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

[Kyle Huey]

On Tue, Jun 27, 2017 at 7:09 PM, Jin, Yao <yao.jin@xxxxxxxxxxxxxxx> wrote:
> Hi,
>
> In theory, the PMI interrupts in skid region should be dropped, right?

No, why would they be dropped?

My understanding of the situation is as follows:

There is some time, call it t_0, where the hardware counter overflows.
The PMU triggers an interrupt, but this is not instantaneous.  Call
the time when the interrupt is actually delivered t_1.  Then t_1 - t_0
is the "skid".

Note that if the counter is `exclude_kernel`, then at t_0 the CPU
*must* be running a userspace program.  But by t_1, the CPU may be
doing something else.  Your patch changed things so that if at t_1 the
CPU is in the kernel, then the interrupt is discarded.  But rr has
programmed the counter to deliver a signal on overflow (via F_SETSIG
on the fd returned by perf_event_open).  This change results in the
signal never being delivered, because the interrupt was ignored.
(More accurately, the signal is delivered the *next* time the counter
overflows, which is far past where we wanted to inject our
asynchronous event into our tracee.

It seems to me that it might be reasonable to ignore the interrupt if
the purpose of the interrupt is to trigger sampling of the CPUs
register state.  But if the interrupt will trigger some other
operation, such as a signal on an fd, then there's no reason to drop
it.

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

I don't understand this question, but hopefully the above clarified things.

- Kyle

> 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/
>
>