Messages in this thread | ![/](/images/icornerl.gif) | | Subject | Re: [REGRESSION] perf/core: PMU interrupts dropped if we entered the kernel in the "skid" region | From | "Jin, Yao" <> | Date | Wed, 28 Jun 2017 10:09:01 +0800 |
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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/
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