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

["Jin, Yao" <yao.jin@linux.intel.com>]

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@kylehuey.com> 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/