On Fri, 8 Jul 2016 12:14:33 -0700 Jacob Pan <[email protected]> wrote:
> On Fri, 8 Jul 2016 14:07:12 +0200 > Ingo Molnar <[email protected]> wrote: > > > > > * 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? > > > That is right, monitor is only partially broken not completely > busted. I don't have the statistics but it is not rare to miss > wakeup. The typical symptom is random slowness and fails to boot, > without this patch. > > So doing both can speed up wake up in some cases. > > Jacob > > Thanks, > > > > Ingo > BTW, I just rebased and sent out v2 to avoid conflict with the commit below. Could you take this patch for v4.8? it is a critical fix for affected CPUs. commit 8709ed4d4b0eab04561c1ec9e6ea50fd1e3897ff Author: Dave Hansen <[email protected]> Date: Fri Jun 17 17:15:03 2016 -0700 x86/cpu: Fix duplicated X86_BUG(9) macro Thanks, Jacob
