[PATCH V7 1/3] tracing: add a possibility of exporting function trace to other places instead of ring buffer only

On 14 November 2016 at 23:59, Steven Rostedt [off-list ref] wrote:

On Fri, 21 Oct 2016 20:13:13 +0800
Chunyan Zhang [off-list ref] wrote:

quoted

On 18 October 2016 at 23:44, Steven Rostedt [off-list ref] wrote:

quoted

On Tue, 18 Oct 2016 16:08:58 +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.

This is getting better, but I still have some nits.

Thanks.

quoted

quoted

Signed-off-by: Chunyan Zhang <redacted>
---
 include/linux/trace.h |  28 +++++++++++
 kernel/trace/trace.c  | 132 +++++++++++++++++++++++++++++++++++++++++++++++++-
 2 files changed, 159 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..eb1c5b8
--- /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 char *, unsigned int);

Why const char*? Why not const void *? This will never be a string.

Will revise this.

quoted

quoted

+};
+
+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..db94ec1 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 +2129,132 @@ 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);
+
+     if (export->write)
+             export->write((char *)entry, size);

Is there ever going to be a time where export->write wont be set?

There hasn't been since only one trace_export (i.e. stm_ftrace) was
added in this patch-set , I just wanted to make sure the write() has
been set before registering trace_export like what I added in 2/3 of
this series.

quoted

And if there is, this can be racy. As in


        CPU 0:                  CPU 1:
        ------                  ------
        if (export->write)

                                export->write = NULL;

Is there going to be this kind of use case? Why some one needs to
change export->write() rather than register a new trace_export?

Then why have a

        if (export->write)


Is there every going to be a case where export will not have a write
function?

There shouldn't be.

I can move this if statement to the register_ftrace_export() to ensure
users won't wrongly use it, that's saying the write() of trace_export
has been set before being registered to 'ftrace_exports_list'.

Thanks,
Chunyan

-- Steve

quoted

I probably haven't understood your point thoroughly, please correct me
if my guess was wrong.


Thanks for the review,
Chunyan

quoted

        export->write(entry, size);

        BOOM!


-- Steve