Messages in this thread | | | Subject | Re: perf_counter: request for three more sample data options | From | Peter Zijlstra <> | Date | Fri, 03 Apr 2009 18:59:14 +0200 |
| |
On Fri, 2009-04-03 at 18:41 +0200, Ingo Molnar wrote: > * Robert Richter <robert.richter@amd.com> wrote: > > > On 03.04.09 19:51:11, Paul Mackerras wrote: > > > Peter Zijlstra writes: > > > > > > > What I was thinking of was re-using some of the cpu_clock() > > > > infrastructure. That provides us with a jiffy based GTOD sample, > > > > cpu_clock() then uses TSC and a few filters to compute a current > > > > timestamp. > > > > > > > > I was thinking about cutting back those filters and thus trusting the > > > > TSC more -- which on x86 can do any random odd thing. So provided the > > > > TSC is not doing funny the results will be ok-ish. > > > > > > > > This does mean however, that its not possible to know when its gone bad. > > > > > > I would expect that perfmon would be just reading the TSC and > > > recording that. If you can read the TSC and do some correction then > > > we're ahead. :) > > > > > > > The question to Paul is, does the powerpc sched_clock() call work in NMI > > > > -- or hard irq disable -- context? > > > > > > Yes - timekeeping is one area where us powerpc guys can be smug. > > > :) We have a per-core, 64-bit timebase register which counts at > > > a constant frequency and is synchronized across all cores. So > > > sched_clock works in any context on powerpc - all it does is > > > read the timebase and do some simple integer arithmetic on it. > > > > Ftrace is using ring_buffer_time_stamp() that finally uses > > sched_clock(). But I am not sure if the time is correct when > > calling from an NMI handler. > > Yeah, that's a bit icky. Right now we have the following > accelerator: > > u64 sched_clock_cpu(int cpu) > { > u64 now, clock, this_clock, remote_clock; > struct sched_clock_data *scd; > > if (sched_clock_stable) > return sched_clock(); > > which works rather well on CPUs that set sched_clock_stable. Do you > think we could set it on Barcelona?
I think you should couple it to the tsc clocksource detection thingy. On all systems the tsc is good enough to use as clocksource, we can short-circuit.
> in the non-stable case we chicken out: > > /* > * Normally this is not called in NMI context - but if it is, > * trying to do any locking here is totally lethal. > */ > if (unlikely(in_nmi())) > return scd->clock; > > as we'd have to take a spinlock which isnt safe from NMI context.
Right, I've been looking at doing cpu_clock() differently, but since its all 64-bit we'd either need to introduce atomic64 into the code, or redo it in the perf counter code.
So for now I've stuck with a plain sched_clock() timestamp.
| |