[patch-RFC 26/26] LTTng Kernel Trace Thread Flag API

[Mathieu Desnoyers]

Add an API to set/clear the kernel wide tracing thread flags. Implemented in
kernel/sched.c. Updates thread flags *asynchronously* while holding the tasklist
lock.

Upon fork, the flag must be re-copied while the tasklist lock is held.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxx>
---
 include/linux/sched.h |    3 ++
 kernel/fork.c         |    9 ++++++++
 kernel/sched.c        |   55 ++++++++++++++++++++++++++++++++++++++++++++++++++
 3 files changed, 67 insertions(+)

Index: linux-2.6-lttng/include/linux/sched.h
===================================================================
--- linux-2.6-lttng.orig/include/linux/sched.h	2007-12-05 20:50:29.000000000 -0500
+++ linux-2.6-lttng/include/linux/sched.h	2007-12-05 20:54:32.000000000 -0500
@@ -2000,6 +2000,9 @@ static inline void migration_init(void)
 }
 #endif
 
+extern void clear_kernel_trace_flag_all_tasks(void);
+extern void set_kernel_trace_flag_all_tasks(void);
+
 #endif /* __KERNEL__ */
 
 #endif
Index: linux-2.6-lttng/kernel/fork.c
===================================================================
--- linux-2.6-lttng.orig/kernel/fork.c	2007-12-05 20:54:00.000000000 -0500
+++ linux-2.6-lttng/kernel/fork.c	2007-12-05 20:54:32.000000000 -0500
@@ -1241,6 +1241,15 @@ static struct task_struct *copy_process(
 			!cpu_online(task_cpu(p))))
 		set_task_cpu(p, smp_processor_id());
 
+	/*
+	 * The state of the parent's TIF_KTRACE flag may have changed
+	 * since it was copied in dup_task_struct() so we re-copy it here.
+	 */
+	if (test_thread_flag(TIF_KERNEL_TRACE))
+		set_tsk_thread_flag(p, TIF_KERNEL_TRACE);
+	else
+		clear_tsk_thread_flag(p, TIF_KERNEL_TRACE);
+
 	/* CLONE_PARENT re-uses the old parent */
 	if (clone_flags & (CLONE_PARENT|CLONE_THREAD))
 		p->real_parent = current->real_parent;
Index: linux-2.6-lttng/kernel/sched.c
===================================================================
--- linux-2.6-lttng.orig/kernel/sched.c	2007-12-05 20:54:00.000000000 -0500
+++ linux-2.6-lttng/kernel/sched.c	2007-12-05 20:54:32.000000000 -0500
@@ -7394,3 +7394,58 @@ struct cgroup_subsys cpuacct_subsys = {
 	.subsys_id = cpuacct_subsys_id,
 };
 #endif	/* CONFIG_CGROUP_CPUACCT */
+
+static DEFINE_MUTEX(kernel_trace_mutex);
+static int kernel_trace_refcount;
+
+/**
+ * clear_kernel_trace_flag_all_tasks - clears all TIF_KERNEL_TRACE thread flags.
+ *
+ * This function iterates on all threads in the system to clear their
+ * TIF_KERNEL_TRACE flag. Setting the TIF_KERNEL_TRACE flag with the
+ * tasklist_lock held in copy_process() makes sure that once we finish clearing
+ * the thread flags, all threads have their flags cleared.
+ */
+void clear_kernel_trace_flag_all_tasks(void)
+{
+	struct task_struct *p;
+	struct task_struct *t;
+
+	mutex_lock(&kernel_trace_mutex);
+	if (--kernel_trace_refcount)
+		goto end;
+	read_lock(&tasklist_lock);
+	do_each_thread(p, t) {
+		clear_tsk_thread_flag(t, TIF_KERNEL_TRACE);
+	} while_each_thread(p, t);
+	read_unlock(&tasklist_lock);
+end:
+	mutex_unlock(&kernel_trace_mutex);
+}
+EXPORT_SYMBOL_GPL(clear_kernel_trace_flag_all_tasks);
+
+/**
+ * set_kernel_trace_flag_all_tasks - sets all TIF_KERNEL_TRACE thread flags.
+ *
+ * This function iterates on all threads in the system to set their
+ * TIF_KERNEL_TRACE flag. Setting the TIF_KERNEL_TRACE flag with the
+ * tasklist_lock held in copy_process() makes sure that once we finish setting
+ * the thread flags, all threads have their flags set.
+ */
+void set_kernel_trace_flag_all_tasks(void)
+{
+	struct task_struct *p;
+	struct task_struct *t;
+
+	mutex_lock(&kernel_trace_mutex);
+	if (kernel_trace_refcount++)
+		goto end;
+	read_lock(&tasklist_lock);
+	do_each_thread(p, t) {
+		set_tsk_thread_flag(t, TIF_KERNEL_TRACE);
+	} while_each_thread(p, t);
+	read_unlock(&tasklist_lock);
+end:
+	mutex_unlock(&kernel_trace_mutex);
+}
+EXPORT_SYMBOL_GPL(set_kernel_trace_flag_all_tasks);

-- 
Mathieu Desnoyers
Computer Engineering Ph.D. Student, Ecole Polytechnique de Montreal
OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F  BA06 3F25 A8FE 3BAE 9A68
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