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SubjectRe: [PATCH 2/2] x86, mce: Add persistent MCE event
On Sat, Mar 24, 2012 at 10:15:01AM +0100, Ingo Molnar wrote:
> * Borislav Petkov <bp@amd64.org> wrote:
>
> > On Sat, Mar 24, 2012 at 08:37:31AM +0100, Ingo Molnar wrote:
> > > I was mainly thinking of reducing this:
> > >
> > > arch/x86/kernel/cpu/mcheck/mce.c | 53 ++++++++++++++++++++++++++++++++++++++
> > > 1 file changed, 53 insertions(+)
> > >
> > > to almost nothing. There doesn't seem to be much MCE specific in
> > > that code, right?
> >
> > Yeah, this could be generalized even more, AFAICT.
> >
> > >
> > > > Btw, the more important question is are we going to need
> > > > persistent events that much so that a generic approach is
> > > > warranted? I guess maybe the black box events recording deal
> > > > would be another user..
> > >
> > > So, here's the big picture as I see it:
> > >
> > > I think tracing could use persistent events: mark all the events
> > > we want to trace as persistent from bootup, and recover the
> > > bootup trace after the system has been booted up.
> >
> > Right, but (more nasty questions):
> >
> > Why would I do this, am I tracing the boot process? [...]
>
> Correct, in essence the MCE persistent event is partially about
> that: we are starting to collect events well before there's any
> user-space available.
>
> > [...] If so, then I need another syntax which enables those
> > events from the kernel command line which gets parsed the
> > moment ftrace and ring buffer get initialized.
>
> Correct. Something really simple like:
>
> boot_trace=<event1>,<event2>...
>
> ... which could be all implicit within MCE too. (So I'm not
> suggesting some boot command trigger to provide the MCE case -
> but for more general boot tracing it would be the right
> solution.)
>
> > IOW, I'd need userspace for perf otherwise but I don't have
> > that before booting...
>
> Correct. In the case of MCE there's no "userspace" really needed
> - we just want to trace early enough. This model carries over to
> later as well: there's no *specific* process we want to attach
> the trace buffer to - we just want a persistent trace buffer
> that essentially never loses MCE events.
>
> > Then, after having booted, do I stop the trace? If no, then I
> > can see the persistency in there so are you saying we want a
> > low overhead, low ressource utilization machinery which runs
> > all the time and traces the system? What are possible real
> > life use cases for that? Scheduler analysis probably,
> > long-term tracing of some stuff people are interested in how
> > it behaves over long periods of time... MCE is one use case,
> > definitely...
>
> Boot tracing is a very real usecase, people use it to reduce
> boot times. Today printk timestamps are used as a substitute.
> (There's also a boot tracer plugin within ftrace, see the
> bootup_tracer.)
>
> > > But other, runtime models of tracing could use it as well:
> > > basically the main difference that ftrace has to perf based
> > > tracing today is a system-wide persistent buffer with no
> > > particular owning process. (The rest is mostly UI and
> > > analysis features and scope of tracing differences, and of
> > > course a lot more love and detail went into ftrace so far.)
> > >
> > > So MCE will in the end be just a minor user of such a
> > > facility - I think you should aim for enabling *any* set of
> > > events to have persistent recording properties, and add the
> > > APIs to recover that information sanely. It should also be
> > > possible for them to record into a shared mmap page in
> > > essence - instead of having per event persistent buffers.
> >
> > Sounds like ftrace. But we have that already, we only need to
> > get to using it perf-side, no...? [...]
>
> What we want is to extend the perf ring-buffer to be persistent
> *as well*. It's an evidently useful model of collecting events.
>
> All the remaining perf tooling can be used after that point - if
> it's a bog-standard perf ring-buffer then it can be saved into a
> perf.data and can be analyzed in a rich fashion, etc.
>
> Think about it: for example we could do not just boot tracing
> but also boot *profiling*, by using the PMU to sample into a
> persistent buffer which after bootup can be put into a perf.data
> and 'perf report' will do the right thing, etc...
>
> Does it overlap with ftrace? Perf overlapped with ftrace from
> day one on and it's starting to become a maintenance problem: we
> want to remove that overlap not by keeping two separate entities
> (both of which suck and rule in their own ways) but having a
> unified facility.

Leaving all of the above for reference.

So, I spent some more nights sleeping on it :-)

Here's what I dreamt of:

* The last thing perf_event_init() does is init the persistent, per-cpu
buffers.

* there's no need for changing TRACE_EVENT: "boot_trace" parameter
parsing code enables those events the moment perf is initialized. We're
doing this anyway because we're enabling the trace_mce_record TP.

It sounds pretty simple to me but the devil is in the details,
especially making the persistent buffers, task-agnostic and generic
enough.

Ingo, Peter, thoughts?

Thanks.

--
Regards/Gruss,
Boris.

Advanced Micro Devices GmbH
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