Date: Tue, 4 Jul 2017 11:06:47 +0200
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)
Message-ID: <20170704090647.dbvrpnah6d47txdx@hirez.programming.kicks-ass.net>
References: <CAP045Ap8cMx6mzSgcQ3n3bnh_8GJuCp7_KZe_5ZTCR_K6cPTLw@mail.gmail.com>
 <CAP045Ao2=6A7GEhBfmY0gZ2g=urL5U4R5ywLsTo4Yb6Tk1M8Cw@mail.gmail.com>
 <2256f9b5-1277-c4b1-1472-61a10cd1db9a@linux.intel.com>
 <CAP045AryPCUTO92S3hmrkag3D7NqgJM-hK82a7iKm9s-rWdn1w@mail.gmail.com>
 <20170628101248.GB5981@leverpostej>
 <20170628105600.GC5981@leverpostej>
 <CAP045ApjOeng6M9kqwbNpv5OTN=_Gkk2MHREztzZV_CecqpmxQ@mail.gmail.com>
 <20170628174900.GG8252@leverpostej>
 <20170629081233.7muy7k447pc5njmg@gmail.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <20170629081233.7muy7k447pc5njmg@gmail.com>
User-Agent: NeoMutt/20170113 (1.7.2)
Sender: linux-kernel-owner@vger.kernel.org
Content-Length: 1396
Lines: 32

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.