From: Ingo Molnar <mingo@kernel.org>
To: Peter Zijlstra <peterz@infradead.org>
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>
Subject: Re: [PATCH] x86: add workaround monitor bug
Date: Fri, 8 Jul 2016 14:07:12 +0200 [thread overview]
Message-ID: <20160708120712.GB27634@gmail.com> (raw)
In-Reply-To: <20160708114558.GI30909@twins.programming.kicks-ass.net>
* Peter Zijlstra <peterz@infradead.org> 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
next prev parent reply other threads:[~2016-07-08 12:07 UTC|newest]
Thread overview: 6+ messages / expand[flat|nested] mbox.gz Atom feed top
2016-07-06 17:01 [PATCH] x86: add workaround monitor bug Jacob Pan
2016-07-08 8:55 ` Ingo Molnar
2016-07-08 11:45 ` Peter Zijlstra
2016-07-08 12:07 ` Ingo Molnar [this message]
2016-07-08 19:14 ` Jacob Pan
2016-07-18 18:45 ` Jacob Pan
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