[PATCH v4 1/3] tracing/hist: Add poll(POLLIN) support on hist file

Masami Hiramatsu (Google)
2024-08-16T02:30:57Z

From: Masami Hiramatsu (Google) <mhiramat@kernel.org>

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 that is not good idea to peek the `trace` for the event randomely
happens. And `trace_pipe` is not coming back until a page is filled
with events.

This allows user to wait for a specific events on `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 you do read() on 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.

Signed-off-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
Reviewed-by: Tom Zanussi <zanussi@kernel.org>
---
include/linux/trace_events.h | 5 +++
kernel/trace/trace_events.c | 18 +++++++++
kernel/trace/trace_events_hist.c | 76 +++++++++++++++++++++++++++++++++++++-
3 files changed, 96 insertions(+), 3 deletions(-)

diff --git a/include/linux/trace_events.h b/include/linux/trace_events.h
index 42bedcddd511..f3ec67d34097 100644
--- a/include/linux/trace_events.h
+++ b/include/linux/trace_events.h
@@ -663,6 +663,11 @@ struct trace_event_file {
struct trace_subsystem_dir *system;
struct list_head triggers;

+#ifdef CONFIG_HIST_TRIGGERS
+ struct irq_work hist_work;
+ wait_queue_head_t hist_wq;
+#endif
+
/*
* 32 bit flags:
* bit 0: enabled
diff --git a/kernel/trace/trace_events.c b/kernel/trace/trace_events.c
index 7266ec2a4eea..0f077b32eea4 100644
--- a/kernel/trace/trace_events.c
+++ b/kernel/trace/trace_events.c
@@ -2972,6 +2972,20 @@ 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_wq from irq_work because the hist trigger
+ * may happen in any context.
+ */
+static void hist_event_irq_work(struct irq_work *work)
+{
+ struct trace_event_file *event_file;
+
+ event_file = container_of(work, struct trace_event_file, hist_work);
+ wake_up_all(&event_file->hist_wq);
+}
+#endif
+
static struct trace_event_file *
trace_create_new_event(struct trace_event_call *call,
struct trace_array *tr)
@@ -3004,6 +3018,10 @@ trace_create_new_event(struct trace_event_call *call,
INIT_LIST_HEAD(&file->triggers);
list_add(&file->list, &tr->events);
refcount_set(&file->ref, 1);
+#ifdef CONFIG_HIST_TRIGGERS
+ init_irq_work(&file->hist_work, hist_event_irq_work);
+ init_waitqueue_head(&file->hist_wq);
+#endif

return file;
}
diff --git a/kernel/trace/trace_events_hist.c b/kernel/trace/trace_events_hist.c
index 5f9119eb7c67..d27b60f54f68 100644
--- a/kernel/trace/trace_events_hist.c
+++ b/kernel/trace/trace_events_hist.c
@@ -5314,6 +5314,9 @@ 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);
+
+ if (hist_data->event_file && wq_has_sleeper(&hist_data->event_file->hist_wq))
+ irq_work_queue(&hist_data->event_file->hist_work);
}

static void hist_trigger_stacktrace_print(struct seq_file *m,
@@ -5593,15 +5596,36 @@ 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, ret = 0;

mutex_lock(&event_mutex);

- event_file = event_file_file(m->private);
+ event_file = event_file_file(hist_file->file);
if (unlikely(!event_file)) {
ret = -ENODEV;
goto out_unlock;
@@ -5611,6 +5635,7 @@ 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);

out_unlock:
mutex_unlock(&event_mutex);
@@ -5618,24 +5643,69 @@ static int hist_show(struct seq_file *m, void *v)
return ret;
}

+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;
+ __poll_t ret = 0;
+
+ mutex_lock(&event_mutex);
+
+ event_file = event_file_data(file);
+ if (!event_file) {
+ ret = EPOLLERR;
+ goto out_unlock;
+ }
+
+ poll_wait(file, &event_file->hist_wq, wait);
+
+ if (hist_file->last_read != get_hist_hit_count(event_file))
+ ret = EPOLLIN | EPOLLRDNORM;
+
+out_unlock:
+ mutex_unlock(&event_mutex);
+
+ return ret;
+}
+
+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