Commit 1bd13edb authored by Masami Hiramatsu (Google)'s avatar Masami Hiramatsu (Google) Committed by Steven Rostedt (Google)
Browse files

tracing/hist: Add poll(POLLIN) support on hist file 

Add poll syscall support on the `hist` file. The Waiter will be waken
up when the histogram is updated with POLLIN.

Currently, there is no way to wait for a specific event in userspace.
So user needs to peek the `trace` periodicaly, or wait on `trace_pipe`.
But it is not a good idea to peek at the `trace` for an event that
randomly happens. And `trace_pipe` is not coming back until a page is
filled with events.

This allows a user to wait for a specific event on the `hist` file. User
can set a histogram trigger on the event which they want to monitor
and poll() on its `hist` file. Since this poll() returns POLLIN, the next
poll() will return soon unless a read() happens on that hist file.

NOTE: To read the hist file again, you must set the file offset to 0,
but just for monitoring the event, you may not need to read the
histogram.

Cc: Shuah Khan <shuah@kernel.org>
Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Link: https://lore.kernel.org/173527247756.464571.14236296701625509931.stgit@devnote2


Signed-off-by: default avatarMasami Hiramatsu (Google) <mhiramat@kernel.org>
Reviewed-by: default avatarTom Zanussi <zanussi@kernel.org>
Signed-off-by: default avatarSteven Rostedt (Google) <rostedt@goodmis.org>
parent 22bec11a

include/linux/trace_events.h

+14 −0
Original line number Diff line number Diff line
@@ -673,6 +673,20 @@ struct trace_event_file { 
	atomic_t		tm_ref;	/* trigger-mode reference counter */

};



#ifdef CONFIG_HIST_TRIGGERS

extern struct irq_work hist_poll_work;

extern wait_queue_head_t hist_poll_wq;



static inline void hist_poll_wakeup(void)

{

	if (wq_has_sleeper(&hist_poll_wq))

		irq_work_queue(&hist_poll_work);

}



#define hist_poll_wait(file, wait)	\

	poll_wait(file, &hist_poll_wq, wait)

#endif



#define __TRACE_EVENT_FLAGS(name, value)				\

	static int __init trace_init_flags_##name(void)			\

	{								\

kernel/trace/trace_events.c

+14 −0
Original line number Diff line number Diff line
@@ -3094,6 +3094,20 @@ static bool event_in_systems(struct trace_event_call *call, 
	return !*p || isspace(*p) || *p == ',';

}



#ifdef CONFIG_HIST_TRIGGERS

/*

 * Wake up waiter on the hist_poll_wq from irq_work because the hist trigger

 * may happen in any context.

 */

static void hist_poll_event_irq_work(struct irq_work *work)

{

	wake_up_all(&hist_poll_wq);

}



DEFINE_IRQ_WORK(hist_poll_work, hist_poll_event_irq_work);

DECLARE_WAIT_QUEUE_HEAD(hist_poll_wq);

#endif



static struct trace_event_file *

trace_create_new_event(struct trace_event_call *call,

		       struct trace_array *tr)

kernel/trace/trace_events_hist.c

+67 −3
Original line number Diff line number Diff line
@@ -5311,6 +5311,8 @@ static void event_hist_trigger(struct event_trigger_data *data, 


	if (resolve_var_refs(hist_data, key, var_ref_vals, true))

		hist_trigger_actions(hist_data, elt, buffer, rec, rbe, key, var_ref_vals);



	hist_poll_wakeup();

}



static void hist_trigger_stacktrace_print(struct seq_file *m,
@@ -5590,15 +5592,36 @@ static void hist_trigger_show(struct seq_file *m, 
		   n_entries, (u64)atomic64_read(&hist_data->map->drops));

}



struct hist_file_data {

	struct file *file;

	u64 last_read;

};



static u64 get_hist_hit_count(struct trace_event_file *event_file)

{

	struct hist_trigger_data *hist_data;

	struct event_trigger_data *data;

	u64 ret = 0;



	list_for_each_entry(data, &event_file->triggers, list) {

		if (data->cmd_ops->trigger_type == ETT_EVENT_HIST) {

			hist_data = data->private_data;

			ret += atomic64_read(&hist_data->map->hits);

		}

	}

	return ret;

}



static int hist_show(struct seq_file *m, void *v)

{

	struct hist_file_data *hist_file = m->private;

	struct event_trigger_data *data;

	struct trace_event_file *event_file;

	int n = 0;



	guard(mutex)(&event_mutex);



	event_file = event_file_file(m->private);

	event_file = event_file_file(hist_file->file);

	if (unlikely(!event_file))

		return -ENODEV;
@@ -5606,27 +5629,68 @@ static int hist_show(struct seq_file *m, void *v) 
		if (data->cmd_ops->trigger_type == ETT_EVENT_HIST)

			hist_trigger_show(m, data, n++);

	}

	hist_file->last_read = get_hist_hit_count(event_file);



	return 0;

}



static __poll_t event_hist_poll(struct file *file, struct poll_table_struct *wait)

{

	struct trace_event_file *event_file;

	struct seq_file *m = file->private_data;

	struct hist_file_data *hist_file = m->private;



	guard(mutex)(&event_mutex);



	event_file = event_file_data(file);

	if (!event_file)

		return EPOLLERR;



	hist_poll_wait(file, wait);



	if (hist_file->last_read != get_hist_hit_count(event_file))

		return EPOLLIN | EPOLLRDNORM;



	return 0;

}



static int event_hist_release(struct inode *inode, struct file *file)

{

	struct seq_file *m = file->private_data;

	struct hist_file_data *hist_file = m->private;



	kfree(hist_file);

	return tracing_single_release_file_tr(inode, file);

}



static int event_hist_open(struct inode *inode, struct file *file)

{

	struct hist_file_data *hist_file;

	int ret;



	ret = tracing_open_file_tr(inode, file);

	if (ret)

		return ret;



	hist_file = kzalloc(sizeof(*hist_file), GFP_KERNEL);

	if (!hist_file)

		return -ENOMEM;

	hist_file->file = file;



	/* Clear private_data to avoid warning in single_open() */

	file->private_data = NULL;

	return single_open(file, hist_show, file);

	ret = single_open(file, hist_show, hist_file);

	if (ret)

		kfree(hist_file);

	return ret;

}



const struct file_operations event_hist_fops = {

	.open = event_hist_open,

	.read = seq_read,

	.llseek = seq_lseek,

	.release = tracing_single_release_file_tr,

	.release = event_hist_release,

	.poll = event_hist_poll,

};



#ifdef CONFIG_HIST_TRIGGERS_DEBUG