Merge branch 'tracing-fixes-for-linus' of...

Reviewers:   Elias Oltmanns, Randy Dunlap, Andrew Morton,

	     John Kacur, and David Teigland.

Written for: 2.6.27-rc1

Written for: 2.6.28-rc2

Introduction

------------

		only be recorded if the latency is greater than

		the value in this file. (in microseconds)

  trace_entries : This sets or displays the number of trace

		entries each CPU buffer can hold. The tracer buffers

		are the same size for each CPU. The displayed number

		is the size of the CPU buffer and not total size. The

  trace_entries: This sets or displays the number of bytes each CPU

		buffer can hold. The tracer buffers are the same size

		for each CPU. The displayed number is the size of the

		 CPU buffer and not total size of all buffers. The

		trace buffers are allocated in pages (blocks of memory

		that the kernel uses for allocation, usually 4 KB in size).

		Since each entry is smaller than a page, if the last

		allocated page has room for more entries than were

		requested, the rest of the page is used to allocate

		entries.

		If the last page allocated has room for more bytes

		than requested, the rest of the page will be used,

		making the actual allocation bigger than requested.

		(Note, the size may not be a multiple of the page size due

		to buffer managment overhead.)

		This can only be updated when the current_tracer

		is set to "none".

		NOTE: It is planned on changing the allocated buffers

		      from being the number of possible CPUS to

		      the number of online CPUS.

		is set to "nop".

  tracing_cpumask: This is a mask that lets the user only trace

		on specified CPUS. The format is a hex string

		be traced. If a function exists in both set_ftrace_filter

		and set_ftrace_notrace,	the function will _not_ be traced.

  available_filter_functions : When a function is encountered the first

		time by the dynamic tracer, it is recorded and

		later the call is converted into a nop. This file

		lists the functions that have been recorded

		by the dynamic tracer and these functions can

		be used to set the ftrace filter by the above

		"set_ftrace_filter" file. (See the section "dynamic ftrace"

		below for more details).

  available_filter_functions: This lists the functions that ftrace

		has processed and can trace. These are the function

		names that you can pass to "set_ftrace_filter" or

		"set_ftrace_notrace". (See the section "dynamic ftrace"

		below for more details.)

The Tracers

Here is the list of current tracers that may be configured.

  ftrace - function tracer that uses mcount to trace all functions.

  function - function tracer that uses mcount to trace all functions.

  sched_switch - traces the context switches between tasks.

		the highest priority task to get scheduled after

		it has been woken up.

  none - This is not a tracer. To remove all tracers from tracing

		simply echo "none" into current_tracer.

  nop - This is not a tracer. To remove all tracers from tracing

		simply echo "nop" into current_tracer.

Examples of using the tracer

Here is an example of the output format of the file "trace"

                             --------

# tracer: ftrace

# tracer: function

#

#           TASK-PID   CPU#    TIMESTAMP  FUNCTION

#              | |      |          |         |

                             --------

A header is printed with the tracer name that is represented by the trace.

In this case the tracer is "ftrace". Then a header showing the format. Task

In this case the tracer is "function". Then a header showing the format. Task

name "bash", the task PID "4251", the CPU that it was running on

"01", the timestamp in <secs>.<usecs> format, the function name that was

traced "path_put" and the parent function that called this function

has been set. We do not see the 'N' until we switch back to the task's

assigned stack.

ftrace

------

function

--------

ftrace is not only the name of the tracing infrastructure, but it

is also a name of one of the tracers. The tracer is the function

tracer. Enabling the function tracer can be done from the

debug file system. Make sure the ftrace_enabled is set otherwise

this tracer is a nop.

This tracer is the function tracer. Enabling the function tracer

can be done from the debug file system. Make sure the ftrace_enabled is

set; otherwise this tracer is a nop.

 # sysctl kernel.ftrace_enabled=1

 # echo ftrace > /debug/tracing/current_tracer

 # echo function > /debug/tracing/current_tracer

 # echo 1 > /debug/tracing/tracing_enabled

 # usleep 1

 # echo 0 > /debug/tracing/tracing_enabled

 # cat /debug/tracing/trace

# tracer: ftrace

# tracer: function

#

#           TASK-PID   CPU#    TIMESTAMP  FUNCTION

#              | |      |          |         |

[...]

Note: ftrace uses ring buffers to store the above entries. The newest data

may overwrite the oldest data. Sometimes using echo to stop the trace

is not sufficient because the tracing could have overwritten the data

that you wanted to record. For this reason, it is sometimes better to

Note: function tracer uses ring buffers to store the above entries.

The newest data may overwrite the oldest data. Sometimes using echo to

stop the trace is not sufficient because the tracing could have overwritten

the data that you wanted to record. For this reason, it is sometimes better to

disable tracing directly from a program. This allows you to stop the

tracing at the point that you hit the part that you are interested in.

To disable the tracing directly from a C program, something like following

of pointing to a simple return. (Enabling FTRACE will include the

-pg switch in the compiling of the kernel.)

When dynamic ftrace is initialized, it calls kstop_machine to make

the machine act like a uniprocessor so that it can freely modify code

without worrying about other processors executing that same code.  At

initialization, the mcount calls are changed to call a "record_ip"

function.  After this, the first time a kernel function is called,

it has the calling address saved in a hash table.

