[PATCH V9 1/6] tracing: add a possibility of exporting function trace to other places instead of ring buffer only

On 23 November 2016 at 06:39, Steven Rostedt [off-list ref] wrote:

On Mon, 21 Nov 2016 15:57:18 +0800
Chunyan Zhang [off-list ref] wrote:

quoted

Currently Function traces can be only exported to ring buffer, this
patch added trace_export concept which can process traces and export
them to a registered destination as an addition to the current only
one output of Ftrace - i.e. ring buffer.

In this way, if we want Function traces to be sent to other destination
rather than ring buffer only, we just need to register a new trace_export
and implement its own .write() function for writing traces to storage.

With this patch, only Function trace (trace type is TRACE_FN)
is supported.

Signed-off-by: Chunyan Zhang <redacted>
---
 include/linux/trace.h |  28 +++++++++++
 kernel/trace/trace.c  | 129 +++++++++++++++++++++++++++++++++++++++++++++++++-
 2 files changed, 156 insertions(+), 1 deletion(-)
 create mode 100644 include/linux/trace.h

diff --git a/include/linux/trace.h b/include/linux/trace.h
new file mode 100644
index 0000000..9330a58
--- /dev/null
+++ b/include/linux/trace.h

@@ -0,0 +1,28 @@
+#ifndef _LINUX_TRACE_H
+#define _LINUX_TRACE_H
+
+#ifdef CONFIG_TRACING
+/*
+ * The trace export - an export of Ftrace output. The trace_export
+ * can process traces and export them to a registered destination as
+ * an addition to the current only output of Ftrace - i.e. ring buffer.
+ *
+ * If you want traces to be sent to some other place rather than ring
+ * buffer only, just need to register a new trace_export and implement
+ * its own .write() function for writing traces to the storage.
+ *
+ * next              - pointer to the next trace_export
+ * write     - copy traces which have been delt with ->commit() to
+ *             the destination
+ */
+struct trace_export {
+     struct trace_export __rcu       *next;
+     void (*write)(const void *, unsigned int);
+};
+
+int register_ftrace_export(struct trace_export *export);
+int unregister_ftrace_export(struct trace_export *export);
+
+#endif       /* CONFIG_TRACING */
+
+#endif       /* _LINUX_TRACE_H */

diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
index 8696ce6..038291d 100644
--- a/kernel/trace/trace.c
+++ b/kernel/trace/trace.c

@@ -40,6 +40,7 @@
 #include <linux/poll.h>
 #include <linux/nmi.h>
 #include <linux/fs.h>
+#include <linux/trace.h>
 #include <linux/sched/rt.h>

 #include "trace.h"

@@ -2128,6 +2129,129 @@ void trace_buffer_unlock_commit_regs(struct trace_array *tr,
      ftrace_trace_userstack(buffer, flags, pc);
 }

+static void
+trace_process_export(struct trace_export *export,
+            struct ring_buffer_event *event)
+{
+     struct trace_entry *entry;
+     unsigned int size = 0;
+
+     entry = ring_buffer_event_data(event);
+     size = ring_buffer_event_length(event);
+     export->write(entry, size);
+}
+
+static DEFINE_MUTEX(ftrace_export_lock);
+
+static struct trace_export __rcu *ftrace_exports_list __read_mostly;
+
+static DEFINE_STATIC_KEY_FALSE(ftrace_exports_enabled);
+
+static inline void ftrace_exports_enable(void)
+{
+     static_branch_enable(&ftrace_exports_enabled);
+}
+
+static inline void ftrace_exports_disable(void)
+{
+     static_branch_disable(&ftrace_exports_enabled);
+}
+
+void ftrace_exports(struct ring_buffer_event *event)

I'm currently testing the patches, but is there a reason that
ftrace_exports() is not static?

At present  ftrace_exports() is only used by function trace though,
but I hope it can be used by other traces when it needed.
So I didn't mark it with static, but if you think it should better be
static for the time being, I can revise that.

Thanks,
Chunyan

-- Steve

quoted

+{
+     struct trace_export *export;
+
+     preempt_disable_notrace();
+
+     export = rcu_dereference_raw_notrace(ftrace_exports_list);
+     while (export) {
+             trace_process_export(export, event);
+             export = rcu_dereference_raw_notrace(export->next);
+     }
+
+     preempt_enable_notrace();
+}
+
+static inline void
+add_trace_export(struct trace_export **list, struct trace_export *export)
+{
+     rcu_assign_pointer(export->next, *list);
+     /*
+      * We are entering export into the list but another
+      * CPU might be walking that list. We need to make sure
+      * the export->next pointer is valid before another CPU sees
+      * the export pointer included into the list.
+      */
+     rcu_assign_pointer(*list, export);
+}
+
+static inline int
+rm_trace_export(struct trace_export **list, struct trace_export *export)
+{
+     struct trace_export **p;
+
+     for (p = list; *p != NULL; p = &(*p)->next)
+             if (*p == export)
+                     break;
+
+     if (*p != export)
+             return -1;
+
+     rcu_assign_pointer(*p, (*p)->next);
+
+     return 0;
+}
+
+static inline void
+add_ftrace_export(struct trace_export **list, struct trace_export *export)
+{
+     if (*list == NULL)
+             ftrace_exports_enable();
+
+     add_trace_export(list, export);
+}
+
+static inline int
+rm_ftrace_export(struct trace_export **list, struct trace_export *export)
+{
+     int ret;
+
+     ret = rm_trace_export(list, export);
+     if (*list == NULL)
+             ftrace_exports_disable();
+
+     return ret;
+}
+
+int register_ftrace_export(struct trace_export *export)
+{
+     if (WARN_ON_ONCE(!export->write))
+             return -1;
+
+     mutex_lock(&ftrace_export_lock);
+
+     add_ftrace_export(&ftrace_exports_list, export);
+
+     mutex_unlock(&ftrace_export_lock);
+
+     return 0;
+}
+EXPORT_SYMBOL_GPL(register_ftrace_export);
+
+int unregister_ftrace_export(struct trace_export *export)
+{
+     int ret;
+
+     mutex_lock(&ftrace_export_lock);
+
+     ret = rm_ftrace_export(&ftrace_exports_list, export);
+
+     mutex_unlock(&ftrace_export_lock);
+
+     return ret;
+}
+EXPORT_SYMBOL_GPL(unregister_ftrace_export);
+
 void
 trace_function(struct trace_array *tr,
             unsigned long ip, unsigned long parent_ip, unsigned long flags,

@@ -2146,8 +2270,11 @@ trace_function(struct trace_array *tr,
      entry->ip                       = ip;
      entry->parent_ip                = parent_ip;

-     if (!call_filter_check_discard(call, entry, buffer, event))
+     if (!call_filter_check_discard(call, entry, buffer, event)) {
+             if (static_branch_unlikely(&ftrace_exports_enabled))
+                     ftrace_exports(event);
              __buffer_unlock_commit(buffer, event);
+     }
 }

 #ifdef CONFIG_STACKTRACE