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diff --git a/Documentation/ftrace.txt b/Documentation/ftrace.txt
index 803b1318b13d..758fb42a1b68 100644
--- a/Documentation/ftrace.txt
+++ b/Documentation/ftrace.txt
@@ -165,6 +165,8 @@ Here is the list of current tracers that may be configured.
165 nop - This is not a tracer. To remove all tracers from tracing 165 nop - This is not a tracer. To remove all tracers from tracing
166 simply echo "nop" into current_tracer. 166 simply echo "nop" into current_tracer.
167 167
168 hw-branch-tracer - traces branches on all cpu's in a circular buffer.
169
168 170
169Examples of using the tracer 171Examples of using the tracer
170---------------------------- 172----------------------------
@@ -1152,6 +1154,78 @@ int main (int argc, char **argv)
1152 return 0; 1154 return 0;
1153} 1155}
1154 1156
1157
1158hw-branch-tracer (x86 only)
1159---------------------------
1160
1161This tracer uses the x86 last branch tracing hardware feature to
1162collect a branch trace on all cpus with relatively low overhead.
1163
1164The tracer uses a fixed-size circular buffer per cpu and only
1165traces ring 0 branches. The trace file dumps that buffer in the
1166following format:
1167
1168# tracer: hw-branch-tracer
1169#
1170# CPU# TO <- FROM
1171 0 scheduler_tick+0xb5/0x1bf <- task_tick_idle+0x5/0x6
1172 2 run_posix_cpu_timers+0x2b/0x72a <- run_posix_cpu_timers+0x25/0x72a
1173 0 scheduler_tick+0x139/0x1bf <- scheduler_tick+0xed/0x1bf
1174 0 scheduler_tick+0x17c/0x1bf <- scheduler_tick+0x148/0x1bf
1175 2 run_posix_cpu_timers+0x9e/0x72a <- run_posix_cpu_timers+0x5e/0x72a
1176 0 scheduler_tick+0x1b6/0x1bf <- scheduler_tick+0x1aa/0x1bf
1177
1178
1179The tracer may be used to dump the trace for the oops'ing cpu on a
1180kernel oops into the system log. To enable this, ftrace_dump_on_oops
1181must be set. To set ftrace_dump_on_oops, one can either use the sysctl
1182function or set it via the proc system interface.
1183
1184 sysctl kernel.ftrace_dump_on_oops=1
1185
1186or
1187
1188 echo 1 > /proc/sys/kernel/ftrace_dump_on_oops
1189
1190
1191Here's an example of such a dump after a null pointer dereference in a
1192kernel module:
1193
1194[57848.105921] BUG: unable to handle kernel NULL pointer dereference at 0000000000000000
1195[57848.106019] IP: [<ffffffffa0000006>] open+0x6/0x14 [oops]
1196[57848.106019] PGD 2354e9067 PUD 2375e7067 PMD 0
1197[57848.106019] Oops: 0002 [#1] SMP
1198[57848.106019] last sysfs file: /sys/devices/pci0000:00/0000:00:1e.0/0000:20:05.0/local_cpus
1199[57848.106019] Dumping ftrace buffer:
1200[57848.106019] ---------------------------------
1201[...]
1202[57848.106019] 0 chrdev_open+0xe6/0x165 <- cdev_put+0x23/0x24
1203[57848.106019] 0 chrdev_open+0x117/0x165 <- chrdev_open+0xfa/0x165
1204[57848.106019] 0 chrdev_open+0x120/0x165 <- chrdev_open+0x11c/0x165
1205[57848.106019] 0 chrdev_open+0x134/0x165 <- chrdev_open+0x12b/0x165
1206[57848.106019] 0 open+0x0/0x14 [oops] <- chrdev_open+0x144/0x165
1207[57848.106019] 0 page_fault+0x0/0x30 <- open+0x6/0x14 [oops]
1208[57848.106019] 0 error_entry+0x0/0x5b <- page_fault+0x4/0x30
1209[57848.106019] 0 error_kernelspace+0x0/0x31 <- error_entry+0x59/0x5b
1210[57848.106019] 0 error_sti+0x0/0x1 <- error_kernelspace+0x2d/0x31
1211[57848.106019] 0 page_fault+0x9/0x30 <- error_sti+0x0/0x1
1212[57848.106019] 0 do_page_fault+0x0/0x881 <- page_fault+0x1a/0x30
1213[...]
1214[57848.106019] 0 do_page_fault+0x66b/0x881 <- is_prefetch+0x1ee/0x1f2
1215[57848.106019] 0 do_page_fault+0x6e0/0x881 <- do_page_fault+0x67a/0x881
1216[57848.106019] 0 oops_begin+0x0/0x96 <- do_page_fault+0x6e0/0x881
1217[57848.106019] 0 trace_hw_branch_oops+0x0/0x2d <- oops_begin+0x9/0x96
1218[...]
1219[57848.106019] 0 ds_suspend_bts+0x2a/0xe3 <- ds_suspend_bts+0x1a/0xe3
1220[57848.106019] ---------------------------------
1221[57848.106019] CPU 0
1222[57848.106019] Modules linked in: oops
1223[57848.106019] Pid: 5542, comm: cat Tainted: G W 2.6.28 #23
1224[57848.106019] RIP: 0010:[<ffffffffa0000006>] [<ffffffffa0000006>] open+0x6/0x14 [oops]
1225[57848.106019] RSP: 0018:ffff880235457d48 EFLAGS: 00010246
1226[...]
1227
1228
1155dynamic ftrace 1229dynamic ftrace
1156-------------- 1230--------------
1157 1231