Messages in this thread | | | Date | Fri, 8 Jul 2016 14:07:12 +0200 | From | Ingo Molnar <> | Subject | Re: [PATCH] x86: add workaround monitor bug |
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* 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
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