perfcounters: add context switch counter

Impact: add new feature, new sw counter

Add a counter that counts the number of context-switches a task
is doing.

Signed-off-by: Ingo Molnar <mingo@elte.hu>

2 files changed, 53 insertions, 2 deletions

diff --git a/include/linux/perf_counter.h b/include/linux/perf_counter.h
index e5d25bf8f74e..d2a16563415f 100644
--- a/include/linux/perf_counter.h
+++ b/include/linux/perf_counter.h
@@ -53,8 +53,8 @@ enum hw_event_types {
         /*
          * Future software events:
          */
-        /* PERF_COUNT_PAGE_FAULTS       = -3,
-           PERF_COUNT_CONTEXT_SWITCHES  = -4, */
+        PERF_COUNT_PAGE_FAULTS          = -3,
+        PERF_COUNT_CONTEXT_SWITCHES     = -4,
 };
 
 /*
diff --git a/kernel/perf_counter.c b/kernel/perf_counter.c
index 1f81cde0dc43..09287091c526 100644
--- a/kernel/perf_counter.c
+++ b/kernel/perf_counter.c
@@ -888,6 +888,54 @@ static const struct hw_perf_counter_ops perf_ops_task_clock = {
         .hw_perf_counter_read           = task_clock_perf_counter_read,
 };
 
+static u64 get_context_switches(void)
+{
+        struct task_struct *curr = current;
+
+        return curr->nvcsw + curr->nivcsw;
+}
+
+static void context_switches_perf_counter_update(struct perf_counter *counter)
+{
+        u64 prev, now;
+        s64 delta;
+
+        prev = atomic64_read(&counter->hw.prev_count);
+        now = get_context_switches();
+
+        atomic64_set(&counter->hw.prev_count, now);
+
+        delta = now - prev;
+        if (WARN_ON_ONCE(delta < 0))
+                delta = 0;
+
+        atomic64_add(delta, &counter->count);
+}
+
+static void context_switches_perf_counter_read(struct perf_counter *counter)
+{
+        context_switches_perf_counter_update(counter);
+}
+
+static void context_switches_perf_counter_enable(struct perf_counter *counter)
+{
+        /*
+         * ->nvcsw + curr->nivcsw is a per-task value already,
+         * so we dont have to clear it on switch-in.
+         */
+}
+
+static void context_switches_perf_counter_disable(struct perf_counter *counter)
+{
+        context_switches_perf_counter_update(counter);
+}
+
+static const struct hw_perf_counter_ops perf_ops_context_switches = {
+        .hw_perf_counter_enable         = context_switches_perf_counter_enable,
+        .hw_perf_counter_disable        = context_switches_perf_counter_disable,
+        .hw_perf_counter_read           = context_switches_perf_counter_read,
+};
+
 static const struct hw_perf_counter_ops *
 sw_perf_counter_init(struct perf_counter *counter)
 {
@@ -900,6 +948,9 @@ sw_perf_counter_init(struct perf_counter *counter)
         case PERF_COUNT_TASK_CLOCK:
                 hw_ops = &perf_ops_task_clock;
                 break;
+        case PERF_COUNT_CONTEXT_SWITCHES:
+                hw_ops = &perf_ops_context_switches;
+                break;
         default:
                 break;
         }