Re: [REGRESSION] perf/core: PMU interrupts dropped if we entered the kernel in the "skid" region

[Kyle Huey]

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/