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

[Jin, Yao]

Hi,

In theory, the PMI interrupts in skid region should be dropped, right?

For a userspace debugger, is it the only choice that relies on the 
*skid* PMI interrupt?

Thanks
Jin Yao

On 6/28/2017 9:01 AM, Kyle Huey wrote:
> 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/