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: Ingo Molnar <mingo@kernel.org>
To: Kyle Huey <me@kylehuey.com>
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: Tue, 11 Jul 2017 11:03:58 +0200	[thread overview]
Message-ID: <20170711090358.6si4un4guz6dbxkz@gmail.com> (raw)
In-Reply-To: <CAP045Aob-1Yk7TY_FzxbDOMDzTsuSqwBF389ZM=PgUedG_eS8g@mail.gmail.com>

* 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.

We should do the original fix as well, but in a version that does not skip the 
sample but zeroes out the RIP and registers (or sets them all to -1LL) - and also 
covers other possible places where skid-RIP is exposed, such as LBR.

Thanks,

	Ingo