Re: [PATCH v8 0/2] Introducing trace buffer mapping by user-space

[Vincent Donnefort <vdonnefort@google.com>]

On Tue, Dec 19, 2023 at 03:39:24PM -0500, Steven Rostedt wrote:
> On Tue, 19 Dec 2023 18:45:54 +0000
> Vincent Donnefort <vdonnefort@google.com> wrote:
> 
> > The tracing ring-buffers can be stored on disk or sent to network
> > without any copy via splice. However the later doesn't allow real time
> > processing of the traces. A solution is to give userspace direct access
> > to the ring-buffer pages via a mapping. An application can now become a
> > consumer of the ring-buffer, in a similar fashion to what trace_pipe
> > offers.
> > 
> > Attached to this cover letter an example of consuming read for a
> > ring-buffer, using libtracefs.
> > 
> 
> I'm still testing this, but I needed to add this patch to fix two bugs. One
> is that you are calling rb_wakeup_waiters() for both the buffer and the
> cpu_buffer, and it needs to know which one to use the container_of() macro.
> 
> The other is a "goto unlock" that unlocks two locks where only one was taken.
> 
> -- Steve
> 
> diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c
> index 35f3736f660b..987ad7bd1e8b 100644
> --- a/kernel/trace/ring_buffer.c
> +++ b/kernel/trace/ring_buffer.c
> @@ -389,6 +389,7 @@ struct rb_irq_work {
>  	bool				waiters_pending;
>  	bool				full_waiters_pending;
>  	bool				wakeup_full;
> +	bool				is_cpu_buffer;
>  };
>  
>  /*
> @@ -771,10 +772,20 @@ static void rb_update_meta_page(struct ring_buffer_per_cpu *cpu_buffer)
>  static void rb_wake_up_waiters(struct irq_work *work)
>  {
>  	struct rb_irq_work *rbwork = container_of(work, struct rb_irq_work, work);
> -	struct ring_buffer_per_cpu *cpu_buffer =
> -		container_of(rbwork, struct ring_buffer_per_cpu, irq_work);
> +	struct ring_buffer_per_cpu *cpu_buffer;
> +	struct trace_buffer *buffer;
> +	int cpu;
>  
> -	rb_update_meta_page(cpu_buffer);
> +	if (rbwork->is_cpu_buffer) {
> +		cpu_buffer = container_of(rbwork, struct ring_buffer_per_cpu, irq_work);
> +		rb_update_meta_page(cpu_buffer);
> +	} else {
> +		buffer = container_of(rbwork, struct trace_buffer, irq_work);
> +		for_each_buffer_cpu(buffer, cpu) {
> +			cpu_buffer = buffer->buffers[cpu];
> +			rb_update_meta_page(cpu_buffer);
> +		}
> +	}

Arg, somehow never reproduced the problem :-\. I suppose you need to cat
trace/trace_pipe and mmap(trace/cpuX/trace_pipe) at the same time?

Updating the meta-page is only useful if the reader we are waking up is a
user-space one, which would only happen with the cpu_buffer version of this
function. We could limit the update of the meta_page only to this case?

>  
>  	wake_up_all(&rbwork->waiters);
>  	if (rbwork->full_waiters_pending || rbwork->wakeup_full) {
> @@ -1569,6 +1580,7 @@ rb_allocate_cpu_buffer(struct trace_buffer *buffer, long nr_pages, int cpu)
>  	init_waitqueue_head(&cpu_buffer->irq_work.waiters);
>  	init_waitqueue_head(&cpu_buffer->irq_work.full_waiters);
>  	mutex_init(&cpu_buffer->mapping_lock);
> +	cpu_buffer->irq_work.is_cpu_buffer = true;
>  
>  	bpage = kzalloc_node(ALIGN(sizeof(*bpage), cache_line_size()),
>  			    GFP_KERNEL, cpu_to_node(cpu));
> @@ -6209,7 +6221,8 @@ int ring_buffer_map(struct trace_buffer *buffer, int cpu)
>  
>  	if (cpu_buffer->mapped) {
>  		WRITE_ONCE(cpu_buffer->mapped, cpu_buffer->mapped + 1);
> -		goto unlock;
> +		mutex_unlock(&cpu_buffer->mapping_lock);
> +		return 0;
>  	}
>  
>  	/* prevent another thread from changing buffer sizes */