Re: [Intel-gfx] [PATCH 03/17] drm/i915: Improve the start alignment of bonded pairs

[Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>]

On 06/03/2020 13:38, Chris Wilson wrote:
> Always wait on the start of the signaler request to reduce the problem
> of dequeueing the bonded pair too early -- we want both payloads to
> start at the same time, with no latency, and yet still allow others to
> make full use of the slack in the system. This reduce the amount of time
> we spend waiting on the semaphore used to synchronise the start of the
> bonded payload.
> 
> Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
> ---
>   drivers/gpu/drm/i915/i915_request.c | 41 +++++++++++++++++++++++++----
>   1 file changed, 36 insertions(+), 5 deletions(-)
> 
> diff --git a/drivers/gpu/drm/i915/i915_request.c b/drivers/gpu/drm/i915/i915_request.c
> index 66efd16c4850..db11006b4ac9 100644
> --- a/drivers/gpu/drm/i915/i915_request.c
> +++ b/drivers/gpu/drm/i915/i915_request.c
> @@ -1128,14 +1128,45 @@ __i915_request_await_execution(struct i915_request *to,
>   					  &from->fence))
>   		return 0;
>   
> -	/* Ensure both start together [after all semaphores in signal] */
> -	if (intel_engine_has_semaphores(to->engine))
> -		err = __emit_semaphore_wait(to, from, from->fence.seqno - 1);
> -	else
> -		err = i915_request_await_start(to, from);
> +	/*
> +	 * Wait until the start of this request.
> +	 *
> +	 * The execution cb fires when we submit the request to HW. But in
> +	 * many cases this may be long before the request itself is ready to
> +	 * run (consider that we submit 2 requests for the same context, where
> +	 * the request of interest is behind an indefinite spinner). So we hook
> +	 * up to both to reduce our queues and keep the execution lag minimised
> +	 * in the worst case, though we hope that the await_start is elided.
> +	 */
> +	err = i915_request_await_start(to, from);
>   	if (err < 0)
>   		return err;
>   
> +	/*
> +	 * Ensure both start together [after all semaphores in signal]
> +	 *
> +	 * Now that we are queued to the HW at roughly the same time (thanks
> +	 * to the execute cb) and are ready to run at roughly the same time
> +	 * (thanks to the await start), our signaler may still be indefinitely
> +	 * delayed by waiting on a semaphore from a remote engine. If our
> +	 * signaler depends on a semaphore, so indirectly do we, and we do not
> +	 * want to start our payload until our signaler also starts theirs.
> +	 * So we wait.
> +	 *
> +	 * However, there is also a second condition for which we need to wait
> +	 * for the precise start of the signaler. Consider that the signaler
> +	 * was submitted in a chain of requests following another context
> +	 * (with just an ordinary intra-engine fence dependency between the
> +	 * two). In this case the signaler is queued to HW, but not for
> +	 * immediate execution, and so we must wait until it reaches the
> +	 * active slot.
> +	 */
> +	if (intel_engine_has_semaphores(to->engine)) {
> +		err = __emit_semaphore_wait(to, from, from->fence.seqno - 1);
> +		if (err < 0)
> +			return err;
> +	}
> +
>   	/* Couple the dependency tree for PI on this exposed to->fence */
>   	if (to->engine->schedule) {
>   		err = i915_sched_node_add_dependency(&to->sched, &from->sched);
> 

Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>

Regards,

Tvrtko
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