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

[...]

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...

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);

-- Steve

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