CHK     include/linux/version.h
  CHK     include/linux/utsrelease.h
  CC      scripts/mod/empty.o
/bin/sh: /usr/src/25/scripts/recordmcount.pl: Permission denied

We shouldn't assume that files have their `x' bits set.  There are various
ways in which file permissions get lost, including use of patch(1).

It might not be correct to assume that perl lives in $PATH?
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NIngo Molnar <mingo@elte.hu>

The --globalize-symbols option came out in objcopy version 2.17.
If the kernel is being compiled on a system with a lower version of
objcopy, then we can not use the globalize / localize trick to
link to symbols pointing to local functions.

This patch tests the version of objcopy and will only use the trick
if the version is greater than or equal to 2.17. Otherwise, if an
object has only local functions within a section, it will give a
nice warning and recommend the user to upgrade their objcopy.

Leaving the symbols unrecorded is not that big of a deal, since the
mcount record method changes the actual mcount code to be a simple
"ret" without recording registers or anything.
Reported-by: NStephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: NSteven Rostedt <srostedt@redhat.com>
Signed-off-by: NIngo Molnar <mingo@elte.hu>

enclose the argument in parenthesis. (especially since we cast it,
which is a high prio operation)
Signed-off-by: NIngo Molnar <mingo@elte.hu>

After disabling FTRACE_MCOUNT_RECORD via a patch, a dormant build
failure surfaced:

 kernel/trace/ftrace.c: In function 'ftrace_record_ip':
 kernel/trace/ftrace.c:416: error: incompatible type for argument 1 of '_spin_lock_irqsave'
 kernel/trace/ftrace.c:433: error: incompatible type for argument 1 of '_spin_lock_irqsave'

Introduced by commit 6dad8e07f4c10b17b038e84d29f3ca41c2e55cd0 ("ftrace:
add necessary locking for ftrace records").
Signed-off-by: NStephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: NIngo Molnar <mingo@elte.hu>

The modification of code is performed either by kstop_machine, before
SMP starts, or on module code before the module is executed. There is
no reason to do the modifications from assembly. The copy to and from
user functions are sufficient and produces cleaner and easier to read
code.

Thanks to Benjamin Herrenschmidt for suggesting the idea.
Signed-off-by: NSteven Rostedt <srostedt@redhat.com>
Signed-off-by: NIngo Molnar <mingo@elte.hu>

During tests and checks, I've discovered that there were failures to
convert mcount callers into nops. Looking deeper into these failures,
code that was attempted to be changed was not an mcount caller.
The current code only updates if the code being changed is what it expects,
but I still investigate any time there is a failure.

What was happening is that a weak symbol was being used as a reference
for other mcount callers. That weak symbol was also referenced elsewhere
so the offsets were using the strong symbol and not the function symbol
that it was referenced from.

This patch changes the setting up of the mcount_loc section to search
for a global function that is not weak. It will pick a local over a weak
but if only a weak is found in a section, a warning is printed and the
mcount location is not recorded (just to be safe).
Signed-off-by: NSteven Rostedt <srostedt@redhat.com>
Signed-off-by: NIngo Molnar <mingo@elte.hu>

I'm trying to keep all the arch changes in recordmcount.pl in one place.
I moved your code into that area, by adding the flags to the commands
that were passed in.
Signed-off-by: NSteven Rostedt <srostedt@redhat.com>
Signed-off-by: NIngo Molnar <mingo@elte.hu>

I'm seeing when I use separate src/build dirs:

make[3]: *** [arch/x86/kernel/time_32.o] Error 1
/bin/sh: scripts/recordmcount.pl: No such file or directory
make[3]: *** [arch/x86/kernel/irq_32.o] Error 1
/bin/sh: scripts/recordmcount.pl: No such file or directory
make[3]: *** [arch/x86/kernel/ldt.o] Error 1
/bin/sh: scripts/recordmcount.pl: No such file or directory
make[3]: *** [arch/x86/kernel/i8259.o] Error 1
/bin/sh: scripts/recordmcount.pl: No such file or directory

This fixes it.
Signed-off-by: NIngo Molnar <mingo@elte.hu>

This patch fixes incorrect comment style of __ftrace_enabled_save().
Signed-off-by: NHuang Ying <ying.huang@intel.com>
Signed-off-by: NIngo Molnar <mingo@elte.hu>

The new design of pre-recorded mcounts and updating the code outside of
kstop_machine has changed the way the records themselves are protected.

