From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755159AbcGHMH0 (ORCPT ); Fri, 8 Jul 2016 08:07:26 -0400 Received: from mail-wm0-f65.google.com ([74.125.82.65]:36306 "EHLO mail-wm0-f65.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754692AbcGHMHQ (ORCPT ); Fri, 8 Jul 2016 08:07:16 -0400 Date: Fri, 8 Jul 2016 14:07:12 +0200 From: Ingo Molnar To: Peter Zijlstra Cc: Jacob Pan , LKML , Ingo Molnar , "H. Peter Anvin" , X86 Kernel , Arjan van de Ven , Len Brown Subject: Re: [PATCH] x86: add workaround monitor bug Message-ID: <20160708120712.GB27634@gmail.com> References: <1467824514-20607-1-git-send-email-jacob.jun.pan@linux.intel.com> <20160708085515.GA4682@gmail.com> <20160708114558.GI30909@twins.programming.kicks-ass.net> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20160708114558.GI30909@twins.programming.kicks-ass.net> User-Agent: Mutt/1.5.24 (2015-08-30) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org * Peter Zijlstra wrote: > On Fri, Jul 08, 2016 at 10:55:15AM +0200, Ingo Molnar wrote: > > > > static inline void mwait_idle_with_hints(unsigned long eax, unsigned long ecx) > > > { > > > - if (!current_set_polling_and_test()) { > > > + if (static_cpu_has_bug(X86_BUG_MONITOR) || !current_set_polling_and_test()) { > > > > Hm, this might be suboptimal: if MONITOR/MWAIT is implemented by setting the > > exclusive flag for the monitored memory address and then snooping for cache > > invalidation requests for that cache line, then not modifying the ->flags value > > with TIF_POLLING_NRFLAG makes MWAIT not wake up - only the IPI would wake it up. > > Confused.. POLLING_NRFLAGS is not used to wake up ever. It is only used > to determine if we want to send IPIs or not. I called the IPI the 'wakeup' - it's the 'CPU wakeup' :-) > And since we _must_ send an IPI in this case, because the monitor is > busted, we cannot set this. > > > I think a better approach would be to still optimistically modify the ->flags > > value _AND_ to also send an IPI, to make sure the wakeup is not lost. This means > > that the woken CPU will wake up much faster (no IPI latency). > > This is exactly what is done. See resched_curr()'s use of > set_nr_and_not_polling(). That does: > > if (!(fetch_or(&flags, NEED_RESCHED) & POLLING_NRFLAG)) > smp_send_reschedule(cpu); > > So we unconditionally set NEED_RESCHED, if, when we set that, POLLING > was set, we skip the IPI. Ah, indeed, we set NEED_RESCHED in the same memory address that __monitor() is watching so all is good. > So again, since monitor is busted, simply setting NEED_RESCHED will not > wake us, we must send the IPI, this is achieved by not setting > POLLING_NRFLAG. Yeah, so I got the impression that it might be broken in only certain circumstances, or is it completely busted? Thanks, Ingo