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

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Date:   Sat, 05 Dec 2020 14:49:04 +1000
From:   Nicholas Piggin <npiggin@...il.com>
To:     Andy Lutomirski <luto@...capital.net>
Cc:     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>,
        Andy Lutomirski <luto@...nel.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

Excerpts from Andy Lutomirski's message of December 5, 2020 12:37 am:
> 
> 
>> 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.

I'd go the other way. The mm_cpumark, TLB, and lazy maintainence is 
different between architectures anyway. I'd keep the simple refcount,
and the pretty simple shoot-lazies approaches for now at least until
a bit more is done on other fronts. If x86 is shooting down lazies on 
the final TLB flush as well, then I might be inclined to think that's
the better way to go in the long term. Shoot-lazies would be a bit of
a bolted on hack for powerpc/hash but it has ~zero impact to core code
really.

Thanks,
Nick