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    SubjectRe: [PATCH 1/3] tracing,x86: add a TSC trace_clock; reset buffer on clock change
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    On Tue, 2012-09-11 at 19:41 -0700, David Sharp wrote:
    > In order to promote interoperability between userspace tracers and ftrace,
    > add a trace_clock that reports raw TSC values which will then be recorded
    > in the ring buffer. Userspace tracers that also record TSCs are then on
    > exactly the same time base as the kernel and events can be unambiguously
    > interlaced.
    >
    > Tested: Enabled a tracepoint and the "tsc" trace_clock and saw very large
    > timestamp values.
    >
    > Google-Bug-Id: 6980623
    > Signed-off-by: David Sharp <dhsharp@google.com>
    > ---
    > include/linux/trace_clock.h | 3 +++
    > kernel/trace/trace.c | 3 +++
    > kernel/trace/trace_clock.c | 16 ++++++++++++++++
    > 3 files changed, 22 insertions(+), 0 deletions(-)
    >
    > diff --git a/include/linux/trace_clock.h b/include/linux/trace_clock.h
    > index 4eb4902..b86c7363 100644
    > --- a/include/linux/trace_clock.h
    > +++ b/include/linux/trace_clock.h
    > @@ -16,5 +16,8 @@ extern u64 notrace trace_clock_local(void);
    > extern u64 notrace trace_clock(void);
    > extern u64 notrace trace_clock_global(void);
    > extern u64 notrace trace_clock_counter(void);
    > +#ifdef CONFIG_X86_TSC
    > +extern u64 notrace trace_clock_tsc(void);
    > +#endif
    >
    > #endif /* _LINUX_TRACE_CLOCK_H */
    > diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
    > index 5c38c81..dc1f1fa 100644
    > --- a/kernel/trace/trace.c
    > +++ b/kernel/trace/trace.c
    > @@ -480,6 +480,9 @@ static struct {
    > { trace_clock_local, "local" },
    > { trace_clock_global, "global" },
    > { trace_clock_counter, "counter" },
    > +#ifdef CONFIG_X86_TSC
    > + { trace_clock_tsc, "tsc" },
    > +#endif
    > };

    I really hate adding arch defs in generic code. Perhaps what we could do
    is add a define here. Something like:

    #ifndef ARCH_TRACE_CLOCKS
    # define ARCH_TRACE_CLOCKS
    #endif

    [...]
    { trace_clock_counter, "counter" },
    ARCH_TRACE_CLOCKS
    };

    and have ARCH_TRACE_CLOCKS defined somewhere in an arch specific header.
    Not sure what header we could use though :-/

    That is, in a header have:

    #define ARCH_TRACE_CLOCKS \
    { trace_clock_x86_tsc, "tsc" },

    and also define trace_clock_x86_tsc in arch/x86/kernel...

    -- Steve


    >
    > int trace_clock_id;
    > diff --git a/kernel/trace/trace_clock.c b/kernel/trace/trace_clock.c
    > index 3947835..1770737 100644
    > --- a/kernel/trace/trace_clock.c
    > +++ b/kernel/trace/trace_clock.c
    > @@ -125,3 +125,19 @@ u64 notrace trace_clock_counter(void)
    > {
    > return atomic64_add_return(1, &trace_counter);
    > }
    > +
    > +#ifdef CONFIG_X86_TSC
    > +/*
    > + * trace_clock_tsc(): A clock that is just the cycle counter.
    > + *
    > + * Unlike the other clocks, this is not in nanoseconds.
    > + */
    > +u64 notrace trace_clock_tsc(void)
    > +{
    > + u64 ret;
    > + rdtsc_barrier();
    > + rdtscll(ret);
    > +
    > + return ret;
    > +}
    > +#endif



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