linux-kernel - Re: [RFC PATCH 1/2] x86/ibpb: Skip IBPB when we switch back to same user process

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Date:   Thu, 25 Jan 2018 10:22:33 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Tim Chen <tim.c.chen@...ux.intel.com>
Cc:     linux-kernel@...r.kernel.org,
        KarimAllah Ahmed <karahmed@...zon.de>,
        Andi Kleen <ak@...ux.intel.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Andy Lutomirski <luto@...nel.org>,
        Arjan van de Ven <arjan@...ux.intel.com>,
        Ashok Raj <ashok.raj@...el.com>,
        Asit Mallick <asit.k.mallick@...el.com>,
        Borislav Petkov <bp@...e.de>,
        Dan Williams <dan.j.williams@...el.com>,
        Dave Hansen <dave.hansen@...el.com>,
        David Woodhouse <dwmw@...zon.co.uk>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "H . Peter Anvin" <hpa@...or.com>, Ingo Molnar <mingo@...hat.com>,
        Janakarajan Natarajan <Janakarajan.Natarajan@....com>,
        Joerg Roedel <joro@...tes.org>,
        Jun Nakajima <jun.nakajima@...el.com>,
        Laura Abbott <labbott@...hat.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Paolo Bonzini <pbonzini@...hat.com>, rkrcmar@...hat.com,
        Thomas Gleixner <tglx@...utronix.de>,
        Tom Lendacky <thomas.lendacky@....com>, x86@...nel.org
Subject: Re: [RFC PATCH 1/2] x86/ibpb: Skip IBPB when we switch back to same
 user process

On Thu, Jan 25, 2018 at 09:58:20AM +0100, Peter Zijlstra wrote:
> On Wed, Jan 24, 2018 at 04:36:41PM -0800, Tim Chen wrote:
> > These two patches provide optimization to skip IBPB for this
> > commonly encountered scenario:
> > We could switch to a kernel idle thread and then back to the original
> > process such as:
> > process A -> idle -> process A
> > 
> > In such scenario, we do not have to do IBPB here even though the process
> > is non-dumpable, as we are switching back to the same process after
> > an hiatus.
> > 
> > The cost is to have an extra pointer to track the last mm we were using before
> > switching to the init_mm used by idle.  But avoiding the extra IBPB
> > is probably worth the extra memory for such a common scenario.
> 
> So we already track active_mm for kernel threads. I can't immediately
> see where this fails for idle and your changelog doesn't say.

idle_task_exit() explicitly switches back to init_mm when we take the
CPU offline, this very much suggests the active_mm thing works for idle
too.

This means that 'A -> idle -> A' should never pass through switch_mm to
begin with.

Please clarify how you think it does.