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tracepoint: Give priority to probes of tracepoints
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In order to guarantee that a probe will be called before other probes that
are attached to a tracepoint, there needs to be a mechanism to provide
priority of one probe over the others.

Adding a prio field to the struct tracepoint_func, which lets the probes be
sorted by the priority set in the structure. If no priority is specified,
then a priority of 10 is given (this is a macro, and perhaps may be changed
in the future).

Now probes may be added to affect other probes that are attached to a
tracepoint with a guaranteed order.

One use case would be to allow tracing of tracepoints be able to filter by
pid. A special (higher priority probe) may be added to the sched_switch
tracepoint and set the necessary flags of the other tracepoints to notify
them if they should be traced or not. In case a tracepoint is enabled at the
sched_switch tracepoint too, the order of the two are not random.

Cc: Mathieu Desnoyers <[email protected]>
Signed-off-by: Steven Rostedt <[email protected]>
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@rostedt
rostedt committed Oct 26, 2015
1 parent 883a1e8 commit 7904b5c
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Showing 2 changed files with 63 additions and 11 deletions.
13 changes: 13 additions & 0 deletions include/linux/tracepoint.h
View file
Original file line number Diff line number Diff line change
Expand Up @@ -26,6 +26,7 @@ struct notifier_block;
struct tracepoint_func {
void *func;
void *data;
int prio;
};

struct tracepoint {
Expand All @@ -42,9 +43,14 @@ struct trace_enum_map {
unsigned long enum_value;
};

#define TRACEPOINT_DEFAULT_PRIO 10

extern int
tracepoint_probe_register(struct tracepoint *tp, void *probe, void *data);
extern int
tracepoint_probe_register_prio(struct tracepoint *tp, void *probe, void *data,
int prio);
extern int
tracepoint_probe_unregister(struct tracepoint *tp, void *probe, void *data);
extern void
for_each_kernel_tracepoint(void (*fct)(struct tracepoint *tp, void *priv),
Expand Down Expand Up @@ -207,6 +213,13 @@ extern void syscall_unregfunc(void);
(void *)probe, data); \
} \
static inline int \
register_trace_prio_##name(void (*probe)(data_proto), void *data,\
int prio) \
{ \
return tracepoint_probe_register_prio(&__tracepoint_##name, \
(void *)probe, data, prio); \
} \
static inline int \
unregister_trace_##name(void (*probe)(data_proto), void *data) \
{ \
return tracepoint_probe_unregister(&__tracepoint_##name,\
Expand Down
61 changes: 50 additions & 11 deletions kernel/tracepoint.c
View file
Original file line number Diff line number Diff line change
Expand Up @@ -91,11 +91,13 @@ static void debug_print_probes(struct tracepoint_func *funcs)
printk(KERN_DEBUG "Probe %d : %p\n", i, funcs[i].func);
}

static struct tracepoint_func *func_add(struct tracepoint_func **funcs,
struct tracepoint_func *tp_func)
static struct tracepoint_func *
func_add(struct tracepoint_func **funcs, struct tracepoint_func *tp_func,
int prio)
{
int nr_probes = 0;
struct tracepoint_func *old, *new;
int nr_probes = 0;
int pos = -1;

if (WARN_ON(!tp_func->func))
return ERR_PTR(-EINVAL);
Expand All @@ -104,18 +106,33 @@ static struct tracepoint_func *func_add(struct tracepoint_func **funcs,
old = *funcs;
if (old) {
/* (N -> N+1), (N != 0, 1) probes */
for (nr_probes = 0; old[nr_probes].func; nr_probes++)
for (nr_probes = 0; old[nr_probes].func; nr_probes++) {
/* Insert before probes of lower priority */
if (pos < 0 && old[nr_probes].prio < prio)
pos = nr_probes;
if (old[nr_probes].func == tp_func->func &&
old[nr_probes].data == tp_func->data)
return ERR_PTR(-EEXIST);
}
}
/* + 2 : one for new probe, one for NULL func */
new = allocate_probes(nr_probes + 2);
if (new == NULL)
return ERR_PTR(-ENOMEM);
if (old)
memcpy(new, old, nr_probes * sizeof(struct tracepoint_func));
new[nr_probes] = *tp_func;
if (old) {
if (pos < 0) {
pos = nr_probes;
memcpy(new, old, nr_probes * sizeof(struct tracepoint_func));
} else {
/* Copy higher priority probes ahead of the new probe */
memcpy(new, old, pos * sizeof(struct tracepoint_func));
/* Copy the rest after it. */
memcpy(new + pos + 1, old + pos,
(nr_probes - pos) * sizeof(struct tracepoint_func));
}
} else
pos = 0;
new[pos] = *tp_func;
new[nr_probes + 1].func = NULL;
*funcs = new;
debug_print_probes(*funcs);
Expand Down Expand Up @@ -174,7 +191,7 @@ static void *func_remove(struct tracepoint_func **funcs,
* Add the probe function to a tracepoint.
*/
static int tracepoint_add_func(struct tracepoint *tp,
struct tracepoint_func *func)
struct tracepoint_func *func, int prio)
{
struct tracepoint_func *old, *tp_funcs;

Expand All @@ -183,7 +200,7 @@ static int tracepoint_add_func(struct tracepoint *tp,

tp_funcs = rcu_dereference_protected(tp->funcs,
lockdep_is_held(&tracepoints_mutex));
old = func_add(&tp_funcs, func);
old = func_add(&tp_funcs, func, prio);
if (IS_ERR(old)) {
WARN_ON_ONCE(1);
return PTR_ERR(old);
Expand Down Expand Up @@ -240,25 +257,47 @@ static int tracepoint_remove_func(struct tracepoint *tp,
* @tp: tracepoint
* @probe: probe handler
* @data: tracepoint data
* @prio: priority of this function over other registered functions
*
* Returns 0 if ok, error value on error.
* Note: if @tp is within a module, the caller is responsible for
* unregistering the probe before the module is gone. This can be
* performed either with a tracepoint module going notifier, or from
* within module exit functions.
*/
int tracepoint_probe_register(struct tracepoint *tp, void *probe, void *data)
int tracepoint_probe_register_prio(struct tracepoint *tp, void *probe,
void *data, int prio)
{
struct tracepoint_func tp_func;
int ret;

mutex_lock(&tracepoints_mutex);
tp_func.func = probe;
tp_func.data = data;
ret = tracepoint_add_func(tp, &tp_func);
tp_func.prio = prio;
ret = tracepoint_add_func(tp, &tp_func, prio);
mutex_unlock(&tracepoints_mutex);
return ret;
}
EXPORT_SYMBOL_GPL(tracepoint_probe_register_prio);

/**
* tracepoint_probe_register - Connect a probe to a tracepoint
* @tp: tracepoint
* @probe: probe handler
* @data: tracepoint data
* @prio: priority of this function over other registered functions
*
* Returns 0 if ok, error value on error.
* Note: if @tp is within a module, the caller is responsible for
* unregistering the probe before the module is gone. This can be
* performed either with a tracepoint module going notifier, or from
* within module exit functions.
*/
int tracepoint_probe_register(struct tracepoint *tp, void *probe, void *data)
{
return tracepoint_probe_register_prio(tp, probe, data, TRACEPOINT_DEFAULT_PRIO);
}
EXPORT_SYMBOL_GPL(tracepoint_probe_register);

/**
Expand Down

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