linux-kernel - Re: [PATCH] x86,switch_mm: skip atomic operations for init

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]

Message-ID: <1527885324.7898.88.camel@surriel.com>
Date:   Fri, 01 Jun 2018 16:35:24 -0400
From:   Rik van Riel <riel@...riel.com>
To:     Andy Lutomirski <luto@...nel.org>
Cc:     Mike Galbraith <efault@....de>,
        LKML <linux-kernel@...r.kernel.org>, songliubraving@...com,
        kernel-team <kernel-team@...com>, Ingo Molnar <mingo@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>, X86 ML <x86@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH] x86,switch_mm: skip atomic operations for init_mm

On Fri, 2018-06-01 at 13:03 -0700, Andy Lutomirski wrote:
> Mike, you never did say: do you have PCID on your CPU?  Also, what is
> your workload doing to cause so many switches back and forth between
> init_mm and a task.
> 
> The point of the optimization is that switching to init_mm() should
> be
> fairly fast on a PCID system, whereas an IPI to do the deferred flush
> is very expensive regardless of PCID. 

While I am sure that bit is true, Song and I
observed about 4x as much CPU use in the atomic
operations in cpumask_clear_cpu and cpumask_set_cpu
(inside switch_mm_irqs_off) as we saw CPU used
in the %cr3 reload itself.

Given how expensive those cpumask updates are,
lazy TLB mode might always be worth it, especially
on larger systems.

-- 
All Rights Reversed.
Download attachment "signature.asc" of type "application/pgp-signature" (489 bytes)