The Linux Kernel Tracepoint API

Author:Jason Baron
Author:William Cohen

Introduction

Tracepoints are static probe points that are located in strategic points throughout the kernel. 'Probes' register/unregister with tracepoints via a callback mechanism. The 'probes' are strictly typed functions that are passed a unique set of parameters defined by each tracepoint.

From this simple callback mechanism, 'probes' can be used to profile, debug, and understand kernel behavior. There are a number of tools that provide a framework for using 'probes'. These tools include Systemtap, ftrace, and LTTng.

Tracepoints are defined in a number of header files via various macros. Thus, the purpose of this document is to provide a clear accounting of the available tracepoints. The intention is to understand not only what tracepoints are available but also to understand where future tracepoints might be added.

The API presented has functions of the form: trace_tracepointname(function parameters). These are the tracepoints callbacks that are found throughout the code. Registering and unregistering probes with these callback sites is covered in the Documentation/trace/* directory.

IRQ

void trace_irq_handler_entry(int irq, struct irqaction * action)

called immediately before the irq action handler

Parameters

int irq
irq number
struct irqaction * action
pointer to struct irqaction

Description

The struct irqaction pointed to by action contains various information about the handler, including the device name, action->name, and the device id, action->dev_id. When used in conjunction with the irq_handler_exit tracepoint, we can figure out irq handler latencies.

void trace_irq_handler_exit(int irq, struct irqaction * action, int ret)

called immediately after the irq action handler returns

Parameters

int irq
irq number
struct irqaction * action
pointer to struct irqaction
int ret
return value

Description

If the ret value is set to IRQ_HANDLED, then we know that the corresponding action->handler successfully handled this irq. Otherwise, the irq might be a shared irq line, or the irq was not handled successfully. Can be used in conjunction with the irq_handler_entry to understand irq handler latencies.

void trace_softirq_entry(unsigned int vec_nr)

called immediately before the softirq handler

Parameters

unsigned int vec_nr
softirq vector number

Description

When used in combination with the softirq_exit tracepoint we can determine the softirq handler routine.

void trace_softirq_exit(unsigned int vec_nr)

called immediately after the softirq handler returns

Parameters

unsigned int vec_nr
softirq vector number

Description

When used in combination with the softirq_entry tracepoint we can determine the softirq handler routine.

void trace_softirq_raise(unsigned int vec_nr)

called immediately when a softirq is raised

Parameters

unsigned int vec_nr
softirq vector number

Description

When used in combination with the softirq_entry tracepoint we can determine the softirq raise to run latency.

SIGNAL

void trace_signal_generate(int sig, struct kernel_siginfo * info, struct task_struct * task, int group, int result)

called when a signal is generated

Parameters

int sig
signal number
struct kernel_siginfo * info
pointer to struct siginfo
struct task_struct * task
pointer to struct task_struct
int group
shared or private
int result
TRACE_SIGNAL_*

Description

Current process sends a 'sig' signal to 'task' process with 'info' siginfo. If 'info' is SEND_SIG_NOINFO or SEND_SIG_PRIV, 'info' is not a pointer and you can't access its field. Instead, SEND_SIG_NOINFO means that si_code is SI_USER, and SEND_SIG_PRIV means that si_code is SI_KERNEL.

void trace_signal_deliver(int sig, struct kernel_siginfo * info, struct k_sigaction * ka)

called when a signal is delivered

Parameters

int sig
signal number
struct kernel_siginfo * info
pointer to struct siginfo
struct k_sigaction * ka
pointer to struct k_sigaction

Description

A 'sig' signal is delivered to current process with 'info' siginfo, and it will be handled by 'ka'. ka->sa.sa_handler can be SIG_IGN or SIG_DFL. Note that some signals reported by signal_generate tracepoint can be lost, ignored or modified (by debugger) before hitting this tracepoint. This means, this can show which signals are actually delivered, but matching generated signals and delivered signals may not be correct.

Block IO

void trace_block_touch_buffer(struct buffer_head * bh)

mark a buffer accessed

Parameters

struct buffer_head * bh
buffer_head being touched

Description

Called from touch_buffer().

void trace_block_dirty_buffer(struct buffer_head * bh)

mark a buffer dirty

Parameters

struct buffer_head * bh
buffer_head being dirtied

Description

Called from mark_buffer_dirty().

void trace_block_rq_requeue(struct request_queue * q, struct request * rq)

place block IO request back on a queue

Parameters

struct request_queue * q
queue holding operation
struct request * rq
block IO operation request

Description

The block operation request rq is being placed back into queue q. For some reason the request was not completed and needs to be put back in the queue.

void trace_block_rq_complete(struct request * rq, int error, unsigned int nr_bytes)

block IO operation completed by device driver

Parameters

struct request * rq
block operations request
int error
status code
unsigned int nr_bytes
number of completed bytes

Description

The block_rq_complete tracepoint event indicates that some portion of operation request has been completed by the device driver. If the rq->bio is NULL, then there is absolutely no additional work to do for the request. If rq->bio is non-NULL then there is additional work required to complete the request.

void trace_block_rq_insert(struct request_queue * q, struct request * rq)

insert block operation request into queue

Parameters

struct request_queue * q
target queue
struct request * rq
block IO operation request

Description

Called immediately before block operation request rq is inserted into queue q. The fields in the operation request rq struct can be examined to determine which device and sectors the pending operation would access.

void trace_block_rq_issue(struct request_queue * q, struct request * rq)

issue pending block IO request operation to device driver

Parameters

struct request_queue * q
queue holding operation
struct request * rq
block IO operation operation request

