linux-kernel - Re: [MOCKUP] x86/mm: Lightweight lazy mm refcounting

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Message-Id: <3B47C470-2900-4A53-9F8E-CB3A003FA361@amacapital.net>
Date:   Thu, 3 Dec 2020 18:17:59 -0800
From:   Andy Lutomirski <luto@...capital.net>
To:     Nicholas Piggin <npiggin@...il.com>
Cc:     Andy Lutomirski <luto@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Anton Blanchard <anton@...abs.org>,
        Arnd Bergmann <arnd@...db.de>,
        Catalin Marinas <catalin.marinas@....com>,
        Dave Hansen <dave.hansen@...el.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>,
        Rik van Riel <riel@...riel.com>,
        Will Deacon <will@...nel.org>, X86 ML <x86@...nel.org>
Subject: Re: [MOCKUP] x86/mm: Lightweight lazy mm refcounting


> On Dec 3, 2020, at 2:13 PM, Nicholas Piggin <npiggin@...il.com> wrote:
> 
> Excerpts from Peter Zijlstra's message of December 3, 2020 6:44 pm:
>>> On Wed, Dec 02, 2020 at 09:25:51PM -0800, Andy Lutomirski wrote:
>>> 
>>> power: same as ARM, except that the loop may be rather larger since
>>> the systems are bigger.  But I imagine it's still faster than Nick's
>>> approach -- a cmpxchg to a remote cacheline should still be faster than
>>> an IPI shootdown. 
>> 
>> While a single atomic might be cheaper than an IPI, the comparison
>> doesn't work out nicely. You do the xchg() on every unlazy, while the
>> IPI would be once per process exit.
>> 
>> So over the life of the process, it might do very many unlazies, adding
>> up to a total cost far in excess of what the single IPI would've been.
> 
> Yeah this is the concern, I looked at things that add cost to the
> idle switch code and it gets hard to justify the scalability improvement
> when you slow these fundmaental things down even a bit.

v2 fixes this and is generally much nicer. I’ll send it out in a couple hours.

> 
> I still think working on the assumption that IPIs = scary expensive 
> might not be correct. An IPI itself is, but you only issue them when 
> you've left a lazy mm on another CPU which just isn't that often.
> 
> Thanks,
> Nick