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From: Kyle Huey <me@kylehuey.com>
Date: Tue, 27 Jun 2017 18:01:00 -0700
Message-ID: <CAP045Ao2=6A7GEhBfmY0gZ2g=urL5U4R5ywLsTo4Yb6Tk1M8Cw@mail.gmail.com>
Subject: Re: [REGRESSION] perf/core: PMU interrupts dropped if we entered the
 kernel in the "skid" region
To: Jin Yao <yao.jin@linux.intel.com>, Ingo Molnar <mingo@kernel.org>
Cc: "Peter Zijlstra (Intel)" <peterz@infradead.org>,
        stable@vger.kernel.org,
        Alexander Shishkin <alexander.shishkin@linux.intel.com>,
        Arnaldo Carvalho de Melo <acme@redhat.com>,
        Jiri Olsa <jolsa@redhat.com>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Namhyung Kim <namhyung@kernel.org>,
        Stephane Eranian <eranian@google.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Vince Weaver <vincent.weaver@maine.edu>, acme@kernel.org,
        jolsa@kernel.org, kan.liang@intel.com,
        Mark Rutland <mark.rutland@arm.com>, Will Deacon <will.deacon@arm.com>,
        gregkh@linuxfoundation.org, "Robert O'Callahan" <robert@ocallahan.org>,
        open list <linux-kernel@vger.kernel.org>
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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/