Re: [PATCH] x86: add workaround monitor bug

[Ingo Molnar <mingo@kernel.org>]

* 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