[Intel-gfx] [PATCH 08/66] drm/i915: Make the stale cached active node available for any timeline

[Tvrtko Ursulin]

On 15/07/2020 12:50, Chris Wilson wrote:
> Rather than require the next timeline after idling to match the MRU
> before idling, reset the index on the node and allow it to match the
> first request. However, this requires cmpxchg(u64) and so is not trivial
> on 32b, so for compatibility we just fallback to keeping the cached node
> pointing to the MRU timeline.
> 
> Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
> ---
>   drivers/gpu/drm/i915/i915_active.c | 21 +++++++++++++++++++--
>   1 file changed, 19 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/gpu/drm/i915/i915_active.c b/drivers/gpu/drm/i915/i915_active.c
> index 0854b1552bc1..6737b5615c0c 100644
> --- a/drivers/gpu/drm/i915/i915_active.c
> +++ b/drivers/gpu/drm/i915/i915_active.c
> @@ -157,6 +157,10 @@ __active_retire(struct i915_active *ref)
>   		rb_link_node(&ref->cache->node, NULL, &ref->tree.rb_node);
>   		rb_insert_color(&ref->cache->node, &ref->tree);
>   		GEM_BUG_ON(ref->tree.rb_node != &ref->cache->node);
> +
> +		/* Make the cached node available for reuse with any timeline */
> +		if (IS_ENABLED(CONFIG_64BIT))
> +			ref->cache->timeline = 0; /* needs cmpxchg(u64) */

Or when fence context wraps shock horror.

>   	}
>   
>   	spin_unlock_irqrestore(&ref->tree_lock, flags);
> @@ -235,9 +239,22 @@ static struct active_node *__active_lookup(struct i915_active *ref, u64 idx)
>   {
>   	struct active_node *it;
>   
> +	GEM_BUG_ON(idx == 0); /* 0 is the unordered timeline, rsvd for cache */
> +
>   	it = READ_ONCE(ref->cache);
> -	if (it && it->timeline == idx)
> -		return it;
> +	if (it) {
> +		u64 cached = READ_ONCE(it->timeline);
> +
> +		if (cached == idx)
> +			return it;
> +
> +#ifdef CONFIG_64BIT /* for cmpxchg(u64) */
> +		if (!cached && !cmpxchg(&it->timeline, 0, idx)) {
> +			GEM_BUG_ON(i915_active_fence_isset(&it->base));
> +			return it;

cpmxchg suggests this needs to be atomic, however above the check for 
equality comes from a separate read.

Since there is a lookup code path under the spinlock, perhaps the 
unlocked lookup could just fail, and then locked lookup could re-assign 
the timeline without the need for cmpxchg?

Regards,

Tvrtko

> +		}
> +#endif
> +	}
>   
>   	BUILD_BUG_ON(offsetof(typeof(*it), node));
>   
>