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

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Date:   Wed, 2 Aug 2023 13:30:56 +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

On Wed, Aug 02, 2023 at 07:45:26AM -0400, Steven Rostedt wrote:
> On Tue, 1 Aug 2023 13:26:03 -0400
> Steven Rostedt <rostedt@...dmis.org> wrote:
> 
> > > +
> > > +	if (READ_ONCE(cpu_buffer->mapped)) {
> > > +		/* Ensure the meta_page is ready */
> > > +		smp_rmb();
> > > +		WRITE_ONCE(cpu_buffer->meta_page->pages_touched,
> > > +			   local_read(&cpu_buffer->pages_touched));
> > > +	}  
> > 
> > I was thinking instead of doing this in the semi fast path, put this logic
> > into the rb_wakeup_waiters() code. That is, if a task is mapped, we call
> > the irq_work() to do this for us. It could even do more, like handle
> > blocked mapped waiters.
> 
> I was thinking how to implement this, and I worry that it may cause an irq
> storm. Let's keep this (and the other locations) as is, where we do the
> updates in place. Then we can look at seeing if it is possible to do it in
> a delayed fashion another time.

I actually looking at this. How about:

On the userspace side, a simple poll:

  static void wait_entries(int fd)
  {
          struct pollfd pollfd = {
                  .fd     = fd,
                  .events = POLLIN,
          };
  
          if (poll(&pollfd, 1, -1) == -1)
                  pdie("poll");
  }

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?

> 
> -- Steve