This patch uses the ftrace_lock to protect the records. Note, the lock
still does not need to be taken within calls that are only called via
kstop_machine, since the that code can not run while the spin lock is held.

Also removed the hash_lock needed for the daemon when MCOUNT_RECORD is
configured. Also did a slight cleanup of an unused variable.
Signed-off-by: NSteven Rostedt <srostedt@redhat.com>
Signed-off-by: NIngo Molnar <mingo@elte.hu>

If one of the self tests of ftrace has disabled the function tracer,
do not run the code to convert the mcount calls in modules.
Signed-off-by: NSteven Rostedt <srostedt@redhat.com>
Signed-off-by: NIngo Molnar <mingo@elte.hu>

This patch fixes some mistakes on the tracer in warning messages when
debugfs fails to create tracing files.
Signed-off-by: NFrederic Weisbecker <fweisbec@gmail.com>
Cc: srostedt@redhat.com
Signed-off-by: NIngo Molnar <mingo@elte.hu>

hack around:

 ld: Relocatable linking with relocations from format elf32-i386 (init/.tmp_gl_calibrate.o) to format elf64-x86-64 (init/.tmp_mx_calibrate.o) i  CC      arch/x86/mm/extable.o
 objcopy: 'init/.tmp_mx_calibrate.o': No such file
 rm: cannot remove `init/.tmp_mx_calibrate.o': No such file or directory
 ld: Relocatable linking with relocations from format elf32-i386 (arch/x86/mm/extable.o) to format elf64-x86-64 (arch/x86/mm/.tmp_mx_extable.o) is not supported
 mv: cannot stat `arch/x86/mm/.tmp_mx_extable.o': No such file or directory
 ld: Relocatable linking with relocations from format elf32-i386 (arch/x86/mm/fault.o) to format elf64-x86-64 (arch/x86/mm/.tmp_mx_fault.o) is not supported
Signed-off-by: NIngo Molnar <mingo@elte.hu>

fix:

 kernel/built-in.o: In function `ftrace_dump':
 (.text+0x2e2ea): undefined reference to `ftrace_kill_atomic'
Signed-off-by: NIngo Molnar <mingo@elte.hu>

fix:

 In file included from init/main.c:65:
 include/linux/ftrace.h:166: error: expected ‘,' or ‘;' before ‘{' token
 make[1]: *** [init/main.o] Error 1
 make: *** [init/main.o] Error 2
Signed-off-by: NIngo Molnar <mingo@elte.hu>

At OLS I had a lot of interest to be able to have the ftrace buffers
dumped on panic.  Usually one would expect to uses kexec and examine
the buffers after a new kernel is loaded. But sometimes the resources
do not permit kdump and kexec, so having an option to still see the
sequence of events up to the crash is very advantageous.

This patch adds the option to have the ftrace buffers dumped to the
console in the latency_trace format on a panic. When the option is set,
the default entries per CPU buffer are lowered to 16384, since the writing
to the serial (if that is the console) may take an awful long time
otherwise.

[
 Changes since -v1:
  Got alpine to send correctly (as well as spell check working).
  Removed config option.
  Moved the static variables into ftrace_dump itself.
  Gave printk a log level.
]
Signed-off-by: NSteven Rostedt <srostedt@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: NIngo Molnar <mingo@elte.hu>

Based on Randy Dunlap's suggestion, the ftrace_printk kernel-doc belongs
with the ftrace_printk macro that should be used. Not with the
__ftrace_printk internal function.
Signed-off-by: NSteven Rostedt <srostedt@redhat.com>
Acked-by: NRandy Dunlap <randy.dunlap@oracle.com>
Signed-off-by: NIngo Molnar <mingo@elte.hu>

This patch adds a feature that can help kernel developers debug their
code using ftrace.

  int ftrace_printk(const char *fmt, ...);

This records into the ftrace buffer using printf formatting. The entry
size in the buffers are still a fixed length. A new type has been added
that allows for more entries to be used for a single recording.

The start of the print is still the same as the other entries.

It returns the number of characters written to the ftrace buffer.

