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

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Date:   Fri, 04 Dec 2020 08:13:40 +1000
From:   Nicholas Piggin <npiggin@...il.com>
To:     Andy Lutomirski <luto@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>
Cc:     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

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.

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