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: <20170711142618.GF2681@leverpostej>
Date:   Tue, 11 Jul 2017 15:26:18 +0100
From:   Mark Rutland <mark.rutland@....com>
To:     Ingo Molnar <mingo@...nel.org>
Cc:     Kyle Huey <me@...ehuey.com>, gregkh@...uxfoundation.org,
        Peter Zijlstra <peterz@...radead.org>,
        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>,
        open list <linux-kernel@...r.kernel.org>,
        Robert O'Callahan <robert@...llahan.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 11, 2017 at 11:03:58AM +0200, Ingo Molnar wrote:
> 
> * Kyle Huey <me@...ehuey.com> wrote:
> 
> > On Wed, Jul 5, 2017 at 10:07 PM, Robert O'Callahan <robert@...llahan.org> wrote:
> > > On Tue, Jul 4, 2017 at 3:21 AM, Mark Rutland <mark.rutland@....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.

Thanks for handling this.

Mark.