Sent again with LKML CCd, sorry for the noise. - Kyle On Tue, Jun 27, 2017 at 5:38 PM, Kyle Huey <me@xxxxxxxxxxxx> wrote: > cc1582c231ea introduced a regression in v4.12.0-rc5, and appears to be > a candidate for backporting to stable branches. > > rr, a userspace record and replay debugger[0], uses the PMU interrupt > to stop a program during replay to inject asynchronous events such as > signals. We are counting retired conditional branches in userspace > only. This changeset causes the kernel to drop interrupts on the > floor if, during the PMU interrupt's "skid" region, the CPU enters > kernel mode for whatever reason. When replaying traces of complex > programs such as Firefox, we intermittently fail to deliver > asynchronous events on time, leading the replay to diverge from the > recorded state. > > It seems like this change should, at a bare minimum, be limited to > counters that actually perform sampling of register state when the > interrupt fires. In our case, with the retired conditional branches > counter restricted to counting userspace events only, it makes no > difference that the PMU interrupt happened to be delivered in the > kernel. > > As this makes rr unusable on complex applications and cannot be > efficiently worked around, we would appreciate this being addressed > before 4.12 is finalized, and the regression not being introduced to > stable branches. > > Thanks, > > - Kyle > > [0] http://rr-project.org/