ftrace: fast, scalable, synchronized timestamps

implement globally synchronized, fast and scalable time source for tracing.

Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

1 files changed, 53 insertions, 1 deletions

diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
index e3778ab0d3f7..9a931c7c2da3 100644
--- a/kernel/trace/trace.c
+++ b/kernel/trace/trace.c
@@ -42,9 +42,61 @@ ns2usecs(cycle_t nsec)
         return nsec;
 }
 
+static const int time_sync_freq_max = 128;
+static const cycle_t time_sync_thresh = 100000;
+
+static DEFINE_PER_CPU(cycle_t, time_offset);
+static DEFINE_PER_CPU(cycle_t, prev_cpu_time);
+static DEFINE_PER_CPU(int, time_sync_count);
+static DEFINE_PER_CPU(int, time_sync_freq);
+
+/*
+ * Global lock which we take every now and then to synchronize
+ * the CPUs time. This method is not warp-safe, but it's good
+ * enough to synchronize slowly diverging time sources and thus
+ * it's good enough for tracing:
+ */
+static DEFINE_SPINLOCK(time_sync_lock);
+static cycle_t prev_global_time;
+
+static notrace cycle_t __ftrace_now_sync(cycles_t time, int cpu)
+{
+        unsigned long flags;
+
+        spin_lock_irqsave(&time_sync_lock, flags);
+
+        /*
+         * Update the synchronization frequency:
+         */
+        if (per_cpu(time_sync_freq, cpu) < time_sync_freq_max)
+                per_cpu(time_sync_freq, cpu) *= 2;
+        per_cpu(time_sync_count, cpu) = per_cpu(time_sync_freq, cpu);
+
+        if (time < prev_global_time) {
+                per_cpu(time_offset, cpu) += prev_global_time - time;
+                time = prev_global_time;
+        } else {
+                prev_global_time = time;
+        }
+
+        spin_unlock_irqrestore(&time_sync_lock, flags);
+
+        return time;
+}
+
 notrace cycle_t ftrace_now(int cpu)
 {
-        return cpu_clock(cpu);
+        cycle_t prev_cpu_time, time, delta_time;
+
+        prev_cpu_time = per_cpu(prev_cpu_time, cpu);
+        time = sched_clock() + per_cpu(time_offset, cpu);
+        delta_time = time-prev_cpu_time;
+
+        if (unlikely(delta_time > time_sync_thresh ||
+                                --per_cpu(time_sync_count, cpu) <= 0))
+                time = __ftrace_now_sync(time, cpu);
+
+        return time;
 }
 
 static atomic_t                 tracer_counter;