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

From: Mark Rutland <mark.rutland@arm.com>
To: Kyle Huey <me@kylehuey.com>
Cc: "Jin, Yao" <yao.jin@linux.intel.com>,
	Ingo Molnar <mingo@kernel.org>,
	"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,
	Will Deacon <will.deacon@arm.com>,
	gregkh@linuxfoundation.org,
	"Robert O'Callahan" <robert@ocallahan.org>,
	open list <linux-kernel@vger.kernel.org>
Subject: Re: [REGRESSION] perf/core: PMU interrupts dropped if we entered the kernel in the "skid" region
Date: Wed, 28 Jun 2017 18:52:41 +0100	[thread overview]
Message-ID: <20170628175241.GH8252@leverpostej> (raw)
In-Reply-To: <CAP045ArpMUYfT2GqdS931xG1nPqeh6z3zTq2RWJAkLXq3Ju6HA@mail.gmail.com>

On Wed, Jun 28, 2017 at 10:36:20AM -0700, Kyle Huey wrote:
> On Wed, Jun 28, 2017 at 10:19 AM, Mark Rutland <mark.rutland@arm.com> wrote:
> > On Wed, Jun 28, 2017 at 09:46:43AM -0700, Kyle Huey wrote:
> >> On Wed, Jun 28, 2017 at 3:12 AM, Mark Rutland <mark.rutland@arm.com> wrote:

> >> > Just to clarify, you don't care about the sample state at all? i.e. you
> >> > don't need the user program counter?
> >>
> >> Right. `sample_regs_user`, `sample_star_user`, `branch_sample_type`,
> >> etc are all 0.
> >> https://github.com/mozilla/rr/blob/cf594dd01f07d96a61409e9f41a29f78c8c51693/src/PerfCounters.cc#L194
> >> is what we do use.
> >
> > Given that, I must be missing something.
> >
> > In __perf_event_overflow(), we already bail out early if
> > !is_sampling_event(event), i.e. when the sample_period is 0.
> >
> > Your attr has a sample_period of zero, so something must be initialising
> > that.
> >
> > Do you always call PERF_EVENT_IOC_PERIOD, or is something in the core
> > fiddling with the sample period behind your back?
> 
> We always either set sample_period or call PERF_EVENT_IOC_PERIOD (with
> an enormous number if we don't actually want an interrupt.  See
> `PerfCounters::reset`, line 446.

Ah, thanks for the pointer.

> > It seems odd that an event without any samples to take has a sample
> > period. I'm surprised that there's not *some* sample_type set.
> 
> Perhaps sample_period is misleadingly named :)  Alternatively, you
> could imagine it as sampling where we're only interested in whether
> the counter passed the sampling value or not.

Sure; it's just that I suspect the existing kernel behviour isn't
*quite* intentional, and I could easily see it getting broken in future,
e.g. if someone were to make is_sampling_event() check the attr for
sample types.

So we need to keep an eye on that, regardless.

Thanks,
Mark.