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)

From: Kyle Huey <me@kylehuey.com>
To: Ingo Molnar <mingo@kernel.org>
Cc: Mark Rutland <mark.rutland@arm.com>,
	gregkh@linuxfoundation.org, Peter Zijlstra <peterz@infradead.org>,
	Vince Weaver <vincent.weaver@maine.edu>,
	"Jin, Yao" <yao.jin@linux.intel.com>,
	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>,
	acme@kernel.org, jolsa@kernel.org, kan.liang@intel.com,
	Will Deacon <will.deacon@arm.com>,
	open list <linux-kernel@vger.kernel.org>,
	"Robert O'Callahan" <robert@ocallahan.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)
Date: Mon, 17 Jul 2017 17:07:26 -0700	[thread overview]
Message-ID: <CAP045AohLGs19aA3zWmiHPx+qeQqtxkW7JGAA=H+9+bTELPJYQ@mail.gmail.com> (raw)
In-Reply-To: <CAP045AofEjymHDOHOZGvEQN+hUS-MQMBiVpySQKoJ5ZGzDSoqA@mail.gmail.com>

On Tue, Jul 11, 2017 at 8:32 AM, Kyle Huey <me@kylehuey.com> wrote:
> On Tue, Jul 11, 2017 at 2:03 AM, Ingo Molnar <mingo@kernel.org> wrote:
>>
>> * Kyle Huey <me@kylehuey.com> wrote:
>>
>>> On Wed, Jul 5, 2017 at 10:07 PM, Robert O'Callahan <robert@ocallahan.org> wrote:
>>> > On Tue, Jul 4, 2017 at 3:21 AM, Mark Rutland <mark.rutland@arm.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.)
>>>
>>> This seems to have stalled out here unfortunately.
>>>
>>> Can we get a consensus (from ingo or peterz?) on Mark's question?  Or,
>>> alternatively, can we move the patch at the top of this thread forward
>>> on the stable branches until we do reach an answer to that question?
>>>
>>> We've abandoned hope of working around this problem in rr and are
>>> currently broken for all of our users with an up-to-date kernel, so
>>> the situation for us is rather dire at the moment I'm afraid.
>>
>> Sorry about that - I've queued up a revert for the original commit and will send
>> the fix to Linus later today. I've added a -stable tag as well so it can be
>> forwarded to Greg the moment it hits upstream.
>
> Great, thank you.
>
> - Kyle

Hi again,

I saw that the revert landed on perf/urgent but it doesn't look like
that got pulled by Linus in time for 4.13-rc1.  Consider this a gentle
poke to please get this merged :)

- Kyle