Re: [PATCH 2/2] tracing/function-graph-tracer: use the more lightweight local clock

Frederic Weisbecker
2009-03-05T14:02:13Z

On Thu, Mar 05, 2009 at 12:56:52PM +0100, Ingo Molnar wrote:
>
> * Frederic Weisbecker <fweisbec@gmail.com> wrote:
>
> > On Thu, Mar 05, 2009 at 11:56:52AM +0100, Ingo Molnar wrote:
> > >
> > > * Frederic Weisbecker <fweisbec@gmail.com> wrote:
> > >
> > > > On Thu, Mar 05, 2009 at 08:30:13AM +0100, Peter Zijlstra wrote:
> > > > > On Thu, 2009-03-05 at 02:19 +0100, Frederic Weisbecker wrote:
> > > > >
> > > > > > It takes 1 ms to execute while tracing.
> > > > > > Considering my frequency is 250 Hz, it means 1/4 of the system is used
> > > > > > on timer interrupt while tracing.
> > > > > >
> > > > > > For now the hang is fixed, but not the awful latency. And I'm just too frightened
> > > > > > to test it on 1000 Hz.
> > > > > >
> > > > > > But I plan to add a kind of watchdog to check how many time we spent inside an
> > > > > > interrupt while graph tracing.
> > > > > > By checking this time against the current Hz value, I could decide to abort the tracing
> > > > > > for all irq.
> > > > >
> > > > > That would basically render the thing useless :-(
> > > >
> > > >
> > > > It would be only for slow machines :-)
> > > > I'm talking about something that happened on a Pentium II.
> > > >
> > > >
> > > > > Is it specifically function_graph that is so expensive? If so, is that
> > > > > because of the function exit hook?
> > > >
> > > >
> > > > Yes, specifically the function_graph, the function tracer is
> > > > not concerned. The function graph tracer takes more than
> > > > double overhead compared to the function tracer.
> > > >
> > > > Usually the function tracer hooks directly the the function
> > > > that insert the event, it's pretty straightforward.
> > > >
> > > > The function graph does much more work:
> > > >
> > > > entry: basic checks, take the time, push the infos on the stack, insert an event
> > > > on the ring-buffer, hook the return value.
> > > > return: pop the infos from stack, insert an event on the ring-buffer, jump
> > > > to the original caller.
> > > >
> > > > It has a high cost... which makes me sad because I plan to
> > > > port it in on Arm and I fear the little Arm boad I recently
> > > > purshased will not let me trace the interrupts without
> > > > hanging...
> > > > :-(
> > > >
> > > > I guess I should start thinking on some optimizations, perhaps
> > > > using perfcounter?
> > >
> > > yeah. perfcounters and KernelTop might not work on a PII CPU out
> > > of box though.
> > >
> > > But hacking perfcounters and looking at perfstat/kerneltop
> > > output is serious amount of fun so if you are interested you
> > > could try to implement support for it. Do you have any box where
> > > perfcounters work? (that would be Core2 Intel boxes or pretty
> > > much any AMD box)
> > >
> > > You could have a look at how oprofile works on your box - the
> > > code for PII CPUs should be in
> > > arch/x86/oprofile/op_model_ppro.c.
> > >
> > > There's also hardcoded support for a single perfcounter in the
> > > nmi_watchdog=2 code, in arch/x86/kernel/cpu/perfctr-watchdog.c,
> > > for pretty much any x86 CPU that has a PMU.
> > >
> > > Plus there's also the CPU documentation on Intel's site. It's
> > > quite well written and pretty well structured. The URL for the
> > > CPU's PMU ("Performance Monitoring") should be:
> > >
> > > http://download.intel.com/design/processor/manuals/253669.pdf
> > >
> > > As a last resort ;-)
> > >
> > > Ingo
> >
> > Ah yes, That could be fun!
> > So, by reading your description, it should work on my labtop I guess?
> >
> > -> Intel(R) Pentium(R) Dual CPU T2310 @ 1.46GHz
>
> Yeah, should work fine there - so that should be a good
> reference point to start off. Let me know if you see any
> bugs/problems.
>
> > Anyway, I will give it a try and see what I can do.
> > Thanks for the pointers.
>
> You are welcome.
>
> Ingo

Ho, that's impressive:

$ ./perfstat /bin/echo 1
1

Performance counter stats for '/bin/echo':

5.681909 task clock ticks (msecs)

2 CPU migrations (events)
9 context switches (events)
422 pagefaults (events)
4986950 CPU cycles (events)
4870587 instructions (events)
62881 cache references (events)
4882 cache misses (events)

Wall-clock time elapsed: 9.046821 msecs

So I guess that for it to be useful on fine grained profiling,
it's better to include the percounters syscalls inside the application
to profile a single function for example?

I've parsed a bit the intel documentation and perfcounters source code.
It looks like the current implementation for intel and amd share enough similar
properties so that they have been factorized in a single file.

This similar property is the use of an msr, while P6 family use the pmc.

Perhaps I can expand the struct pmc_x86_ops to guess whether we want to use
rdmsr or rdpmc.

Hmm it seems the implementation would be enough different to deserve a new file for
P6.
Will see...