[PATCH 5/4] dma-fence: Have dma_fence_signal call signal_locked

[Chris Wilson]

Quoting Chris Wilson (2019-08-15 20:03:13)
> Quoting Daniel Vetter (2019-08-15 19:48:42)
> > On Thu, Aug 15, 2019 at 8:46 PM Chris Wilson <chris at chris-wilson.co.uk> wrote:
> > > Quoting Daniel Vetter (2019-08-14 18:20:53)
> > > > On Sun, Aug 11, 2019 at 10:15:23AM +0100, Chris Wilson wrote:
> > > > > Now that dma_fence_signal always takes the spinlock to flush the
> > > > > cb_list, simply take the spinlock and call dma_fence_signal_locked() to
> > > > > avoid code repetition.
> > > > >
> > > > > Suggested-by: Christian König <christian.koenig at amd.com>
> > > > > Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
> > > > > Cc: Christian König <christian.koenig at amd.com>
> > > >
> > > > Hm, I think this largely defeats the point of having the lockless signal
> > > > enabling trickery in dma_fence. Maybe that part isn't needed by anyone,
> > > > but feels like a thing that needs a notch more thought. And if we need it,
> > > > maybe a bit more cleanup.
> > >
> > > You mean dma_fence_enable_sw_signaling(). The only user appears to be to
> > > flush fences, which is actually the intent of always notifying the signal
> > > cb. By always doing the callbacks, we can avoid installing the interrupt
> > > and completely saturating CPUs with irqs, instead doing a batch in a
> > > leisurely timer callback if not flushed naturally.
> > 
> > Yeah I'm not against ditching this,
> 
> I was just thinking aloud working out what the current use case in ttm
> was for.
> 
> > but can't we ditch a lot more if
> > we just always take the spinlock in those paths now anyways? Kinda not
> > worth having the complexity anymore.
> 
> You would be able to drop the was_set from dma_fence_add_callback. Say
> 
> diff --git a/drivers/dma-buf/dma-fence.c b/drivers/dma-buf/dma-fence.c
> index 59ac96ec7ba8..e566445134a2 100644
> --- a/drivers/dma-buf/dma-fence.c
> +++ b/drivers/dma-buf/dma-fence.c
> @@ -345,38 +345,31 @@ int dma_fence_add_callback(struct dma_fence *fence, struct dma_fence_cb *cb,
>                            dma_fence_func_t func)
>  {
>         unsigned long flags;
> -       int ret = 0;
> -       bool was_set;
> +       int ret = -ENOENT;
> 
>         if (WARN_ON(!fence || !func))
>                 return -EINVAL;
> 
> -       if (test_bit(DMA_FENCE_FLAG_SIGNALED_BIT, &fence->flags)) {
> -               INIT_LIST_HEAD(&cb->node);
> +       INIT_LIST_HEAD(&cb->node);
> +       cb->func = func;
> +
> +       if (test_bit(DMA_FENCE_FLAG_SIGNALED_BIT, &fence->flags))
>                 return -ENOENT;
> -       }
> 
>         spin_lock_irqsave(fence->lock, flags);
> -
> -       was_set = test_and_set_bit(DMA_FENCE_FLAG_ENABLE_SIGNAL_BIT,
> -                                  &fence->flags);
> -
> -       if (test_bit(DMA_FENCE_FLAG_SIGNALED_BIT, &fence->flags))
> -               ret = -ENOENT;
> -       else if (!was_set && fence->ops->enable_signaling) {
> +       if (!test_bit(DMA_FENCE_FLAG_SIGNALED_BIT, &fence->flags) &&
> +           !test_and_set_bit(DMA_FENCE_FLAG_ENABLE_SIGNAL_BIT,
> +                             &fence->flags)) {
>                 trace_dma_fence_enable_signal(fence);
> 
> -               if (!fence->ops->enable_signaling(fence)) {
> +               if (!fence->ops->enable_signaling ||
> +                   fence->ops->enable_signaling(fence)) {
> +                       list_add_tail(&cb->node, &fence->cb_list);
> +                       ret = 0;
> +               } else {
>                         dma_fence_signal_locked(fence);
> -                       ret = -ENOENT;
>                 }
>         }
> -
> -       if (!ret) {
> -               cb->func = func;
> -               list_add_tail(&cb->node, &fence->cb_list);
> -       } else
> -               INIT_LIST_HEAD(&cb->node);
>         spin_unlock_irqrestore(fence->lock, flags);
> 
>         return ret;
> 
> Not a whole lot changes in terms of branches and serialising
> instructions. One less baffling sequence to worry about.

Fwiw,
Function                                     old     new   delta
dma_fence_add_callback                       338     302     -36

Almost certainly more shaving if you stare.
-Chris