[patch 3/4] x86, hw-branch-tracer: keep resources on stop

[Markus Metzger]

Distinguish init/reset and start/stop:
init/reset will allocate and release bts tracing resources
stop/start will suspend and resume bts tracing

Return an error on init() if no cpu can be traced.


Signed-off-by: Markus Metzger <markus.t.metzger@xxxxxxxxx>
---

Index: gits/kernel/trace/trace_hw_branches.c
===================================================================
--- gits.orig/kernel/trace/trace_hw_branches.c	2009-03-02 16:15:11.000000000 +0100
+++ gits/kernel/trace/trace_hw_branches.c	2009-03-03 10:17:31.000000000 +0100
@@ -38,17 +38,18 @@ static DEFINE_PER_CPU(unsigned char[SIZE
 #define this_tracer per_cpu(tracer, smp_processor_id())
 #define this_buffer per_cpu(buffer, smp_processor_id())
 
-static int __read_mostly trace_hw_branches_enabled;
+static int trace_hw_branches_enabled __read_mostly;
+static int trace_hw_branches_suspended __read_mostly;
 static struct trace_array *hw_branch_trace __read_mostly;
 
 
 /*
- * Start tracing on the current cpu.
+ * Initialize the tracer for the current cpu.
  * The argument is ignored.
  *
  * pre: bts_tracer_lock must be locked.
  */
-static void bts_trace_start_cpu(void *arg)
+static void bts_trace_init_cpu(void *arg)
 {
 	if (this_tracer)
 		ds_release_bts(this_tracer);
@@ -63,23 +64,39 @@ static void bts_trace_start_cpu(void *ar
 	}
 }
 
-static void bts_trace_start(struct trace_array *tr)
+static int bts_trace_init(struct trace_array *tr)
 {
+	int cpu, avail;
+
 	spin_lock(&bts_tracer_lock);
 
-	on_each_cpu(bts_trace_start_cpu, NULL, 1);
-	trace_hw_branches_enabled = 1;
+	hw_branch_trace = tr;
+
+	on_each_cpu(bts_trace_init_cpu, NULL, 1);
+
+	/* Check on how many cpus we could enable tracing */
+	avail = 0;
+	for_each_online_cpu(cpu)
+		if (per_cpu(tracer, cpu))
+			avail++;
+
+	trace_hw_branches_enabled = (avail ? 1 : 0);
+	trace_hw_branches_suspended = 0;
 
 	spin_unlock(&bts_tracer_lock);
+
+
+	/* If we could not enable tracing on a single cpu, we fail. */
+	return avail ? 0 : -EOPNOTSUPP;
 }
 
 /*
- * Stop tracing on the current cpu.
+ * Release the tracer for the current cpu.
  * The argument is ignored.
  *
  * pre: bts_tracer_lock must be locked.
  */
-static void bts_trace_stop_cpu(void *arg)
+static void bts_trace_release_cpu(void *arg)
 {
 	if (this_tracer) {
 		ds_release_bts(this_tracer);
@@ -87,12 +104,57 @@ static void bts_trace_stop_cpu(void *arg
 	}
 }
 
-static void bts_trace_stop(struct trace_array *tr)
+static void bts_trace_reset(struct trace_array *tr)
 {
 	spin_lock(&bts_tracer_lock);
 
+	on_each_cpu(bts_trace_release_cpu, NULL, 1);
 	trace_hw_branches_enabled = 0;
-	on_each_cpu(bts_trace_stop_cpu, NULL, 1);
+	trace_hw_branches_suspended = 0;
+
+	spin_unlock(&bts_tracer_lock);
+}
+
+/*
+ * Resume tracing on the current cpu.
+ * The argument is ignored.
+ *
+ * pre: bts_tracer_lock must be locked.
+ */
+static void bts_trace_resume_cpu(void *arg)
+{
+	if (this_tracer)
+		ds_resume_bts(this_tracer);
+}
+
+static void bts_trace_start(struct trace_array *tr)
+{
+	spin_lock(&bts_tracer_lock);
+
+	on_each_cpu(bts_trace_resume_cpu, NULL, 1);
+	trace_hw_branches_suspended = 0;
+
+	spin_unlock(&bts_tracer_lock);
+}
+
+/*
+ * Suspend tracing on the current cpu.
+ * The argument is ignored.
+ *
+ * pre: bts_tracer_lock must be locked.
+ */
+static void bts_trace_suspend_cpu(void *arg)
+{
+	if (this_tracer)
+		ds_suspend_bts(this_tracer);
+}
+
+static void bts_trace_stop(struct trace_array *tr)
+{
+	spin_lock(&bts_tracer_lock);
+
+	on_each_cpu(bts_trace_suspend_cpu, NULL, 1);
+	trace_hw_branches_suspended = 1;
 
 	spin_unlock(&bts_tracer_lock);
 }
@@ -110,10 +172,14 @@ static int __cpuinit bts_hotcpu_handler(
 	switch (action) {
 	case CPU_ONLINE:
 	case CPU_DOWN_FAILED:
-		smp_call_function_single(cpu, bts_trace_start_cpu, NULL, 1);
+		smp_call_function_single(cpu, bts_trace_init_cpu, NULL, 1);
+
+		if (trace_hw_branches_suspended)
+			smp_call_function_single(cpu, bts_trace_suspend_cpu,
+						 NULL, 1);
 		break;
 	case CPU_DOWN_PREPARE:
-		smp_call_function_single(cpu, bts_trace_stop_cpu, NULL, 1);
+		smp_call_function_single(cpu, bts_trace_release_cpu, NULL, 1);
 		break;
 	}
 
@@ -126,20 +192,6 @@ static struct notifier_block bts_hotcpu_
 	.notifier_call = bts_hotcpu_handler
 };
 
-static int bts_trace_init(struct trace_array *tr)
-{
-	hw_branch_trace = tr;
-
-	bts_trace_start(tr);
-
-	return 0;
-}
-
-static void bts_trace_reset(struct trace_array *tr)
-{
-	bts_trace_stop(tr);
-}
-
 static void bts_trace_print_header(struct seq_file *m)
 {
 	seq_puts(m, "# CPU#        TO  <-  FROM\n");
@@ -276,7 +328,8 @@ void trace_hw_branch_oops(void)
 {
 	spin_lock(&bts_tracer_lock);
 
-	trace_bts_cpu(hw_branch_trace);
+	if (trace_hw_branches_enabled)
+		trace_bts_cpu(hw_branch_trace);
 
 	spin_unlock(&bts_tracer_lock);
 }
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