Re: [Intel-gfx] [PATCH] drm/i915/gt: Only swap to a random sibling once upon creation

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

On 13/07/2020 17:05, Chris Wilson wrote:
> The danger in switching at random upon intel_context_pin is that the
> context may still actually be inflight, as it will not be scheduled out
> until a context switch after it is complete -- that is after we do a
> final intel_context_unpin.
> 
> Closes: https://gitlab.freedesktop.org/drm/intel/-/issues/2118
> Fixes: 6d06779e8672 ("drm/i915: Load balancing across a virtual engine")
> Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
> Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
> Cc: <stable@vger.kernel.org> # v5.3+
> ---
>   drivers/gpu/drm/i915/gt/intel_lrc.c | 18 ++++--------------
>   1 file changed, 4 insertions(+), 14 deletions(-)
> 
> diff --git a/drivers/gpu/drm/i915/gt/intel_lrc.c b/drivers/gpu/drm/i915/gt/intel_lrc.c
> index cd4262cc96e2..f4658849b08a 100644
> --- a/drivers/gpu/drm/i915/gt/intel_lrc.c
> +++ b/drivers/gpu/drm/i915/gt/intel_lrc.c
> @@ -5435,13 +5435,8 @@ static void virtual_engine_initial_hint(struct virtual_engine *ve)
>   	 * typically be the first we inspect for submission.
>   	 */
>   	swp = prandom_u32_max(ve->num_siblings);
> -	if (!swp)
> -		return;
> -
> -	swap(ve->siblings[swp], ve->siblings[0]);
> -	if (!intel_engine_has_relative_mmio(ve->siblings[0]))
> -		virtual_update_register_offsets(ve->context.lrc_reg_state,
> -						ve->siblings[0]);
> +	if (swp)
> +		swap(ve->siblings[swp], ve->siblings[0]);
>   }
>   
>   static int virtual_context_alloc(struct intel_context *ce)
> @@ -5454,15 +5449,9 @@ static int virtual_context_alloc(struct intel_context *ce)
>   static int virtual_context_pin(struct intel_context *ce)
>   {
>   	struct virtual_engine *ve = container_of(ce, typeof(*ve), context);
> -	int err;
>   
>   	/* Note: we must use a real engine class for setting up reg state */
> -	err = __execlists_context_pin(ce, ve->siblings[0]);
> -	if (err)
> -		return err;
> -
> -	virtual_engine_initial_hint(ve);
> -	return 0;
> +	return __execlists_context_pin(ce, ve->siblings[0]);
>   }
>   
>   static void virtual_context_enter(struct intel_context *ce)
> @@ -5808,6 +5797,7 @@ intel_execlists_create_virtual(struct intel_engine_cs **siblings,
>   
>   	ve->base.flags |= I915_ENGINE_IS_VIRTUAL;
>   
> +	virtual_engine_initial_hint(ve);
>   	return &ve->context;
>   
>   err_put:
> 

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

Regards,

Tvrtko