linux-kernel - 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)

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Message-ID: <20170704090647.dbvrpnah6d47txdx@hirez.programming.kicks-ass.net>
Date:   Tue, 4 Jul 2017 11:06:47 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Ingo Molnar <mingo@...nel.org>
Cc:     Mark Rutland <mark.rutland@....com>, Kyle Huey <me@...ehuey.com>,
        Vince Weaver <vincent.weaver@...ne.edu>,
        "Jin, Yao" <yao.jin@...ux.intel.com>, stable@...r.kernel.org,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Arnaldo Carvalho de Melo <acme@...hat.com>,
        Jiri Olsa <jolsa@...hat.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Namhyung Kim <namhyung@...nel.org>,
        Stephane Eranian <eranian@...gle.com>,
        Thomas Gleixner <tglx@...utronix.de>, acme@...nel.org,
        jolsa@...nel.org, kan.liang@...el.com,
        Will Deacon <will.deacon@....com>, gregkh@...uxfoundation.org,
        Robert O'Callahan <robert@...llahan.org>,
        open list <linux-kernel@...r.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)

On Thu, Jun 29, 2017 at 10:12:33AM +0200, Ingo Molnar wrote:
> 
> * Mark Rutland <mark.rutland@....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.