For example:

Having a module with the following code:

static int __init ftrace_print_test(void)
{
        ftrace_printk("jiffies are %ld\n", jiffies);
        return 0;
}

Gives me:

  insmod-5441  3...1 7569us : ftrace_print_test: jiffies are 4296626666

for the latency_trace file and:

          insmod-5441  [03]  1959.370498: ftrace_print_test jiffies are 4296626666

for the trace file.

Note: Only the infrastructure should go into the kernel. It is to help
facilitate debugging for other kernel developers. Calls to ftrace_printk
is not intended to be left in the kernel, and should be frowned upon just
like scattering printks around in the code.

But having this easily at your fingertips helps the debugging go faster
and bugs be solved quicker.

Maybe later on, we can hook this with markers and have their printf format
be sucked into ftrace output.
Signed-off-by: NSteven Rostedt <srostedt@redhat.com>
Signed-off-by: NIngo Molnar <mingo@elte.hu>

Some tracers will need to work with more than one entry. In order to do this
the trace_entry structure was split into two fields. One for the start of
all entries, and one to continue an existing entry.

The trace_entry structure now has a "field" entry that consists of the previous
content of the trace_entry, and a "cont" entry that is just a string buffer
the size of the "field" entry.

Thanks to Andrew Morton for suggesting this idea.
Signed-off-by: NSteven Rostedt <srostedt@redhat.com>
Signed-off-by: NIngo Molnar <mingo@elte.hu>

When a mcount pointer is recorded into a table, it is used to add or
remove calls to mcount (replacing them with nops). If the code is removed
via removing a module, the pointers still exist.  At modifying the code
a check is always made to make sure the code being replaced is the code
expected. In-other-words, the code being replaced is compared to what
it is expected to be before being replaced.

There is a very small chance that the code being replaced just happens
to look like code that calls mcount (very small since the call to mcount
is relative). To remove this chance, this patch adds ftrace_release to
allow module unloading to remove the pointers to mcount within the module.

Another change for init calls is made to not trace calls marked with
__init. The tracing can not be started until after init is done anyway.
Signed-off-by: NSteven Rostedt <srostedt@redhat.com>
Signed-off-by: NIngo Molnar <mingo@elte.hu>

The notrace define belongs in compiler.h so that it can be used in
init.h
Signed-off-by: NSteven Rostedt <srostedt@redhat.com>
Signed-off-by: NIngo Molnar <mingo@elte.hu>

Seems that freed records can appear in the available_filter_functions list.
This patch fixes that.
Signed-off-by: NSteven Rostedt <srostedt@redhat.com>
Signed-off-by: NIngo Molnar <mingo@elte.hu>

Mathieu Desnoyers revealed a bug in the original code. The nop that is
used to relpace the mcount caller can be a two part nop. This runs the
risk where a process can be preempted after executing the first nop, but
before the second part of the nop.

The ftrace code calls kstop_machine to keep multiple CPUs from executing
code that is being modified, but it does not protect against a task preempting
in the middle of a two part nop.

If the above preemption happens and the tracer is enabled, after the
kstop_machine runs, all those nops will be calls to the trace function.
If the preempted process that was preempted between the two nops is executed
again, it will execute half of the call to the trace function, and this
might crash the system.

This patch instead uses what both the latest Intel and AMD spec suggests.
That is the P6_NOP5 sequence of "0x0f 0x1f 0x44 0x00 0x00".

Note, some older CPUs and QEMU might fault on this nop, so this nop
is executed with fault handling first. If it detects a fault, it will then
use the code "0x66 0x66 0x66 0x66 0x90". If that faults, it will then
default to a simple "jmp 1f; .byte 0x00 0x00 0x00; 1:". The jmp is
not optimal but will do if the first two can not be executed.

TODO: Examine the cpuid to determine the nop to use.
Signed-off-by: NSteven Rostedt <srostedt@redhat.com>
Signed-off-by: NIngo Molnar <mingo@elte.hu>

x86 now sets up the mcount locations through the build and no longer
needs to record the ip when the function is executed. This patch changes
the initial mcount to simply return. There's no need to do any other work.
If the ftrace start up test fails, the original mcount will be what everything
will use, so having this as fast as possible is a good thing.
Signed-off-by: NSteven Rostedt <srostedt@redhat.com>
Signed-off-by: NIngo Molnar <mingo@elte.hu>

Enable the use of the __mcount_loc infrastructure on x86_64 and i386.
Signed-off-by: NSteven Rostedt <srostedt@redhat.com>
Signed-off-by: NIngo Molnar <mingo@elte.hu>

When enabling or disabling CONFIG_FTRACE_MCOUNT_RECORD, we want a full
kernel compile to handle the adding of the __mcount_loc sections.
Signed-off-by: NSteven Rostedt <srostedt@redhat.com>
Signed-off-by: NIngo Molnar <mingo@elte.hu>

This patch enables the loading of the __mcount_section of modules and
changing all the callers of mcount into nops.

