On Sun, 18 Aug 2024 22:18:34 +0900
"Masami Hiramatsu (Google)" <mhiramat@xxxxxxxxxx> wrote:
> From: Masami Hiramatsu (Google) <mhiramat@xxxxxxxxxx>
>
> 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@xxxxxxxxxx>
> Reviewed-by: Tom Zanussi <zanussi@xxxxxxxxxx>
> ---
> 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
I really hate to add this to the trace_event_file, as that is created for
every event. If there are 1853 events (that's what my machine shows), and
sizeof(struct irq_work) = 32 bytes, and sizeof(wait_queue_head_t) is 24
bytes, this ends up adding 103,768 bytes to every instance!
That's a bit of bloat.
Can we make this a global work queue, and perhaps add a flag to flags that
states there's a waiter? Then on poll wakeup, we can check if the change
happened to hist that the waiter is waiting on (which it actually already does!)
> +
> /*
> * 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);
Instead of using wq_has_sleeper() we can use the event_file->flags instead.
> }
>
> 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;
Even if this gets woken up early, it will still not go back to user space.
I don't expect a lot of hist waiters so this should not be an issue.
I don't see an issue if we just have the hist wait queue global.
-- Steve
> +
> +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
