Re: [PATCH v5 1/2] ring-buffer: Introducing ring-buffer mapping functions

On Thu, 3 Aug 2023 11:33:39 +0100
Vincent Donnefort [off-list ref] wrote:

[...]

quoted

quoted

And on the kernel side, just a function to update the "writer fields" of the
meta-page:

   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);
  +
  +       rb_update_meta_page(cpu_buffer);
   
          wake_up_all(&rbwork->waiters);

That would rate limit the number of updates to the meta-page without any irq storm?
  

Is poll an issue? It requires user space to do a system call to see if
there's more data? But I guess that's not too much of an issue, as it needs
to do the ioctl to get the reader page.  

I don't think there's any problem with this approach, beside the extra system
call...

quoted

We could also add an option to the ioctl to block, or have the ioctl honor
the NON_BLOCK flags of the fd?  

... but indeed, we could block there. The userspace interface would be even simpler.
How about?

  +++ b/kernel/trace/trace.c
  @@ -8499,12 +8499,22 @@ static long tracing_buffers_ioctl(struct file *file, unsigned int cmd, unsigned
   {
          struct ftrace_buffer_info *info = file->private_data;
          struct trace_iterator *iter = &info->iter;
  +       int err;
  +
  +       if (cmd == TRACE_MMAP_IOCTL_GET_READER_PAGE) {
  +               if (!(file->f_flags & O_NONBLOCK)) {
  +                       err = ring_buffer_wait(iter->array_buffer->buffer,
  +                                              iter->cpu_file,
  +                                              iter->tr->buffer_percent);
  +                       if (err)
  +                               return err;
  +               }
   
  -       if (cmd == TRACE_MMAP_IOCTL_GET_READER_PAGE)
                  return ring_buffer_map_get_reader_page(iter->array_buffer->buffer,
                                                         iter->cpu_file);

Looks good to me.

-- Steve