The modification is done before the init_module function is called, so
again, we do not need to use kstop_machine to make these changes.
Signed-off-by: NSteven Rostedt <srostedt@redhat.com>
Signed-off-by: NIngo Molnar <mingo@elte.hu>

This is the infrastructure to the converting the mcount call sites
recorded by the __mcount_loc section into nops on boot. It also allows
for using these sites to enable tracing as normal. When the __mcount_loc
section is used, the "ftraced" kernel thread is disabled.

This uses the current infrastructure to record the mcount call sites
as well as convert them to nops. The mcount function is kept as a stub
on boot up and not converted to the ftrace_record_ip function. We use the
ftrace_record_ip to only record from the table.

This patch does not handle modules. That comes with a later patch.
Signed-off-by: NSteven Rostedt <srostedt@redhat.com>
Signed-off-by: NIngo Molnar <mingo@elte.hu>

This patch creates a section in the kernel called "__mcount_loc".
This will hold a list of pointers to the mcount relocation for
each call site of mcount.

For example:

objdump -dr init/main.o
[...]
Disassembly of section .text:

0000000000000000 <do_one_initcall>:
   0:   55                      push   %rbp
[...]
000000000000017b <init_post>:
 17b:   55                      push   %rbp
 17c:   48 89 e5                mov    %rsp,%rbp
 17f:   53                      push   %rbx
 180:   48 83 ec 08             sub    $0x8,%rsp
 184:   e8 00 00 00 00          callq  189 <init_post+0xe>
                        185: R_X86_64_PC32      mcount+0xfffffffffffffffc
[...]

We will add a section to point to each function call.

   .section __mcount_loc,"a",@progbits
[...]
   .quad .text + 0x185
[...]

The offset to of the mcount call site in init_post is an offset from
the start of the section, and not the start of the function init_post.
The mcount relocation is at the call site 0x185 from the start of the
.text section.

  .text + 0x185  == init_post + 0xa

We need a way to add this __mcount_loc section in a way that we do not
lose the relocations after final link.  The .text section here will
be attached to all other .text sections after final link and the
offsets will be meaningless.  We need to keep track of where these
.text sections are.

To do this, we use the start of the first function in the section.
do_one_initcall.  We can make a tmp.s file with this function as a reference
to the start of the .text section.

   .section __mcount_loc,"a",@progbits
[...]
   .quad do_one_initcall + 0x185
[...]

Then we can compile the tmp.s into a tmp.o

  gcc -c tmp.s -o tmp.o

And link it into back into main.o.

  ld -r main.o tmp.o -o tmp_main.o
  mv tmp_main.o main.o

But we have a problem.  What happens if the first function in a section
is not exported, and is a static function. The linker will not let
the tmp.o use it.  This case exists in main.o as well.

Disassembly of section .init.text:

0000000000000000 <set_reset_devices>:
   0:   55                      push   %rbp
   1:   48 89 e5                mov    %rsp,%rbp
   4:   e8 00 00 00 00          callq  9 <set_reset_devices+0x9>
                        5: R_X86_64_PC32        mcount+0xfffffffffffffffc

The first function in .init.text is a static function.

00000000000000a8 t __setup_set_reset_devices
000000000000105f t __setup_str_set_reset_devices
0000000000000000 t set_reset_devices

The lowercase 't' means that set_reset_devices is local and is not exported.
If we simply try to link the tmp.o with the set_reset_devices we end
up with two symbols: one local and one global.

 .section __mcount_loc,"a",@progbits
 .quad set_reset_devices + 0x10

00000000000000a8 t __setup_set_reset_devices
000000000000105f t __setup_str_set_reset_devices
0000000000000000 t set_reset_devices
                 U set_reset_devices

We still have an undefined reference to set_reset_devices, and if we try
to compile the kernel, we will end up with an undefined reference to
set_reset_devices, or even worst, it could be exported someplace else,
and then we will have a reference to the wrong location.

