On Tue, Nov 20, 2018 at 01:46:05PM -0800, Kyle Huey wrote: > On Tue, Nov 20, 2018 at 1:18 PM Andi Kleen <a...@linux.intel.com> wrote: > > > > > I suppose that's fair that it's better for some use cases. The flip > > > side is that it's no longer possible to get exactly accurate counts > > > from user space if you're using the PMI (because any events between > > > the overflow itself and the transition to the PMI handler are > > > permanently lost) which is catastrophically bad for us :) > > > > Yes that's a fair point. For most usages it doesn't matter. > > > > I suspect that's a case for supporting opt-out for freezing > > per perf event, and rr using that. > > I don't see how you could easily opt-out on a per perf event basis. If > I'm reading the SDM correctly the Freeze_PerfMon_On_PMI setting is > global and affects all counters on that CPU. Even counters that don't > use the PMI at all will still be frozen if another counter overflows > and counter freezing is enabled. It would seem that a counter that > wants to use counter freezing and a counter that wants the behavior we > want would be mutually exclusive. I suppose the kernel could handle > all of that but it's a bit involved.
Yes it's a per CPU setting. You wouldn't be able to opt-in. If anyone opts out on a CPU it would be disabled on that CPU while that event is active. -Andi