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

[kan . liang]

From: Kan Liang <kan.liang@xxxxxxxxx>

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(-)

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2.4.3