Re: [PATCH] x86 idle: repair large-server 50-watt idle-powerregression

From: Peter Zijlstra
Date: Thu Dec 19 2013 - 12:25:58 EST


On Thu, Dec 19, 2013 at 06:07:41PM +0100, Ingo Molnar wrote:
>
> * H. Peter Anvin <hpa@xxxxxxxxx> wrote:
>
> > On 12/19/2013 08:21 AM, Peter Zijlstra wrote:
> > >
> > > What's that mb for?
> > >
> >
> > It already exists in mwait_idle_with_hints(); I just moved it into
> > this common function. It is a bit odd, I have to admit; it seems
> > like it should be *before* the monitor (and possibly we should have
> > one after the CLFLUSH as well?)
>
> Yes, I think we need a barrier before the CLFLUSH, because according
> to my reading of the Intel documentation CLFLUSH has no implicit
> ordering so it might get reordered with the store to ->flags in
> current_set_polling_and_test(), which might result in spurious wakeup
> problems again.

No it cannot; since current_set_polling_and_test() already has a barrier
to prevent that.

Also, the location patched by hpa doesn't actually call that at all.

That said, I would find it very strange indeed if a CLFLUSH doesn't also
flush the store buffer.

> (And CLFLUSH is a store in a sense, so special in that the regular
> ordering for stores does not apply.)
>
> Likewise, having a barrier before the MONITOR looks sensible as well.
> Having it _after_ monitor looks weird and is probably wrong. [It might
> have been the effects of someone seeing the spurious wakeup problems
> with realizing the true source, or so.]

I again have to disagree, one would expect monitor to flush all that is
required to start the monitor -- and it actually does so. As is
testified by this extra CLFLUSH being called a bug workaround.
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