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: Peter Zijlstra <peterz@infradead.org>
To: Ingo Molnar <mingo@kernel.org>
Cc: Mark Rutland <mark.rutland@arm.com>, Kyle Huey <me@kylehuey.com>,
	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>,
	gregkh@linuxfoundation.org,
	"Robert O'Callahan" <robert@ocallahan.org>,
	open list <linux-kernel@vger.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)
Date: Tue, 4 Jul 2017 11:06:47 +0200	[thread overview]
Message-ID: <20170704090647.dbvrpnah6d47txdx@hirez.programming.kicks-ass.net> (raw)
In-Reply-To: <20170629081233.7muy7k447pc5njmg@gmail.com>

On Thu, Jun 29, 2017 at 10:12:33AM +0200, Ingo Molnar wrote:
> 
> * Mark Rutland <mark.rutland@arm.com> wrote:
> 
> > It still seems wrong to make up data, though.
> 
> So what we have here is a hardware quirk: we asked for user-space samples, but 
> didn't get them and we cannot expose the kernel-internal address.
> 
> The question is, how do we handle the hardware quirk. Since we cannot fix the 
> hardware on existing systems there's really just two choices:
> 
>  - Lose the sample (and signal it as a lost sample)
> 
>  - Keep the sample but change the sensitive kernel-internal address to something 
>    that is not sensitive: 0 or -1 works, but we could perhaps also return a 
>    well-known user-space address such as the vDSO syscall trampoline or such?
> 
> there's no other option really.
> 
> I'd lean towards Vince's take: losing samples is more surprising than getting the 
> occasional sample with some sanitized data in it.
> 
> If we make the artificial data still a meaningful user-space address, related to 
> kernel entries, then it might even be a bonus, as users would learn to recognize 
> it as: 'oh, skid artifact, I know about that'.

So while we could easily fake SAMPLE_IP to do as you suggest, other
entries might be much harder to fake. That said, I have no problems with
just 0 stuffing them.

The only real problem is determining how much to stuff I suppose.