linux-kernel - Re: [patch 17/24] x86/speculation: Move IBPB control out of switch

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Date:   Thu, 22 Nov 2018 08:42:33 +0100 (CET)
From:   Jiri Kosina <jikos@...nel.org>
To:     Andi Kleen <ak@...ux.intel.com>
cc:     Thomas Gleixner <tglx@...utronix.de>,
        LKML <linux-kernel@...r.kernel.org>, x86@...nel.org,
        Peter Zijlstra <peterz@...radead.org>,
        Andy Lutomirski <luto@...nel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Tom Lendacky <thomas.lendacky@....com>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        David Woodhouse <dwmw@...zon.co.uk>,
        Dave Hansen <dave.hansen@...el.com>,
        Casey Schaufler <casey.schaufler@...el.com>,
        Asit Mallick <asit.k.mallick@...el.com>,
        Arjan van de Ven <arjan@...ux.intel.com>,
        Jon Masters <jcm@...hat.com>,
        Waiman Long <longman9394@...il.com>,
        Greg KH <gregkh@...uxfoundation.org>,
        Dave Stewart <david.c.stewart@...el.com>,
        Kees Cook <keescook@...omium.org>
Subject: Re: [patch 17/24] x86/speculation: Move IBPB control out of
 switch_mm()

On Wed, 21 Nov 2018, Andi Kleen wrote:

> > +		 * This could be optimized by keeping track of the last
> > +		 * user task per cpu and avoiding the barrier when the task
> > +		 * is immediately scheduled back and the thread inbetween
> > +		 * was a kernel thread. It's dubious whether that'd be
> > +		 * worth the extra load/store and conditional operations.
> > +		 * Keep it optimized for the common case where the TIF bit
> > +		 * is not set.
> > +		 */
> 
> The optimization was there before and you removed it?
> 
> It's quite important for switching to idle and back. With your variant short IOs
> that do short idle waits will be badly impacted. 

The question is what scenario to optimize for.

Either you penalize everybody in the default prctl+seccomp setup 
(irrespective of it's TIF flag value), as you have the extra overhead on 
each and every switch_to() (to check exactly for this back-to-back 
scheduling), or you penalize only those tasks that are penalized anyway by 
the IBPB flush.

I think the latter (which is what this patch implements) makes more sense. 

Thanks,

-- 
Jiri Kosina
SUSE Labs