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Subject[PATCH] perf events: Add stalled cycles generic event - PERF_COUNT_HW_STALLED_CYCLES

* Ingo Molnar <mingo@elte.hu> wrote:

> > [...] If there is an expensive load, you'll see that the load instruction
> > takes many cycles and you can infer that it's a cache miss.
> >
> > Questions app developers typically ask me:
> >
> > * If I fix all my top 5 L3 misses how much faster will my app go?
>
> This has come up: we could add a 'stalled/idle-cycles' generic event - i.e.
> cycles spent without performing useful work in the pipelines. (Resource-stall
> events on Intel CPUs.)

How about something like the patch below?

Ingo
---
Subject: perf events: Add stalled cycles generic event - PERF_COUNT_HW_STALLED_CYCLES
From: Ingo Molnar <mingo@elte.hu>
The new PERF_COUNT_HW_STALLED_CYCLES event tries to approximate
cycles the CPU does nothing useful, because it is stalled on a
cache-miss or some other condition.

Note: this is still a incomplete and will work on Intel Nehalem
CPUs for now, the intel_perfmon_event_map[] needs to be
properly split between the major models.

Also update 'perf stat' to print:

611,527 cycles
400,553 instructions # ( 0.7 instructions per cycle )
77,809 stalled-cycles # ( 12.7% of all cycles )

0.000610987 seconds time elapsed

Signed-off-by: Ingo Molnar <mingo@elte.hu>
---
arch/x86/kernel/cpu/perf_event_intel.c | 2 ++
include/linux/perf_event.h | 1 +
tools/perf/builtin-stat.c | 11 +++++++++--
tools/perf/util/parse-events.c | 1 +
tools/perf/util/python.c | 1 +
5 files changed, 14 insertions(+), 2 deletions(-)
Index: linux/arch/x86/kernel/cpu/perf_event_intel.c
===================================================================
--- linux.orig/arch/x86/kernel/cpu/perf_event_intel.c
+++ linux/arch/x86/kernel/cpu/perf_event_intel.c
@@ -34,6 +34,8 @@ static const u64 intel_perfmon_event_map
[PERF_COUNT_HW_BRANCH_INSTRUCTIONS] = 0x00c4,
[PERF_COUNT_HW_BRANCH_MISSES] = 0x00c5,
[PERF_COUNT_HW_BUS_CYCLES] = 0x013c,
+ [PERF_COUNT_HW_STALLED_CYCLES] = 0xffa2, /* 0xff: All reasons, 0xa2: Resource stalls */
+
};

static struct event_constraint intel_core_event_constraints[] =
Index: linux/include/linux/perf_event.h
===================================================================
--- linux.orig/include/linux/perf_event.h
+++ linux/include/linux/perf_event.h
@@ -52,6 +52,7 @@ 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_MAX, /* non-ABI */
};
Index: linux/tools/perf/builtin-stat.c
===================================================================
--- linux.orig/tools/perf/builtin-stat.c
+++ linux/tools/perf/builtin-stat.c
@@ -442,7 +442,7 @@ static void abs_printout(int cpu, struct
if (total)
ratio = avg / total;

- fprintf(stderr, " # %10.3f IPC ", ratio);
+ fprintf(stderr, " # ( %3.1f instructions per cycle )", ratio);
} else if (perf_evsel__match(evsel, HARDWARE, HW_BRANCH_MISSES) &&
runtime_branches_stats[cpu].n != 0) {
total = avg_stats(&runtime_branches_stats[cpu]);
@@ -450,7 +450,7 @@ static void abs_printout(int cpu, struct
if (total)
ratio = avg * 100 / total;

- fprintf(stderr, " # %10.3f %% ", ratio);
+ fprintf(stderr, " # %10.3f %%", ratio);

} else if (runtime_nsecs_stats[cpu].n != 0) {
total = avg_stats(&runtime_nsecs_stats[cpu]);
@@ -459,6 +459,13 @@ static void abs_printout(int cpu, struct
ratio = 1000.0 * avg / total;

fprintf(stderr, " # %10.3f M/sec", ratio);
+ } else if (perf_evsel__match(evsel, HARDWARE, HW_STALLED_CYCLES)) {
+ total = avg_stats(&runtime_cycles_stats[cpu]);
+
+ if (total)
+ ratio = avg / total * 100.0;
+
+ fprintf(stderr, " # (%5.1f%% of all cycles )", ratio);
}
}

Index: linux/tools/perf/util/parse-events.c
===================================================================
--- linux.orig/tools/perf/util/parse-events.c
+++ linux/tools/perf/util/parse-events.c
@@ -38,6 +38,7 @@ static struct event_symbol event_symbols
{ CHW(BRANCH_INSTRUCTIONS), "branch-instructions", "branches" },
{ CHW(BRANCH_MISSES), "branch-misses", "" },
{ CHW(BUS_CYCLES), "bus-cycles", "" },
+ { CHW(STALLED_CYCLES), "stalled-cycles", "" },

{ CSW(CPU_CLOCK), "cpu-clock", "" },
{ CSW(TASK_CLOCK), "task-clock", "" },
Index: linux/tools/perf/util/python.c
===================================================================
--- linux.orig/tools/perf/util/python.c
+++ linux/tools/perf/util/python.c
@@ -798,6 +798,7 @@ static struct {
{ "COUNT_HW_BRANCH_INSTRUCTIONS", PERF_COUNT_HW_BRANCH_INSTRUCTIONS },
{ "COUNT_HW_BRANCH_MISSES", PERF_COUNT_HW_BRANCH_MISSES },
{ "COUNT_HW_BUS_CYCLES", PERF_COUNT_HW_BUS_CYCLES },
+ { "COUNT_HW_STALLED_CYCLES", PERF_COUNT_HW_STALLED_CYCLES },
{ "COUNT_HW_CACHE_L1D", PERF_COUNT_HW_CACHE_L1D },
{ "COUNT_HW_CACHE_L1I", PERF_COUNT_HW_CACHE_L1I },
{ "COUNT_HW_CACHE_LL", PERF_COUNT_HW_CACHE_LL },

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