Later on the ftraced kernel thread is awoken and will again call

kstop_machine if new functions have been recorded. The ftraced thread

will change all calls to mcount to "nop".  Just calling mcount

and having mcount return has shown a 10% overhead. By converting

it to a nop, there is no measurable overhead to the system.

At compile time every C file object is run through the

recordmcount.pl script (located in the scripts directory). This

script will process the C object using objdump to find all the

locations in the .text section that call mcount. (Note, only

the .text section is processed, since processing other sections

like .init.text may cause races due to those sections being freed).

A new section called "__mcount_loc" is created that holds references

to all the mcount call sites in the .text section. This section is

compiled back into the original object. The final linker will add

all these references into a single table.

On boot up, before SMP is initialized, the dynamic ftrace code

scans this table and updates all the locations into nops. It also

records the locations, which are added to the available_filter_functions

list.  Modules are processed as they are loaded and before they are

executed.  When a module is unloaded, it also removes its functions from

the ftrace function list. This is automatic in the module unload

code, and the module author does not need to worry about it.

When tracing is enabled, kstop_machine is called to prevent races

with the CPUS executing code being modified (which can cause the

CPU to do undesireable things), and the nops are patched back

to calls. But this time, they do not call mcount (which is just

a function stub). They now call into the ftrace infrastructure.

One special side-effect to the recording of the functions being

traced is that we can now selectively choose which functions we

We can see that there's no more lock or preempt tracing.

ftraced

-------

As mentioned above, when dynamic ftrace is configured in, a kernel

thread wakes up once a second and checks to see if there are mcount

calls that need to be converted into nops. If there are not any, then

it simply goes back to sleep. But if there are some, it will call

kstop_machine to convert the calls to nops.

There may be a case in which you do not want this added latency.

Perhaps you are doing some audio recording and this activity might

cause skips in the playback. There is an interface to disable

and enable the "ftraced" kernel thread.

 # echo 0 > /debug/tracing/ftraced_enabled

This will disable the calling of kstop_machine to update the

mcount calls to nops. Remember that there is a large overhead

to calling mcount. Without this kernel thread, that overhead will

exist.

If there are recorded calls to mcount, any write to the ftraced_enabled

file will cause the kstop_machine to run. This means that a

user can manually perform the updates when they want to by simply

echoing a '0' into the ftraced_enabled file.

The updates are also done at the beginning of enabling a tracer

that uses ftrace function recording.

trace_pipe

----------

This means that subsequent reads will be different. The trace

is live.

 # echo ftrace > /debug/tracing/current_tracer

 # echo function > /debug/tracing/current_tracer

 # cat /debug/tracing/trace_pipe > /tmp/trace.out &

[1] 4153

 # echo 1 > /debug/tracing/tracing_enabled

 # usleep 1

 # echo 0 > /debug/tracing/tracing_enabled

 # cat /debug/tracing/trace

# tracer: ftrace

# tracer: function

#

#           TASK-PID   CPU#    TIMESTAMP  FUNCTION

#              | |      |          |         |

Note, reading the trace_pipe file will block until more input is added.

By changing the tracer, trace_pipe will issue an EOF. We needed

to set the ftrace tracer _before_ cating the trace_pipe file.

to set the function tracer _before_ we "cat" the trace_pipe file.

trace entries

65620

Note, to modify this, you must have tracing completely disabled. To do that,

echo "none" into the current_tracer. If the current_tracer is not set

to "none", an EINVAL error will be returned.

echo "nop" into the current_tracer. If the current_tracer is not set

to "nop", an EINVAL error will be returned.

 # echo none > /debug/tracing/current_tracer

 # echo nop > /debug/tracing/current_tracer

 # echo 100000 > /debug/tracing/trace_entries

 # cat /debug/tracing/trace_entries

100045

		/* Did the write stamp get updated already? */

		if (unlikely(ts < cpu_buffer->write_stamp))

			goto again;

			delta = 0;

		if (test_time_stamp(delta)) {

		return TRACE_TYPE_HANDLED;

	SEQ_PUT_FIELD_RET(s, entry->pid);

	SEQ_PUT_FIELD_RET(s, iter->cpu);

	SEQ_PUT_FIELD_RET(s, entry->cpu);

	SEQ_PUT_FIELD_RET(s, iter->ts);

	switch (entry->type) {

{

	unsigned long val;

	char buf[64];

	int ret;

	int ret, cpu;

	struct trace_array *tr = filp->private_data;

	if (cnt >= sizeof(buf))

		goto out;

	}

	/* disable all cpu buffers */

	for_each_tracing_cpu(cpu) {

		if (global_trace.data[cpu])

			atomic_inc(&global_trace.data[cpu]->disabled);

		if (max_tr.data[cpu])

			atomic_inc(&max_tr.data[cpu]->disabled);

	}

	if (val != global_trace.entries) {

		ret = ring_buffer_resize(global_trace.buffer, val);

		if (ret < 0) {

	if (tracing_disabled)

		cnt = -ENOMEM;

 out:

	for_each_tracing_cpu(cpu) {

		if (global_trace.data[cpu])

			atomic_dec(&global_trace.data[cpu]->disabled);

		if (max_tr.data[cpu])

			atomic_dec(&max_tr.data[cpu]->disabled);

	}

	max_tr.entries = global_trace.entries;

	mutex_unlock(&trace_types_lock);