way to mitigate that, but I think that my approach may be better overall for keeping the core code uniform and relatively straightforward.

> 
> For powerpc I don't think we'd be inclined to go that way, so don't feel 
> the need to add this complexity for us alone -- we'd be more inclined to 
> move the exit lazy to the final TLB shootdown path, which we're slowly 
> getting more infrastructure in place to do.
> 


> 
> There's a few nits but I don't think I can see a fundamental problem 
> yet.

Thanks!

I can polish the patch, but I want to be sure the memory ordering parts are clear.

> 
> Thanks,
> Nick