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Message-ID: <20090116120354.A30366@sedona.ch.intel.com>
Date: Fri, 16 Jan 2009 12:03:54 +0100
From: Markus Metzger <markus.t.metzger@...el.com>
To: linux-kernel@...r.kernel.org, mingo@...e.hu, tglx@...utronix.de,
hpa@...or.com
Cc: markus.t.metzger@...el.com, markus.t.metzger@...il.com,
srostedt@...hat.com
Subject: [patch 5/5] x86, ftrace, hw-branch-tracer: documentation
Document the hw-branch-tracer in the ftrace documentation.
Signed-off-by: Markus Metzger <markus.t.metzger@...el.com>
---
Index: ftrace/Documentation/ftrace.txt
===================================================================
--- ftrace.orig/Documentation/ftrace.txt 2008-12-11 11:23:48.000000000 +0100
+++ ftrace/Documentation/ftrace.txt 2009-01-16 10:58:07.000000000 +0100
@@ -165,6 +165,8 @@
nop - This is not a tracer. To remove all tracers from tracing
simply echo "nop" into current_tracer.
+ hw-branch-tracer - traces branches on all cpu's in a circular buffer.
+
Examples of using the tracer
----------------------------
@@ -1152,6 +1154,78 @@
return 0;
}
+
+hw-branch-tracer (x86 only)
+---------------------------
+
+This tracer uses the x86 last branch tracing hardware feature to
+collect a branch trace on all cpus with relatively low overhead.
+
+The tracer uses a fixed-size circular buffer per cpu and only
+traces ring 0 branches. The trace file dumps that buffer in the
+following format:
+
+# tracer: hw-branch-tracer
+#
+# CPU# TO <- FROM
+ 0 scheduler_tick+0xb5/0x1bf <- task_tick_idle+0x5/0x6
+ 2 run_posix_cpu_timers+0x2b/0x72a <- run_posix_cpu_timers+0x25/0x72a
+ 0 scheduler_tick+0x139/0x1bf <- scheduler_tick+0xed/0x1bf
+ 0 scheduler_tick+0x17c/0x1bf <- scheduler_tick+0x148/0x1bf
+ 2 run_posix_cpu_timers+0x9e/0x72a <- run_posix_cpu_timers+0x5e/0x72a
+ 0 scheduler_tick+0x1b6/0x1bf <- scheduler_tick+0x1aa/0x1bf
+
+
+The tracer may be used to dump the trace for the oops'ing cpu on a
+kernel oops into the system log. To enable this, ftrace_dump_on_oops
+must be set. To set ftrace_dump_on_oops, one can either use the sysctl
+function or set it via the proc system interface.
+
+ sysctl kernel.ftrace_dump_on_oops=1
+
+or
+
+ echo 1 > /proc/sys/kernel/ftrace_dump_on_oops
+
+
+Here's an example of such a dump after a null pointer dereference in a
+kernel module:
+
+[57848.105921] BUG: unable to handle kernel NULL pointer dereference at 0000000000000000
+[57848.106019] IP: [<ffffffffa0000006>] open+0x6/0x14 [oops]
+[57848.106019] PGD 2354e9067 PUD 2375e7067 PMD 0
+[57848.106019] Oops: 0002 [#1] SMP
+[57848.106019] last sysfs file: /sys/devices/pci0000:00/0000:00:1e.0/0000:20:05.0/local_cpus
+[57848.106019] Dumping ftrace buffer:
+[57848.106019] ---------------------------------
+[...]
+[57848.106019] 0 chrdev_open+0xe6/0x165 <- cdev_put+0x23/0x24
+[57848.106019] 0 chrdev_open+0x117/0x165 <- chrdev_open+0xfa/0x165
+[57848.106019] 0 chrdev_open+0x120/0x165 <- chrdev_open+0x11c/0x165
+[57848.106019] 0 chrdev_open+0x134/0x165 <- chrdev_open+0x12b/0x165
+[57848.106019] 0 open+0x0/0x14 [oops] <- chrdev_open+0x144/0x165
+[57848.106019] 0 page_fault+0x0/0x30 <- open+0x6/0x14 [oops]
+[57848.106019] 0 error_entry+0x0/0x5b <- page_fault+0x4/0x30
+[57848.106019] 0 error_kernelspace+0x0/0x31 <- error_entry+0x59/0x5b
+[57848.106019] 0 error_sti+0x0/0x1 <- error_kernelspace+0x2d/0x31
+[57848.106019] 0 page_fault+0x9/0x30 <- error_sti+0x0/0x1
+[57848.106019] 0 do_page_fault+0x0/0x881 <- page_fault+0x1a/0x30
+[...]
+[57848.106019] 0 do_page_fault+0x66b/0x881 <- is_prefetch+0x1ee/0x1f2
+[57848.106019] 0 do_page_fault+0x6e0/0x881 <- do_page_fault+0x67a/0x881
+[57848.106019] 0 oops_begin+0x0/0x96 <- do_page_fault+0x6e0/0x881
+[57848.106019] 0 trace_hw_branch_oops+0x0/0x2d <- oops_begin+0x9/0x96
+[...]
+[57848.106019] 0 ds_suspend_bts+0x2a/0xe3 <- ds_suspend_bts+0x1a/0xe3
+[57848.106019] ---------------------------------
+[57848.106019] CPU 0
+[57848.106019] Modules linked in: oops
+[57848.106019] Pid: 5542, comm: cat Tainted: G W 2.6.28 #23
+[57848.106019] RIP: 0010:[<ffffffffa0000006>] [<ffffffffa0000006>] open+0x6/0x14 [oops]
+[57848.106019] RSP: 0018:ffff880235457d48 EFLAGS: 00010246
+[...]
+
+
dynamic ftrace
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