To handle this case, we make an intermediate step using objcopy.
We convert set_reset_devices into a global exported symbol before linking
it with tmp.o and set it back afterwards.

00000000000000a8 t __setup_set_reset_devices
000000000000105f t __setup_str_set_reset_devices
0000000000000000 T set_reset_devices

00000000000000a8 t __setup_set_reset_devices
000000000000105f t __setup_str_set_reset_devices
0000000000000000 T set_reset_devices

00000000000000a8 t __setup_set_reset_devices
000000000000105f t __setup_str_set_reset_devices
0000000000000000 t set_reset_devices

Now we have a section in main.o called __mcount_loc that we can place
somewhere in the kernel using vmlinux.ld.S and access it to convert
all these locations that call mcount into nops before starting SMP
and thus, eliminating the need to do this with kstop_machine.

Note, A well documented perl script (scripts/recordmcount.pl) is used
to do all this in one location.
Signed-off-by: NSteven Rostedt <srostedt@redhat.com>
Signed-off-by: NIngo Molnar <mingo@elte.hu>

it can be called in the NMI path:

[    0.645999] calling  ftrace_dynamic_init+0x0/0xd6
[    0.647521] ------------[ cut here ]------------
[    0.647521] WARNING: at kernel/trace/ftrace.c:348 ftrace_record_ip+0x4e/0x252()
[    0.647521] Modules linked in:
[    0.647521] Pid: 15, comm: kstop1 Not tainted 2.6.27-rc1-tip #22686
[    0.647521]
[    0.647521] Call Trace:
[    0.647521]  <NMI>  [<ffffffff8024593f>] warn_on_slowpath+0x5d/0x84
[    0.647521]  [<ffffffff80220b99>] ? lapic_wd_event+0xb/0x5c
[    0.647521]  [<ffffffff80287b3b>] ftrace_record_ip+0x4e/0x252
[    0.647521]  [<ffffffff80211274>] mcount_call+0x5/0x31
[    0.647521]  [<ffffffff80220b9e>] ? lapic_wd_event+0x10/0x5c
[    0.647521]  [<ffffffff8083f3ec>] nmi_watchdog_tick+0x19d/0x1ad
[    0.647521]  [<ffffffff8083e875>] default_do_nmi+0x75/0x1e3
[    0.647521]  [<ffffffff8083f0b3>] do_nmi+0x5d/0x94
[    0.647521]  [<ffffffff8083e2d2>] nmi+0xa2/0xc2
[    0.647521]  [<ffffffff802b48c3>] ? check_bytes_and_report+0x11/0xcc
[    0.647521]  <<EOE>>  [<ffffffff80211274>] ? mcount_call+0x5/0x31
[    0.647521]  [<ffffffff802b49df>] check_object+0x61/0x1b0
[    0.647521]  [<ffffffff802b502a>] __slab_free+0x169/0x2ae
[    0.647521]  [<ffffffff80242dbf>] ? __cleanup_sighand+0x25/0x27
[    0.647521]  [<ffffffff80242dbf>] ? __cleanup_sighand+0x25/0x27
[    0.647521]  [<ffffffff802b60cd>] kmem_cache_free+0x85/0xb9
[    0.647521]  [<ffffffff80242dbf>] __cleanup_sighand+0x25/0x27
[    0.647521]  [<ffffffff80247b3d>] release_task+0x256/0x339
[    0.647521]  [<ffffffff802490b4>] do_exit+0x764/0x7ef
[    0.647521]  [<ffffffff8027624c>] __xchg+0x0/0x38
[    0.647521]  [<ffffffff8027619a>] ? stop_cpu+0x0/0xb2
[    0.647521]  [<ffffffff8027619a>] ? stop_cpu+0x0/0xb2
[    0.647521]  [<ffffffff8025922f>] kthread+0x4e/0x7b
[    0.647521]  [<ffffffff80212979>] child_rip+0xa/0x11
[    0.647521]  [<ffffffff80211c17>] ? restore_args+0x0/0x30
[    0.647521]  [<ffffffff802283a5>] ? native_load_tls+0x14/0x2e
[    0.647521]  [<ffffffff802591e1>] ? kthread+0x0/0x7b
[    0.647521]  [<ffffffff8021296f>] ? child_rip+0x0/0x11
[    0.647521]
[    0.647521] ---[ end trace 4eaa2a86a8e2da22 ]---
[    0.672032] initcall ftrace_dynamic_init+0x0/0xd6 returned 0 after 19 msecs

also mark it no-kprobes while at it.
Signed-off-by: NIngo Molnar <mingo@elte.hu>

kprobes already has an extensive list of annotations for functions
that should not be instrumented. Add notrace annotations to these
functions as well.

