Messages in this thread | | | Date | Thu, 19 Dec 2013 18:25:35 +0100 | From | Peter Zijlstra <> | Subject | Re: [PATCH] x86 idle: repair large-server 50-watt idle-power regression |
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On Thu, Dec 19, 2013 at 06:07:41PM +0100, Ingo Molnar wrote: > > * H. Peter Anvin <hpa@zytor.com> 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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