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

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Date:   Thu, 3 Aug 2023 11:33:39 +0100
From:   Vincent Donnefort <vdonnefort@...gle.com>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     mhiramat@...nel.org, linux-kernel@...r.kernel.org,
        linux-trace-kernel@...r.kernel.org, kernel-team@...roid.com
Subject: Re: [PATCH v5 1/2] ring-buffer: Introducing ring-buffer mapping
 functions

[...]

> > 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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>