Re: [PATCH] x86: add workaround monitor bug

From: Peter Zijlstra
Date: Fri Jul 08 2016 - 07:46:13 EST

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))

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