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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Date:   Tue, 4 Jul 2017 11:21:59 +0100
From:   Mark Rutland <mark.rutland@....com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Kyle Huey <me@...ehuey.com>,
        Vince Weaver <vincent.weaver@...ne.edu>,
        "Jin, Yao" <yao.jin@...ux.intel.com>,
        Ingo Molnar <mingo@...nel.org>, 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 Tue, Jul 04, 2017 at 10:33:45AM +0100, Mark Rutland wrote:
> On Tue, Jul 04, 2017 at 11:03:13AM +0200, Peter Zijlstra wrote:
> > Faking data gets a wee bit tricky in how much data we need to clear
> > through, its not only IP, pretty much everything we get from the
> > interrupt context, like the branch stack and registers is also suspect.
> 
> Indeed. I'll take a run through __perf_event_output() and callees, and
> see what we need to drop.

Looking at perf_event_sample_format in uapi/linux/perf_event.h, there
are samples that are obviously sensitive, and should be dropped:

* PERF_SAMPLE_IP
* PERF_SAMPLE_CALLCHAIN
* PERF_SAMPLE_BRANCH_STACK
* PERF_SAMPLE_REGS_INTR

... samples that look benign:

* PERF_SAMPLE_TID
* PERF_SAMPLE_TIME
* PERF_SAMPLE_CPU
* PERF_SAMPLE_PERIOD
* PERF_SAMPLE_REGS_USER
* PERF_SAMPLE_STACK_USER
* PERF_SAMPLE_READ
* PERF_SAMPLE_ID
* PERF_SAMPLE_STREAM_ID
* PERF_SAMPLE_IDENTIFIER

.. and samples that I have no idea about:

* PERF_SAMPLE_ADDR
* PERF_SAMPLE_RAW
* PERF_SAMPLE_WEIGHT
* PERF_SAMPLE_DATA_SRC
* PERF_SAMPLE_TRANSACTION

Should any of those be moved into the "should be dropped" pile?

Thanks,
Mark.