Re: [PATCH v9 1/2] ring-buffer: Introducing ring-buffer mapping functions

[Steven Rostedt]

On Thu, 21 Dec 2023 17:35:22 +0000
Vincent Donnefort <vdonnef...@google.com> wrote:

> @@ -739,6 +747,22 @@ static __always_inline bool full_hit(struct trace_buffer 
> *buffer, int cpu, int f
>       return (dirty * 100) > (full * nr_pages);
>  }
>  
> +static void rb_update_meta_page(struct ring_buffer_per_cpu *cpu_buffer)
> +{
> +     if (unlikely(READ_ONCE(cpu_buffer->mapped))) {
> +             /* Ensure the meta_page is ready */
> +             smp_rmb();
> +             WRITE_ONCE(cpu_buffer->meta_page->entries,
> +                        local_read(&cpu_buffer->entries));
> +             WRITE_ONCE(cpu_buffer->meta_page->overrun,
> +                        local_read(&cpu_buffer->overrun));
> +             WRITE_ONCE(cpu_buffer->meta_page->subbufs_touched,
> +                        local_read(&cpu_buffer->pages_touched));
> +             WRITE_ONCE(cpu_buffer->meta_page->subbufs_lost,
> +                        local_read(&cpu_buffer->pages_lost));
> +     }
> +}
> +
>  /*
>   * rb_wake_up_waiters - wake up tasks waiting for ring buffer input
>   *
> @@ -749,6 +773,18 @@ static void rb_wake_up_waiters(struct irq_work *work)
>  {
>       struct rb_irq_work *rbwork = container_of(work, struct rb_irq_work, 
> work);
>  
> +     if (rbwork->is_cpu_buffer) {
> +             struct ring_buffer_per_cpu *cpu_buffer;
> +
> +             cpu_buffer = container_of(rbwork, struct ring_buffer_per_cpu,
> +                                       irq_work);
> +             /*
> +              * If the waiter is a cpu_buffer, this might be due to a
> +              * userspace mapping. Let's update the meta-page.
> +              */
> +             rb_update_meta_page(cpu_buffer);
> +     }
> +
>       wake_up_all(&rbwork->waiters);
>       if (rbwork->full_waiters_pending || rbwork->wakeup_full) {
>               rbwork->wakeup_full = false;


I think this code would be cleaner if we did:

static void rb_update_meta_page(strucrt rb_irq_work *rbwork)
{
        struct ring_buffer_per_cpu *cpu_buffer;

        if (!rbwork->is_cpu_buffer)
                return;

        /*
         * If the waiter is a cpu_buffer, this might be due to a
         * userspace mapping. Let's update the meta-page.
         */
        cpu_buffer = container_of(rbwork, struct ring_buffer_per_cpu,
                                  irq_work);

        if (unlikely(READ_ONCE(cpu_buffer->mapped))) {

// I don't think we need the "unlikely"

                /* Ensure the meta_page is ready */
                smp_rmb();
                WRITE_ONCE(cpu_buffer->meta_page->entries,
                           local_read(&cpu_buffer->entries));
                WRITE_ONCE(cpu_buffer->meta_page->overrun,
                           local_read(&cpu_buffer->overrun));
                WRITE_ONCE(cpu_buffer->meta_page->subbufs_touched,
                           local_read(&cpu_buffer->pages_touched));
                WRITE_ONCE(cpu_buffer->meta_page->subbufs_lost,
                           local_read(&cpu_buffer->pages_lost));
        }
}

/*
 * rb_wake_up_waiters - wake up tasks waiting for ring buffer input
 *
 * Schedules a delayed work to wake up any task that is blocked on the
 * ring buffer waiters queue.
 */
static void rb_wake_up_waiters(struct irq_work *work)
{
        struct rb_irq_work *rbwork = container_of(work, struct rb_irq_work, 
work);

        rb_update_meta_page(cpu_buffer);

        wake_up_all(&rbwork->waiters);
        if (rbwork->full_waiters_pending || rbwork->wakeup_full) {
                rbwork->wakeup_full = false;
                rbwork->full_waiters_pending = false;
                wake_up_all(&rbwork->full_waiters);
        }
}


-- Steve