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.

Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Link: http://lkml.kernel.org/n/tip-7y40wib8n000io7hjpn1dsrm@git.kernel.org
Signed-off-by: Ingo Molnar <mingo@elte.hu>

arch/x86/kernel/cpu/perf_event_intel.c

@@ -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

@@ -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 */
 };