Description

Called when block operation request rq from queue q is sent to a device driver for processing.

void trace_block_bio_bounce(struct request_queue * q, struct bio * bio)

used bounce buffer when processing block operation

Parameters

struct request_queue * q
queue holding the block operation
struct bio * bio
block operation

Description

A bounce buffer was used to handle the block operation bio in q. This occurs when hardware limitations prevent a direct transfer of data between the bio data memory area and the IO device. Use of a bounce buffer requires extra copying of data and decreases performance.

void trace_block_bio_complete(struct request_queue * q, struct bio * bio, int error)

completed all work on the block operation

Parameters

struct request_queue * q
queue holding the block operation
struct bio * bio
block operation completed
int error
io error value

Description

This tracepoint indicates there is no further work to do on this block IO operation bio.

void trace_block_bio_backmerge(struct request_queue * q, struct request * rq, struct bio * bio)

merging block operation to the end of an existing operation

Parameters

struct request_queue * q
queue holding operation
struct request * rq
request bio is being merged into
struct bio * bio
new block operation to merge

Description

Merging block request bio to the end of an existing block request in queue q.

void trace_block_bio_frontmerge(struct request_queue * q, struct request * rq, struct bio * bio)

merging block operation to the beginning of an existing operation

Parameters

struct request_queue * q
queue holding operation
struct request * rq
request bio is being merged into
struct bio * bio
new block operation to merge

Description

Merging block IO operation bio to the beginning of an existing block operation in queue q.

void trace_block_bio_queue(struct request_queue * q, struct bio * bio)

putting new block IO operation in queue

Parameters

struct request_queue * q
queue holding operation
struct bio * bio
new block operation

Description

About to place the block IO operation bio into queue q.

void trace_block_getrq(struct request_queue * q, struct bio * bio, int rw)

get a free request entry in queue for block IO operations

Parameters

struct request_queue * q
queue for operations
struct bio * bio
pending block IO operation (can be NULL)
int rw
low bit indicates a read (0) or a write (1)

Description

A request struct for queue q has been allocated to handle the block IO operation bio.

void trace_block_sleeprq(struct request_queue * q, struct bio * bio, int rw)

waiting to get a free request entry in queue for block IO operation

Parameters

struct request_queue * q
queue for operation
struct bio * bio
pending block IO operation (can be NULL)
int rw
low bit indicates a read (0) or a write (1)

Description

In the case where a request struct cannot be provided for queue q the process needs to wait for an request struct to become available. This tracepoint event is generated each time the process goes to sleep waiting for request struct become available.

void trace_block_plug(struct request_queue * q)

keep operations requests in request queue

Parameters

struct request_queue * q
request queue to plug

Description

Plug the request queue q. Do not allow block operation requests to be sent to the device driver. Instead, accumulate requests in the queue to improve throughput performance of the block device.

void trace_block_unplug(struct request_queue * q, unsigned int depth, bool explicit)

release of operations requests in request queue

Parameters

struct request_queue * q
request queue to unplug
unsigned int depth
number of requests just added to the queue
bool explicit
whether this was an explicit unplug, or one from schedule()

Description

Unplug request queue q because device driver is scheduled to work on elements in the request queue.

void trace_block_split(struct request_queue * q, struct bio * bio, unsigned int new_sector)

split a single bio struct into two bio structs

Parameters

struct request_queue * q
queue containing the bio
struct bio * bio
block operation being split
unsigned int new_sector
The starting sector for the new bio

Description

The bio request bio in request queue q needs to be split into two bio requests. The newly created bio request starts at new_sector. This split may be required due to hardware limitation such as operation crossing device boundaries in a RAID system.

void trace_block_bio_remap(struct request_queue * q, struct bio * bio, dev_t dev, sector_t from)

map request for a logical device to the raw device

Parameters

struct request_queue * q
queue holding the operation
struct bio * bio
revised operation
dev_t dev
device for the operation
sector_t from
original sector for the operation

Description

An operation for a logical device has been mapped to the raw block device.

void trace_block_rq_remap(struct request_queue * q, struct request * rq, dev_t dev, sector_t from)

map request for a block operation request

Parameters

struct request_queue * q
queue holding the operation
struct request * rq
block IO operation request
dev_t dev
device for the operation
sector_t from
original sector for the operation

Description

The block operation request rq in q has been remapped. The block operation request rq holds the current information and from hold the original sector.

Workqueue

void trace_workqueue_queue_work(unsigned int req_cpu, struct pool_workqueue * pwq, struct work_struct * work)

called when a work gets queued

Parameters

unsigned int req_cpu
the requested cpu
struct pool_workqueue * pwq
pointer to struct pool_workqueue
struct work_struct * work
pointer to struct work_struct

Description

This event occurs when a work is queued immediately or once a delayed work is actually queued on a workqueue (ie: once the delay has been reached).

void trace_workqueue_activate_work(struct work_struct * work)

called when a work gets activated

Parameters

struct work_struct * work
pointer to struct work_struct

Description

This event occurs when a queued work is put on the active queue, which happens immediately after queueing unless max_active limit is reached.

void trace_workqueue_execute_start(struct work_struct * work)

called immediately before the workqueue callback

Parameters

struct work_struct * work
pointer to struct work_struct

Description

Allows to track workqueue execution.

void trace_workqueue_execute_end(struct work_struct * work, work_func_t function)

called immediately after the workqueue callback

Parameters

struct work_struct * work
pointer to struct work_struct
work_func_t function
pointer to worker function

Description

Allows to track workqueue execution.