perf events: Add generic front-end and back-end stalled cycle event definitions
Subsystems (2)
Commit-ID: 8f62242246351b5a4bc0c1f00c0c7003edea128a Gitweb: http://git.kernel.org/tip/8f62242246351b5a4bc0c1f00c0c7003edea128a Author: Ingo Molnar <[email protected]> AuthorDate: Fri, 29 Apr 2011 13:19:47 +0200 Committer: Ingo Molnar <[email protected]> CommitDate: Fri, 29 Apr 2011 14:23:58 +0200
perf events: Add generic front-end and back-end stalled cycle event definitions
Add two generic hardware events: front-end and back-end stalled cycles.
These events measure conditions when the CPU is executing code but its capabilities are not fully utilized. Understanding such situations and analyzing them is an important sub-task of code optimization workflows.
Both events limit performance: most front end stalls tend to be caused by branch misprediction or instruction fetch cachemisses, backend stalls can be caused by various resource shortages or inefficient instruction scheduling.
Front-end stalls are the more important ones: code cannot run fast if the instruction stream is not being kept up.
An over-utilized back-end can cause front-end stalls and thus has to be kept an eye on as well.
The exact composition is very program logic and instruction mix dependent.
We use the terms 'stall', 'front-end' and 'back-end' loosely and try to use the best available events from specific CPUs that approximate these concepts.
--- arch/x86/kernel/cpu/perf_event_intel.c | 2 +- include/linux/perf_event.h | 3 ++- 2 files changed, 3 insertions(+), 2 deletions(-)
arch/x86/kernel/cpu/perf_event_intel.c +1 -1
@@ -1414,7 +1414,7 @@ static __init int intel_pmu_init(void) x86_pmu.extra_regs = intel_nehalem_extra_regs;/* Install the stalled-cycles event: UOPS_EXECUTED.CORE_ACTIVE_CYCLES,c=1,i=1 */ - intel_perfmon_event_map[PERF_COUNT_HW_STALLED_CYCLES ] =0x1803fb1 ; + intel_perfmon_event_map[PERF_COUNT_HW_STALLED_CYCLES_BACKEND ] =0x1803fb1 ;if (ebx &0x40 ) {/*
include/linux/perf_event.h +2 -1
@@ -52,7 +52,8 @@ enum perf_hw_id {PERF_COUNT_HW_BRANCH_INSTRUCTIONS =4 ,PERF_COUNT_HW_BRANCH_MISSES =5 ,PERF_COUNT_HW_BUS_CYCLES =6 , -PERF_COUNT_HW_STALLED_CYCLES =7 , +PERF_COUNT_HW_STALLED_CYCLES_FRONTEND =7 , +PERF_COUNT_HW_STALLED_CYCLES_BACKEND =8 , PERF_COUNT_HW_MAX, /* non-ABI */ };