[PATCH] Simple LTTng userspace events through debugfs

From: Mathieu Desnoyers
Date: Mon Dec 15 2008 - 18:10:57 EST

Hi,

I looked at kmemtrace and noticed that the presence of a debugfs
"marker" file, which lets userspace write events into the trace buffers.

So I just did the same with a

/debugfs/ltt/write_event

file in LTTng. This should be OK for userspace tracing which does not
require high throughput. It allows userspace to write a string up to
4095-bytes long into the channel named "userspace", event named "event".

This new feature is available in LTTng 0.68 (for kernel 2.6.27.9). For
the curious, I provide the patch applied on top of the LTTng tree below.

Mathieu


LTTng userspace event

Add userspace event support to LTTng.

Simply has to write to :

/debugfs/ltt/write_event

E.g. :

echo "Error X happened !" > /debugfs/ltt/write_event

(assuming debugfs is mounted under /debugfs)

Todo :
Maybe use ltt_relay_user_blocking to block if channel is full rather than losing
an event ? Be careful about effect of stopped tracing on userspace...

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxx>
---
ltt/Kconfig | 9 +++
ltt/Makefile | 1
ltt/ltt-userspace-event.c | 120 ++++++++++++++++++++++++++++++++++++++++++++++
3 files changed, 130 insertions(+)

Index: linux-2.6-lttng/ltt/Kconfig
===================================================================
--- linux-2.6-lttng.orig/ltt/Kconfig 2008-12-15 16:18:04.000000000 -0500
+++ linux-2.6-lttng/ltt/Kconfig 2008-12-15 16:18:08.000000000 -0500
@@ -125,6 +125,15 @@ config LTT_DEBUG_EVENT_SIZE
help
Tracer-internal option to help debugging event type encoding problems.

+config LTT_USERSPACE_EVENT
+ tristate "Support logging events from userspace"
+ depends on LTT_TRACER
+ depends on LTT_TRACEPROBES
+ default m
+ help
+ This option lets userspace write text events in
+ /debugfs/ltt/write_event.
+
config LTT_VMCORE
bool "Support trace extraction from crash dump"
default y
Index: linux-2.6-lttng/ltt/Makefile
===================================================================
--- linux-2.6-lttng.orig/ltt/Makefile 2008-12-15 16:18:04.000000000 -0500
+++ linux-2.6-lttng/ltt/Makefile 2008-12-15 16:18:08.000000000 -0500
@@ -5,6 +5,7 @@
obj-$(CONFIG_MARKERS) += ltt-channels.o
obj-$(CONFIG_LTT) += ltt-core.o
obj-$(CONFIG_LTT_TRACER) += ltt-tracer.o
+obj-$(CONFIG_LTT_USERSPACE_EVENT) += ltt-userspace-event.o
obj-$(CONFIG_LTT_RELAY) += ltt-relay.o
obj-$(CONFIG_LTT_RELAY_LOCKED) += ltt-relay-locked.o
obj-$(CONFIG_LTT_RELAY_ALLOC) += ltt-relay-alloc.o
Index: linux-2.6-lttng/ltt/ltt-userspace-event.c
===================================================================
--- /dev/null 1970-01-01 00:00:00.000000000 +0000
+++ linux-2.6-lttng/ltt/ltt-userspace-event.c 2008-12-15 17:19:33.000000000 -0500
@@ -0,0 +1,120 @@
+/*
+ * Copyright (C) 2008 Mathieu Desnoyers
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation; either version 2 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program; if not, write to the Free Software
+ * Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA.
+ */
+
+#include <linux/module.h>
+#include <linux/marker.h>
+#include <linux/uaccess.h>
+#include <linux/gfp.h>
+#include <linux/fs.h>
+#include <linux/debugfs.h>
+
+#include "probes/ltt-type-serializer.h"
+
+#define LTT_WRITE_EVENT_FILE "write_event"
+
+DEFINE_MARKER(userspace, event, "string %s");
+static struct dentry *ltt_event_file;
+
+/**
+ * write_event: write a userspace string into the trace stream
+ * @file: file pointer
+ * @user_buf: user string
+ * @count: length to copy, including the final NULL
+ * @ppos: unused
+ *
+ * Copy at most a 4095-bytes string into a trace event, in channel "userspace",
+ * event "event". Returns -E2BIG if string is larger than 4095kB. Last byte is
+ * kept for NULL character.
+ * On success, returns the number of bytes copied (without NULL character).
+ */
+static ssize_t write_event(struct file *file, const char __user *user_buf,
+ size_t count, loff_t *ppos)
+{
+ struct marker *marker;
+ char *page;
+ int ret;
+
+ if (count > PAGE_SIZE - 1) {
+ ret = -E2BIG;
+ goto string_out;
+ }
+ page = (char *)__get_free_page(GFP_TEMPORARY);
+ if (!page) {
+ ret = -ENOMEM;
+ goto string_out;
+ }
+ ret = copy_from_user(page, user_buf, count);
+ if (ret) {
+ ret = -EFAULT;
+ goto string_err;
+ }
+ page[count] = '\0';
+
+ marker = &GET_MARKER(userspace, event);
+ ltt_specialized_trace(marker, marker->single.probe_private,
+ page, count + 1, sizeof(char));
+ ret = count;
+string_err:
+ free_page((unsigned long)page);
+string_out:
+ return ret;
+}
+
+static struct file_operations ltt_userspace_operations = {
+ .write = write_event,
+};
+
+static int __init ltt_userspace_init(void)
+{
+ struct dentry *ltt_root_dentry;
+ int err = 0;
+
+ ltt_root_dentry = get_ltt_root();
+ if (!ltt_root_dentry) {
+ err = -ENOENT;
+ goto err_no_root;
+ }
+
+ ltt_event_file = debugfs_create_file(LTT_WRITE_EVENT_FILE,
+ S_IWUGO,
+ ltt_root_dentry,
+ NULL,
+ &ltt_userspace_operations);
+ if (IS_ERR(ltt_event_file) || !ltt_event_file) {
+ printk(KERN_ERR
+ "ltt_userspace_init: failed to create file %s\n",
+ LTT_WRITE_EVENT_FILE);
+ err = -EPERM;
+ goto err_no_file;
+ }
+err_no_file:
+err_no_root:
+ return err;
+}
+
+static void __exit ltt_userspace_exit(void)
+{
+ debugfs_remove(ltt_event_file);
+}
+
+module_init(ltt_userspace_init);
+module_exit(ltt_userspace_exit);
+
+MODULE_LICENSE("GPL");
+MODULE_AUTHOR("Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxx>");
+MODULE_DESCRIPTION("Linux Trace Toolkit Userspace Event");


--
Mathieu Desnoyers
OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F BA06 3F25 A8FE 3BAE 9A68
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