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

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Date:   Thu, 3 Aug 2023 10:52:42 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Vincent Donnefort <vdonnefort@...gle.com>
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

On Thu, 3 Aug 2023 11:33:39 +0100
Vincent Donnefort <vdonnefort@...gle.com> wrote:

> [...]
> 
> > > 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);
> 

Looks good to me.

-- Steve