Re: [PATCH v4 2/2] tracing: add per-CPU memory usage statistics in tracefs

[Vincent Donnefort <vdonnefort@google.com>]

On Mon, Sep 21, 2026 at 07:30:46PM +0800, Xiang Gao wrote:
> Provide a per-CPU view of the tracing ring-buffer memory usage under
> trace_stats/per_cpu/cpuN/memory_usage_kb, using the same format as the
> aggregate file. Each file reports only the buffers belonging to that CPU
> across the global trace array and all tracing instances.
> 
> Signed-off-by: Xiang Gao <gaoxiang17@xiaomi.com>
> ---
>  Documentation/trace/ftrace.rst |  3 ++
>  kernel/trace/trace.c           | 67 +++++++++++++++++++++++++++++++++-
>  2 files changed, 69 insertions(+), 1 deletion(-)
> 
> diff --git a/Documentation/trace/ftrace.rst b/Documentation/trace/ftrace.rst
> index 99ddfe26b7cd..e53cdd8d6702 100644
> --- a/Documentation/trace/ftrace.rst
> +++ b/Documentation/trace/ftrace.rst
> @@ -230,6 +230,9 @@ of ftrace. Here is a list of some of the key files:
>  	    buffers: ...
>  	    snapshot_buffers: ...
>  
> +	The same file is provided under trace_stats/per_cpu/cpuN/, reporting
> +	only the buffers belonging to that CPU.
> +
>    buffer_subbuf_size_kb:

Could you split the documentation part into a different commit?

>  
>  	This sets or displays the sub buffer size. The ring buffer is broken up
> diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
> index d4a913ff8a69..4010f28f3ebf 100644
> --- a/kernel/trace/trace.c
> +++ b/kernel/trace/trace.c
> @@ -5844,6 +5844,52 @@ static const struct file_operations trace_mem_fops = {
>  	.release	= single_release,
>  };
>  
> +static struct trace_mem_stats trace_buffers_memory_cpu(int cpu)
> +{
> +	struct trace_mem_stats stats = {};
> +	struct trace_array *tr;
> +
> +	guard(mutex)(&trace_types_lock);
> +
> +	list_for_each_entry(tr, &ftrace_trace_arrays, list)
> +		trace_array_buffer_memory(tr, cpu, &stats.buffers,
> +					  &stats.snapshot);
> +
> +	/* Account for the bootstrapping temp_buffer as well. */
> +	if (temp_buffer)
> +		stats.buffers += ring_buffer_memory_size(temp_buffer, cpu);
> +
> +	return stats;
> +}
> +
> +static int trace_mem_per_cpu_show(struct seq_file *m, void *v)
> +{
> +	struct trace_mem_stats stats = trace_buffers_memory_cpu((long)m->private);
> +
> +	seq_printf(m, "buffers: %lu\n", stats.buffers >> 10);
> +	seq_printf(m, "snapshot_buffers: %lu\n", stats.snapshot >> 10);
> +
> +	return 0;
> +}
> +
> +static int trace_mem_per_cpu_open(struct inode *inode, struct file *file)
> +{
> +	int ret;
> +
> +	ret = tracing_check_open_get_tr(NULL);
> +	if (ret)
> +		return ret;
> +
> +	return single_open(file, trace_mem_per_cpu_show, inode->i_private);
> +}
> +
> +static const struct file_operations trace_mem_per_cpu_fops = {
> +	.open		= trace_mem_per_cpu_open,
> +	.read		= seq_read,
> +	.llseek		= seq_lseek,
> +	.release	= single_release,
> +};
> +
>  #define LAST_BOOT_HEADER ((void *)1)
>  
>  static void *l_next(struct seq_file *m, void *v, loff_t *pos)
> @@ -9360,7 +9406,9 @@ static struct notifier_block trace_module_nb = {
>  
>  static __init void init_trace_stats_tracefs(void)
>  {
> -	struct dentry *stats_dir;
> +	struct dentry *stats_dir, *per_cpu_dir, *cpu_dir;
> +	char cpu_dir_name[30];
> +	int cpu;
>  
>  	/*
>  	 * tracer_alloc_buffers() frees tracing_buffer_mask and temp_buffer
> @@ -9376,6 +9424,23 @@ static __init void init_trace_stats_tracefs(void)
>  
>  	trace_create_file("memory_usage_kb", TRACE_MODE_READ, stats_dir,
>  			  NULL, &trace_mem_fops);
> +
> +	per_cpu_dir = tracefs_create_dir("per_cpu", stats_dir);
> +	if (!per_cpu_dir)
> +		return;
> +
> +	for_each_tracing_cpu(cpu) {
> +		snprintf(cpu_dir_name, 30, "cpu%d", cpu);
> +		cpu_dir = tracefs_create_dir(cpu_dir_name, per_cpu_dir);
> +		if (!cpu_dir) {
> +			pr_warn("Could not create tracefs '%s' entry\n",
> +				cpu_dir_name);
> +			continue;
> +		}
> +
> +		trace_create_file("memory_usage_kb", TRACE_MODE_READ, cpu_dir,
> +				  (void *)(long)cpu, &trace_mem_per_cpu_fops);
> +	}
>  }
>  
>  static __init void tracer_init_tracefs_work_func(struct work_struct *work)
> -- 
> 2.34.1
> 

-- 
Vincent