On Tue, Nov 20, 2018 at 12:11:44PM -0800, Andi Kleen wrote: > > > > Given that we're already at rc3, and that this renders rr unusable, > > > > we'd ask that counter freezing be disabled for the 4.20 release. > > > > > > The boot option should be good enough for the release? > > > > I'm not entirely sure what you mean here. We want you to flip the > > default boot option so this feature is off for this release. i.e. rr > > should work by default on 4.20 and people should have to opt into the > > inaccurate behavior if they want faster PMI servicing. > > I don't think it's inaccurate, it's just different > than what you are used to. > > For profiling including the kernel it's actually far more accurate > because the count is stopped much earlier near the sampling > point. Otherwise there is a considerable over count into > the PMI handler. > > In your case you limit the count to ring 3 so it's always cut off > at the transition point into the kernel, while with freezing > it's at the overflow point.
Ooh, so the thing does FREEZE_ON_OVERFLOW _not_ FREEZE_ON_PMI. Yes, that can be a big difference. See, FREEZE_ON_PMI, as advertised by the name, should have no observable effect on counters limited to USR. But something like FREEZE_ON_OVERFLOW will loose everything between the overflow and the eventual PMI, and by freezing early we can't even compensate for it anymore either, introducing drift in the period. And I don't buy the over-count argument, the counter register shows how far over you are; it triggers the overflow when we cross 0, it then continues counting. So if you really care, you can throw away the 'over-count' at PMI time. That doesn't make it more reliable. We don't magically get pt_regs from earlier on or any other state. The only thing where it might make a difference is if you're running multiple counters (groups in perf speak) and want to correlate the count values. Then, and only then, does it matter. Bah.