On Mon, 2015-01-05 at 13:00 +0000, Peter Zijlstra wrote:
> On Thu, Nov 06, 2014 at 04:51:56PM +0000, Pawel Moll wrote:
> > Documentation/kernel-parameters.txt | 9 +++++++++
> > kernel/events/core.c | 37
> > +++++++++++++++++++++++++++++++++++++
> > 2 files changed, 46 insertions(+)
>
> > diff --git a/Documentation/kernel-parameters.txt
> > b/Documentation/kernel-parameters.txt
> > index 4c81a86..8ead8d8 100644
> > --- a/Documentation/kernel-parameters.txt
> > +++ b/Documentation/kernel-parameters.txt
>
> > @@ -2763,6 +2764,14 @@ bytes respectively. Such letter suffixes can also be
> > entirely omitted.
> > allocator. This parameter is primarily for debugging
> > and performance comparison.
> >
> > + perf_use_local_clock
> > + [PERF]
> > + Use local_clock() as a source for perf timestamps
> > + generation. This was be the default behaviour and
> > + this parameter can be used to maintain backward
> > + compatibility or on older hardware with expensive
> > + monotonic clock source.
> > +
> > pf. [PARIDE]
> > See Documentation/blockdev/paride.txt.
>
> So I'm always terminally confused on the naming of kernel parameters,
> sometimes things are modules (even when they're not actually =m capable)
> and get a module::foo naming or so and sometimes they're not.
I guess you mean module.foo?
> So we want to use the module naming thing or not?
Honestly, I don't mind either way. For one thing ftrace doesn't bother
and just uses __setup() as well.
> > diff --git a/kernel/events/core.c b/kernel/events/core.c
> > index 2b02c9f..5d0aa03 100644
> > --- a/kernel/events/core.c
> > +++ b/kernel/events/core.c
>
> > @@ -322,8 +323,41 @@ extern __weak const char *perf_pmu_name(void)
> > return "pmu";
> > }
> >
> > +static bool perf_use_local_clock;
> > +static int __init perf_use_local_clock_setup(char *__unused)
> > +{
> > + perf_use_local_clock = true;
> > + return 1;
> > +}
> > +__setup("perf_use_local_clock", perf_use_local_clock_setup);
>
> > static inline u64 perf_clock(void)
> > {
> > + if (likely(!perf_use_local_clock))
> > + return ktime_get_mono_fast_ns();
> > +
> > return local_clock();
> > }
>
> Since this all is boot time, should we not use things like static_key
> and avoid the 'pointless' conditional at runtime?
Right. it's good to learn new stuff - I didn't know there was
architecture-agnostic support for jump labels. Definitely worth the
effort, will give it a try and spin the patch.
Pawel
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