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Message-Id: <20241227120406.48efd9faf4ef187493a52f64@kernel.org>
Date: Fri, 27 Dec 2024 12:04:06 +0900
From: Masami Hiramatsu (Google) <mhiramat@...nel.org>
To: Steven Rostedt <rostedt@...dmis.org>
Cc: Shuah Khan <shuah@...nel.org>, Tom Zanussi <zanussi@...nel.org>, Mathieu
Desnoyers <mathieu.desnoyers@...icios.com>, linux-kernel@...r.kernel.org,
linux-trace-kernel@...r.kernel.org, linux-kselftest@...r.kernel.org
Subject: Re: [PATCH v6 1/3] tracing/hist: Add poll(POLLIN) support on hist
file
On Thu, 19 Dec 2024 11:23:52 -0500
Steven Rostedt <rostedt@...dmis.org> wrote:
> On Wed, 16 Oct 2024 19:49:24 +0900
> "Masami Hiramatsu (Google)" <mhiramat@...nel.org> wrote:
>
> > From: Masami Hiramatsu (Google) <mhiramat@...nel.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.
>
> I would reword the above to:
>
> 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 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.
>
> And that to:
>
> 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.
OK, I will update the description.
Thank you!
>
> >
> > 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@...nel.org>
> > Reviewed-by: Tom Zanussi <zanussi@...nel.org>
> > ---
> > Changes in v6:
> > - Use a global poll irq_work and wait_queue.
> > ---
> > include/linux/trace_events.h | 14 +++++++
> > kernel/trace/trace_events.c | 14 +++++++
> > kernel/trace/trace_events_hist.c | 75 ++++++++++++++++++++++++++++++++++++--
> > 3 files changed, 100 insertions(+), 3 deletions(-)
> >
> > diff --git a/include/linux/trace_events.h b/include/linux/trace_events.h
> > index 42bedcddd511..46c771a61f2a 100644
> > --- a/include/linux/trace_events.h
> > +++ b/include/linux/trace_events.h
> > @@ -685,6 +685,20 @@ 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) \
> > { \
> > diff --git a/kernel/trace/trace_events.c b/kernel/trace/trace_events.c
> > index 7266ec2a4eea..0a7cb30043ef 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_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)
> > diff --git a/kernel/trace/trace_events_hist.c b/kernel/trace/trace_events_hist.c
> > index 5f9119eb7c67..107eaa0f40f1 100644
> > --- a/kernel/trace/trace_events_hist.c
> > +++ b/kernel/trace/trace_events_hist.c
> > @@ -5314,6 +5314,8 @@ 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,
> > @@ -5593,15 +5595,36 @@ 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 +5634,7 @@ 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 +5642,69 @@ 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);
>
> Let's start using guard(mutex)(&event_mutex);
>
> I'm working on changing the other locations in this file with a separate
> patch. I don't want to add new ones.
>
> -- Steve
>
> > +
> > + event_file = event_file_data(file);
> > + if (!event_file) {
> > + ret = EPOLLERR;
> > + goto out_unlock;
> > + }
> > +
> > + hist_poll_wait(file, 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
>
--
Masami Hiramatsu (Google) <mhiramat@...nel.org>
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