linux-kernel - Re: [PATCH] perf/core: generate overflow signal when samples are dropped (WAS: Re: [REGRESSION] perf/core: PMU interrupts dropped if we entered the kernel in the "skid" region)

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Message-ID: <CAOp6jLZjGDRiHaitwcV0Jz86WbL=8UocEFCGd_ZXAEYRwtf6nA@mail.gmail.com>
Date:   Wed, 5 Jul 2017 22:07:59 -0700
From:   "Robert O'Callahan" <robert@...llahan.org>
To:     Mark Rutland <mark.rutland@....com>
Cc:     Peter Zijlstra <peterz@...radead.org>, Kyle Huey <me@...ehuey.com>,
        Vince Weaver <vincent.weaver@...ne.edu>,
        "Jin, Yao" <yao.jin@...ux.intel.com>,
        Ingo Molnar <mingo@...nel.org>, stable@...r.kernel.org,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Arnaldo Carvalho de Melo <acme@...hat.com>,
        Jiri Olsa <jolsa@...hat.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Namhyung Kim <namhyung@...nel.org>,
        Stephane Eranian <eranian@...gle.com>,
        Thomas Gleixner <tglx@...utronix.de>, acme@...nel.org,
        jolsa@...nel.org, kan.liang@...el.com,
        Will Deacon <will.deacon@....com>, gregkh@...uxfoundation.org,
        open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] perf/core: generate overflow signal when samples are
 dropped (WAS: Re: [REGRESSION] perf/core: PMU interrupts dropped if we
 entered the kernel in the "skid" region)

On Tue, Jul 4, 2017 at 3:21 AM, Mark Rutland <mark.rutland@....com> wrote:
> Should any of those be moved into the "should be dropped" pile?

Why not be conservative and clear every sample you're not sure about?

We'd appreciate a fix sooner rather than later here, since rr is
currently broken on every stable Linux kernel and our attempts to
implement a workaround have failed.

(We have separate "interrupt" and "measure" counters, and I thought we
might work around this regression by programming the "interrupt"
counter to count kernel events as well as user events (interrupting
early is OK), but that caused our (completely separate) "measure"
counter to report off-by-one results (!), which seems to be a
different bug present on a range of older kernels.)

Thanks,
Rob
-- 
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