Messages in this thread | | | From | Stephane Eranian <> | Subject | [PATCH 0/4] perf_events: add support for Intel reference cycles event | Date | Sun, 11 Dec 2011 00:28:50 +0100 |
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The unhalted_reference_cycles is a very useful event which counts cycles at the reference CPU clock speed (same as TSC if processors supports constant_tsc). That means, it keeps a constant correlation with wall-clock time while the processor is not halted, unlike its counter-part unhalted_core_cycles. The latter event, used by generic PMU event cycles in the perf tool is subject to frequency down-scaling AND Turbo mode fluctuations.
The unhlated_reference_cycles is NOT equivalent to wall clock time because it counts only when the processor is not in halted state. But this limitation impacts only system-wide measurements.
The unhalted_reference_cycles can only be counted on fixed counter 2. As such, it does not have an event encoding. Fixed counters can only measure one (hardwired) event. Yet, with perf_events user manipulates events and not counters, thus each event needs an encoding. That encoding is then used by the kernel to identify the event and schedule it on the proper counter.
Up until now, there was no encoding chosen for unhalted_reference_cycles, thus it could not be scheduled properly and thus it was not supported.
This patch uses the encoding 0x0300. That means event code 0x00, umask 0x3. That event code is not used by Intel on any existing processor. We have also checked with Intel and it seems that they don't plan on using it in the future.
With this patch, it is possible to see the effect of Turbo mode:
$ perf stat -e unhalted_reference_cycles,unhalted_core_cycles ./noploop 1 noploop for 1 seconds
Performance counter stats for './noploop 1':
3,197,640,960 unhalted_reference_cycles 3,595,043,997 unhalted_core_cycles
1.000732233 seconds time elapsed
Signed-off-by: Stephane Eranian <eranian@google.com> ---
Stephane Eranian (4): perf_events: enable raw event support for Intel unhalted_reference_cycles event perf_event: add PERF_COUNT_HW_REF_CPU_CYCLES generic PMU event perf_events: add Intel X86 mapping for PERF_COUNT_HW_REF_CPU_CYCLES perf: add support for PERF_HW_COUNT_REF_CPU_CYCLES
arch/x86/include/asm/perf_event.h | 15 ++++++++------- arch/x86/kernel/cpu/perf_event.c | 8 +++++++- arch/x86/kernel/cpu/perf_event_intel.c | 16 ++++++---------- include/linux/perf_event.h | 1 + tools/perf/util/parse-events.c | 2 ++ 5 files changed, 24 insertions(+), 18 deletions(-)
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