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Subject[PATCH 2/5] perf_counter: optimize perf_counter_task_tick()
perf_counter_task_tick() does way too much work to find out there's nothing to
do. Provide an easy short-circuit for the normal case where there are no
counters on the system.

Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
---
kernel/perf_counter.c | 13 +++++++++++--
1 file changed, 11 insertions(+), 2 deletions(-)
Index: linux-2.6/kernel/perf_counter.c
===================================================================
--- linux-2.6.orig/kernel/perf_counter.c
+++ linux-2.6/kernel/perf_counter.c
@@ -39,6 +39,7 @@ int perf_max_counters __read_mostly = 1;
static int perf_reserved_percpu __read_mostly;
static int perf_overcommit __read_mostly = 1;

+static atomic_t nr_counters __read_mostly;
static atomic_t nr_mmap_tracking __read_mostly;
static atomic_t nr_munmap_tracking __read_mostly;
static atomic_t nr_comm_tracking __read_mostly;
@@ -1076,8 +1077,14 @@ static void rotate_ctx(struct perf_count

void perf_counter_task_tick(struct task_struct *curr, int cpu)
{
- struct perf_cpu_context *cpuctx = &per_cpu(perf_cpu_context, cpu);
- struct perf_counter_context *ctx = &curr->perf_counter_ctx;
+ struct perf_cpu_context *cpuctx;
+ struct perf_counter_context *ctx;
+
+ if (!atomic_read(&nr_counters))
+ return;
+
+ cpuctx = &per_cpu(perf_cpu_context, cpu);
+ ctx = &curr->perf_counter_ctx;

perf_counter_cpu_sched_out(cpuctx);
perf_counter_task_sched_out(curr, cpu);
@@ -1197,6 +1204,7 @@ static void free_counter(struct perf_cou
{
perf_pending_sync(counter);

+ atomic_dec(&nr_counters);
if (counter->hw_event.mmap)
atomic_dec(&nr_mmap_tracking);
if (counter->hw_event.munmap)
@@ -2861,6 +2869,7 @@ done:

counter->pmu = pmu;

+ atomic_inc(&nr_counters);
if (counter->hw_event.mmap)
atomic_inc(&nr_mmap_tracking);
if (counter->hw_event.munmap)
--



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