Messages in this thread | | | Date | Tue, 15 May 2012 17:32:48 +0200 | From | Borislav Petkov <> | Subject | Re: [PATCH 2/2] x86, mce: Add persistent MCE event |
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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.
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