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

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Date:   Fri, 04 Dec 2020 17:54:40 +1000
From:   Nicholas Piggin <npiggin@...il.com>
To:     Andy Lutomirski <luto@...nel.org>
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>,
        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 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.

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.

(The powerpc hash MMU code which we're slowly moving away from might 
never get that capability because it's complex there and hard to do with
that virtualisation model so current big systems (and radix MMU until we
finish the TLB flushing stuff) want something here, but for those the
shoot-lazies could quite likely be sufficient)

But if core code was moved over to something like this for the benefit of
others archs we'd probably just as happily do that.

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

Thanks,
Nick