This is particularly useful for functions called by the NMI path.
Signed-off-by: NIngo Molnar <mingo@elte.hu>

Steven Rostedt suggested:

| Wouldn't it look nicer to have: (TRACEPOINT_TABLE_SIZE - 1) ?
Signed-off-by: NMathieu Desnoyers <mathieu.desnoyers@polymtl.ca>
Cc: Steven Rostedt <rostedt@goodmis.org>
Signed-off-by: NIngo Molnar <mingo@elte.hu>

When SIL, DIL, BPL or SPL registers were used in MMIO, the datum
was extracted from AH, BH, CH, or DH, which are incorrect.
Signed-off-by: NPekka Paalanen <pq@iki.fi>
Cc: "Vegard Nossum" <vegard.nossum@gmail.com>
Cc: "Steven Rostedt" <srostedt@redhat.com>
Cc: proski@gnu.org
Cc: "Pekka Enberg"
	<penberg@cs.helsinki.fi>
Signed-off-by: NIngo Molnar <mingo@elte.hu>

do not expose users to CONFIG_TRACEPOINTS - tracers can select it
just fine.

update ftrace to select CONFIG_TRACEPOINTS.
Signed-off-by: NIngo Molnar <mingo@elte.hu>

make it a bit more structured hence more readable.
Signed-off-by: NIngo Molnar <mingo@elte.hu>

while it's arguably low overhead, we dont enable new features by default.
Signed-off-by: NIngo Molnar <mingo@elte.hu>

Porting the trace_mark() used by ftrace to tracepoints. (cleanup)

Changelog :
- Change error messages : marker -> tracepoint

[ mingo@elte.hu: conflict resolutions ]
Signed-off-by: NMathieu Desnoyers <mathieu.desnoyers@polymtl.ca>
Acked-by: N'Peter Zijlstra' <peterz@infradead.org>
Signed-off-by: NIngo Molnar <mingo@elte.hu>

Instrument the scheduler activity (sched_switch, migration, wakeups,
wait for a task, signal delivery) and process/thread
creation/destruction (fork, exit, kthread stop). Actually, kthread
creation is not instrumented in this patch because it is architecture
dependent. It allows to connect tracers such as ftrace which detects
scheduling latencies, good/bad scheduler decisions. Tools like LTTng can
export this scheduler information along with instrumentation of the rest
of the kernel activity to perform post-mortem analysis on the scheduler
activity.

About the performance impact of tracepoints (which is comparable to
markers), even without immediate values optimizations, tests done by
Hideo Aoki on ia64 show no regression. His test case was using hackbench
on a kernel where scheduler instrumentation (about 5 events in code
scheduler code) was added. See the "Tracepoints" patch header for
performance result detail.

Changelog :

- Change instrumentation location and parameter to match ftrace
  instrumentation, previously done with kernel markers.

[ mingo@elte.hu: conflict resolutions ]
Signed-off-by: NMathieu Desnoyers <mathieu.desnoyers@polymtl.ca>
Acked-by: N'Peter Zijlstra' <peterz@infradead.org>
Signed-off-by: NIngo Molnar <mingo@elte.hu>

Tracepoint example code under samples/.
Signed-off-by: NMathieu Desnoyers <mathieu.desnoyers@polymtl.ca>
Acked-by: N'Peter Zijlstra' <peterz@infradead.org>
Signed-off-by: NIngo Molnar <mingo@elte.hu>

Documentation of tracepoint usage.
Signed-off-by: NMathieu Desnoyers <mathieu.desnoyers@polymtl.ca>
Acked-by: N'Peter Zijlstra' <peterz@infradead.org>
Signed-off-by: NIngo Molnar <mingo@elte.hu>