From mboxrd@z Thu Jan 1 00:00:00 1970 From: Ingo Molnar Subject: Re: [PATCH] x86 idle: repair large-server 50-watt idle-power regression Date: Thu, 19 Dec 2013 18:07:41 +0100 Message-ID: <20131219170741.GB30382@gmail.com> References: <20131219122257.GC11279@gmail.com> <52B316FF.50906@zytor.com> <20131219160210.GA28426@gmail.com> <52B31B21.6010901@zytor.com> <20131219162136.GM16438@laptop.programming.kicks-ass.net> <52B323BE.7090108@zytor.com> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Return-path: Content-Disposition: inline In-Reply-To: <52B323BE.7090108@zytor.com> Sender: linux-kernel-owner@vger.kernel.org To: "H. Peter Anvin" Cc: Peter Zijlstra , Len Brown , x86@kernel.org, linux-pm@vger.kernel.org, linux-kernel@vger.kernel.org, Len Brown , stable@vger.kernel.org, Linus Torvalds , Thomas Gleixner , Mike Galbraith , Borislav Petkov List-Id: linux-pm@vger.kernel.org * H. Peter Anvin 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. (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.] Thanks, Ingo