linux-kernel - Re: [PATCH 8/8] tracing: Update modules to persistent instances when loaded

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Message-Id: <20250206190124.1518c3bf070e862da7993d9e@kernel.org>
Date: Thu, 6 Feb 2025 19:01:24 +0900
From: Masami Hiramatsu (Google) <mhiramat@...nel.org>
To: Steven Rostedt <rostedt@...dmis.org>
Cc: linux-kernel@...r.kernel.org, linux-trace-kernel@...r.kernel.org, Masami
 Hiramatsu <mhiramat@...nel.org>, Mark Rutland <mark.rutland@....com>,
 Mathieu Desnoyers <mathieu.desnoyers@...icios.com>, Andrew Morton
 <akpm@...ux-foundation.org>
Subject: Re: [PATCH 8/8] tracing: Update modules to persistent instances
 when loaded

On Wed, 05 Feb 2025 17:50:39 -0500
Steven Rostedt <rostedt@...dmis.org> wrote:

> From: Steven Rostedt <rostedt@...dmis.org>
> 
> When a module is loaded and a persistent buffer is actively tracing, add
> it to the list of modules in the persistent memory.
> 
> Signed-off-by: Steven Rostedt (Google) <rostedt@...dmis.org>
> ---
>  kernel/trace/trace.c        | 25 +++++++++++++++++++++++
>  kernel/trace/trace.h        |  2 ++
>  kernel/trace/trace_events.c | 40 ++++++++++++++++++++++++++-----------
>  3 files changed, 55 insertions(+), 12 deletions(-)
> 
> diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
> index 7b4027804cd4..443f2bc5b856 100644
> --- a/kernel/trace/trace.c
> +++ b/kernel/trace/trace.c
> @@ -10088,6 +10088,30 @@ static void trace_module_remove_evals(struct module *mod)
>  static inline void trace_module_remove_evals(struct module *mod) { }
>  #endif /* CONFIG_TRACE_EVAL_MAP_FILE */
>  
> +static bool trace_array_active(struct trace_array *tr)
> +{
> +	if (tr->current_trace != &nop_trace)
> +		return true;
> +
> +	/* 0 is no events, 1 is all disabled */
> +	return trace_events_enabled(tr, NULL) > 1;
> +}
> +
> +static void trace_module_record(struct module *mod)
> +{
> +	struct trace_array *tr;
> +
> +	list_for_each_entry(tr, &ftrace_trace_arrays, list) {
> +		/* Update any persistent trace array that has already been started */
> +		if ((tr->flags & (TRACE_ARRAY_FL_BOOT | TRACE_ARRAY_FL_LAST_BOOT)) ==
> +		    TRACE_ARRAY_FL_BOOT) {
> +			/* Only update if the trace array is active */
> +			if (trace_array_active(tr))

Do we really need this check? It seems that we can just save_mod() if the
above condition is true.

> +				save_mod(mod, tr);
> +		}
> +	}
> +}
> +
>  static int trace_module_notify(struct notifier_block *self,
>  			       unsigned long val, void *data)
>  {
> @@ -10096,6 +10120,7 @@ static int trace_module_notify(struct notifier_block *self,
>  	switch (val) {
>  	case MODULE_STATE_COMING:
>  		trace_module_add_evals(mod);
> +		trace_module_record(mod);
>  		break;
>  	case MODULE_STATE_GOING:
>  		trace_module_remove_evals(mod);

Don't we need to remove the module entry when a module is removed?
(everytime we remove a module, trace data is cleared?)

Thank you,

> diff --git a/kernel/trace/trace.h b/kernel/trace/trace.h
> index 3a020fb82a34..90493220c362 100644
> --- a/kernel/trace/trace.h
> +++ b/kernel/trace/trace.h
> @@ -786,6 +786,8 @@ extern void trace_find_cmdline(int pid, char comm[]);
>  extern int trace_find_tgid(int pid);
>  extern void trace_event_follow_fork(struct trace_array *tr, bool enable);
>  
> +extern int trace_events_enabled(struct trace_array *tr, const char *system);
> +
>  #ifdef CONFIG_DYNAMIC_FTRACE
>  extern unsigned long ftrace_update_tot_cnt;
>  extern unsigned long ftrace_number_of_pages;
> diff --git a/kernel/trace/trace_events.c b/kernel/trace/trace_events.c
> index 4cb275316e51..107767afe0ab 100644
> --- a/kernel/trace/trace_events.c
> +++ b/kernel/trace/trace_events.c
> @@ -1811,28 +1811,28 @@ event_enable_write(struct file *filp, const char __user *ubuf, size_t cnt,
>  	return cnt;
>  }
>  
> -static ssize_t
> -system_enable_read(struct file *filp, char __user *ubuf, size_t cnt,
> -		   loff_t *ppos)
> +/*
> + * Returns:
> + *   0 : no events exist?
> + *   1 : all events are disabled
> + *   2 : all events are enabled
> + *   3 : some events are enabled and some are enabled
> + */
> +int trace_events_enabled(struct trace_array *tr, const char *system)
>  {
> -	const char set_to_char[4] = { '?', '0', '1', 'X' };
> -	struct trace_subsystem_dir *dir = filp->private_data;
> -	struct event_subsystem *system = dir->subsystem;
>  	struct trace_event_call *call;
>  	struct trace_event_file *file;
> -	struct trace_array *tr = dir->tr;
> -	char buf[2];
>  	int set = 0;
> -	int ret;
>  
> -	mutex_lock(&event_mutex);
> +	guard(mutex)(&event_mutex);
> +
>  	list_for_each_entry(file, &tr->events, list) {
>  		call = file->event_call;
>  		if ((call->flags & TRACE_EVENT_FL_IGNORE_ENABLE) ||
>  		    !trace_event_name(call) || !call->class || !call->class->reg)
>  			continue;
>  
> -		if (system && strcmp(call->class->system, system->name) != 0)
> +		if (system && strcmp(call->class->system, system) != 0)
>  			continue;
>  
>  		/*
> @@ -1848,7 +1848,23 @@ system_enable_read(struct file *filp, char __user *ubuf, size_t cnt,
>  		if (set == 3)
>  			break;
>  	}
> -	mutex_unlock(&event_mutex);
> +
> +	return set;
> +}
> +
> +static ssize_t
> +system_enable_read(struct file *filp, char __user *ubuf, size_t cnt,
> +		   loff_t *ppos)
> +{
> +	const char set_to_char[4] = { '?', '0', '1', 'X' };
> +	struct trace_subsystem_dir *dir = filp->private_data;
> +	struct event_subsystem *system = dir->subsystem;
> +	struct trace_array *tr = dir->tr;
> +	char buf[2];
> +	int set;
> +	int ret;
> +
> +	set = trace_events_enabled(tr, system ? system->name : NULL);
>  
>  	buf[0] = set_to_char[set];
>  	buf[1] = '\n';
> -- 
> 2.45.2
> 
> 
> 


-- 
Masami Hiramatsu (Google) <mhiramat@...nel.org>