RE: [PATCH V3 0/6] export perf overheads information (kernel)

["Liang, Kan" <kan.liang@intel.com>]

Hi Peter,

Any comments?

Thanks,
Kan

> Subject: RE: [PATCH V3 0/6] export perf overheads information (kernel)
> 
> 
> Ping.
> Any comments for the series?
> 
> Thanks,
> Kan
> 
> > Subject: [PATCH V3 0/6] export perf overheads information (kernel)
> >
> > From: Kan Liang <kan.liang@intel.com>
> >
> > This series only include the kernel related patches.
> >
> > Profiling brings additional overhead. High overhead may impacts the
> > behavior of the profiling object, impacts the accuracy of the
> > profiling result, and even hang the system.
> > Currently, perf has dynamic interrupt throttle mechanism to lower the
> > sample rate and overhead. But it has limitations.
> >  - The mechanism only focus in the sampling overhead. However, there
> >    are other parts which also bring big overhead. E.g, multiplexing.
> >  - The hint from the mechanism doesn't work on fixed period.
> >  - The system changes which caused by the mechanism are not recorded
> >    in the perf.data. Users have no idea about the overhead and its
> >    impact.
> > Actually, any passive ways like dynamic interrupt throttle mechanism
> > are only palliative. The best way is to export overhead information,
> > provide more hints, and help the users design more proper perf
> command.
> >
> > For kernel, three parts can bring obvious overhead.
> >   - sample overhead: For x86, it's the time cost in perf NMI handler.
> >   - multiplexing overhead: The time cost spends on rotate context.
> >   - side-band events overhead: The time cost spends on iterating all
> >     events that need to receive side-band events.
> > The time cost of those parts are stored in pmu's per-cpu cpuctx.
> > The tool can call PERF_EVENT_IOC_STAT when it's 'done'. Then the
> > kernel generates the overhead record PERF_RECORD_OVERHEAD.
> >
> > User can use the overhead information to refine their perf command and
> > get accurate profiling result. For example, if there is high overhead
> > warning, user may reduce the number of events/increase the
> > period/decrease the frequency.
> > Developer can also use the overhead information to find bugs.
> >
> > Changes since V2:
> >  - Separate kernel patches from the previous version
> >  - Add PERF_EVENT_IOC_STAT to control overhead statistics
> >  - Collect per pmu overhead information
> >  - Store the overhead information in pmu's cpuctx
> >  - Add CPU information in overhead record
> >
> > Changes since V1:
> >  - fix u32 holes and remove duplicate CPU information
> >  - configurable overhead logging
> >  - Introduce the concept of PMU specific overhead and common core
> >    overhead. Rename NMI overhead to PMU sample overhead
> >  - Add log_overhead in perf_event_context to indicate the logging of
> >    overhead
> >  - Refine the output of overhead information
> >  - Use perf CPU time to replace perf write data overhead
> >  - Refine the formula of overhead evaluation
> >  - Refine perf script
> >
> > Kan Liang (6):
> >   perf/core: Introduce PERF_RECORD_OVERHEAD
> >   perf/core: Add PERF_EVENT_IOC_STAT to control overhead statistics
> >   perf/x86: implement overhead stat for x86 pmu
> >   perf/core: calculate multiplexing overhead
> >   perf/core: calculate side-band events overhead
> >   perf/x86: calculate sampling overhead
> >
> >  arch/x86/events/core.c          | 45 +++++++++++++++++++++++-
> >  include/linux/perf_event.h      | 12 +++++++
> >  include/uapi/linux/perf_event.h | 44 ++++++++++++++++++++++-
> >  kernel/events/core.c            | 77
> > ++++++++++++++++++++++++++++++++++++++++-
> >  4 files changed, 175 insertions(+), 3 deletions(-)
> >
> > --
> > 2.4.3