* Peter Zijlstra <[email protected]> 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
