Re: [PATCH] x86: add workaround monitor bug

Path	csiph.com!news.mixmin.net!aioe.org!gothmog.csi.it!bofh.it!news.nic.it!robomod
From	Peter Zijlstra <peterz@infradead.org>
Newsgroups	linux.kernel
Subject	Re: [PATCH] x86: add workaround monitor bug
Date	Fri, 08 Jul 2016 13:50:01 +0200
Message-ID	<rSCs9-4e3-27@gated-at.bofh.it> (permalink)
References	<rRYuK-3wy-33@gated-at.bofh.it> <rSzND-2nt-3@gated-at.bofh.it>
MIME-Version	1.0
Content-Type	text/plain; charset=us-ascii
Content-Disposition	inline
User-Agent	Mutt/1.5.23.1 (2014-03-12)
Sender	robomod@news.nic.it
List-ID	<linux-kernel.vger.kernel.org>
X-Mailing-List	linux-kernel@vger.kernel.org
Approved	robomod@news.nic.it
Lines	34
Organization	linux.* mail to news gateway
X-Original-Cc	Jacob Pan <jacob.jun.pan@linux.intel.com>, LKML <linux-kernel@vger.kernel.org>, Ingo Molnar <mingo@redhat.com>, "H. Peter Anvin" <hpa@zytor.com>, X86 Kernel <x86@kernel.org>, Arjan van de Ven <arjan@linux.intel.com>, Len Brown <lenb@kernel.org>
X-Original-Date	Fri, 8 Jul 2016 13:45:58 +0200
X-Original-Message-ID	<20160708114558.GI30909@twins.programming.kicks-ass.net>
X-Original-References	<1467824514-20607-1-git-send-email-jacob.jun.pan@linux.intel.com> <20160708085515.GA4682@gmail.com>
X-Original-Sender	linux-kernel-owner@vger.kernel.org
Xref	csiph.com linux.kernel:1439343

Show key headers only | View raw

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.

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.

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.

Thread

[PATCH] x86: add workaround monitor bug Jacob Pan <jacob.jun.pan@linux.intel.com> - 2016-07-06 19:10 +0200
  Re: [PATCH] x86: add workaround monitor bug Ingo Molnar <mingo@kernel.org> - 2016-07-08 11:00 +0200
    Re: [PATCH] x86: add workaround monitor bug Peter Zijlstra <peterz@infradead.org> - 2016-07-08 13:50 +0200
      Re: [PATCH] x86: add workaround monitor bug Ingo Molnar <mingo@kernel.org> - 2016-07-08 14:10 +0200
        Re: [PATCH] x86: add workaround monitor bug Jacob Pan <jacob.jun.pan@linux.intel.com> - 2016-07-08 21:20 +0200

csiph-web