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

[Ingo Molnar]

* Peter Zijlstra <peterz@xxxxxxxxxxxxx> 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
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