linux-kernel - Re: 50 Watt idle power regression bisected to Linux-3.10

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:	Thu, 12 Dec 2013 14:28:46 +0100
From:	Ingo Molnar <mingo@...nel.org>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	"H. Peter Anvin" <hpa@...or.com>, Borislav Petkov <bp@...en8.de>,
	Mike Galbraith <bitbucket@...ine.de>,
	Thomas Gleixner <tglx@...utronix.de>,
	Len Brown <lenb@...nel.org>,
	Linux PM list <linux-pm@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Jeremy Eder <jeder@...hat.com>, x86@...nel.org
Subject: Re: 50 Watt idle power regression bisected to Linux-3.10


* Peter Zijlstra <peterz@...radead.org> wrote:

> On Wed, Dec 11, 2013 at 03:08:35PM -0800, H. Peter Anvin wrote:
> > On 12/11/2013 09:50 AM, Ingo Molnar wrote:
> > > 
> > > Well, availability could be a problem too, if some CPU (real or 
> > > virtual) implements MWAIT but not CLFLUSH.
> > > 
> > > In theory we could make mwait an alternatives variant and patch in the 
> > > right combination of instructions? The CLFLUSH goes to the same 
> > > address as on which the monitoring happens, so it could be considered 
> > > one meta-instruction.
> > > 
> > 
> > The first thing to do is probably to drop the use of thread_info as a
> > wakeup doorbell.  It seemed like a good idea at the time -- after all,
> > there is one for each thread -- but it is extremely likely to be dirty
> > in the cache, which is (presumably) what causes these kinds of bugs to
> > be maximally likely.  Even if we don't do the CLFLUSH it is likely that
> > the hardware has to do something expensive behind the scenes.
> > 
> > So I would like to propose that we switch to using a percpu variable
> > which is a single cache line of nothing at all.  It would only ever be
> > touched by MONITOR and for explicit wakeup.  Hopefully that will resolve
> > this problem without the need for the CLFLUSH.
> 
> The reason we use thread_info::flags is because we need to write
> TIF_NEED_RESCHED into it to wake up anyhow.
> 
> Using another cacheline would mean the wakeup path would need to write a
> second cross cpu cacheline -- that is badness too.
> 
> So no, I don't think we want to listen to another line.

Seconded ...

Thanks,

	Ingo
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/