Re: [Intel-gfx] [PATCH] drm/i915/gem: Utilize rcu iteration of context engines

[Mika Kuoppala <mika.kuoppala@linux.intel.com>]

Chris Wilson <chris@chris-wilson.co.uk> writes:

> Now that we can peek at GEM->engines[] and obtain a reference to them
> using RCU, do so for instances where we can safely iterate the
> potentially old copy of the engines. For setting, we can do this when we
> know the engine properties are copied over before swapping, so we know
> the new engines already have the global property and we update the old
> before they are discarded. For reading, we only need to be safe; as we
> do so on behalf of the user, their races are their own problem.
>
> Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>

Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>

> ---
>  drivers/gpu/drm/i915/gem/i915_gem_context.c | 59 +++++++++++----------
>  1 file changed, 31 insertions(+), 28 deletions(-)
>
> diff --git a/drivers/gpu/drm/i915/gem/i915_gem_context.c b/drivers/gpu/drm/i915/gem/i915_gem_context.c
> index 50e7580f9337..2b6dd08de6f1 100644
> --- a/drivers/gpu/drm/i915/gem/i915_gem_context.c
> +++ b/drivers/gpu/drm/i915/gem/i915_gem_context.c
> @@ -757,21 +757,46 @@ __create_context(struct drm_i915_private *i915)
>  	return ERR_PTR(err);
>  }
>  
> +static inline struct i915_gem_engines *
> +__context_engines_await(const struct i915_gem_context *ctx)
> +{
> +	struct i915_gem_engines *engines;
> +
> +	rcu_read_lock();
> +	do {
> +		engines = rcu_dereference(ctx->engines);
> +		GEM_BUG_ON(!engines);
> +
> +		if (unlikely(!i915_sw_fence_await(&engines->fence)))
> +			continue;
> +
> +		if (likely(engines == rcu_access_pointer(ctx->engines)))
> +			break;
> +
> +		i915_sw_fence_complete(&engines->fence);
> +	} while (1);
> +	rcu_read_unlock();
> +
> +	return engines;
> +}
> +
>  static int
>  context_apply_all(struct i915_gem_context *ctx,
>  		  int (*fn)(struct intel_context *ce, void *data),
>  		  void *data)
>  {
>  	struct i915_gem_engines_iter it;
> +	struct i915_gem_engines *e;
>  	struct intel_context *ce;
>  	int err = 0;
>  
> -	for_each_gem_engine(ce, i915_gem_context_lock_engines(ctx), it) {
> +	e = __context_engines_await(ctx);
> +	for_each_gem_engine(ce, e, it) {
>  		err = fn(ce, data);
>  		if (err)
>  			break;
>  	}
> -	i915_gem_context_unlock_engines(ctx);
> +	i915_sw_fence_complete(&e->fence);
>  
>  	return err;
>  }
> @@ -786,11 +811,13 @@ static int __apply_ppgtt(struct intel_context *ce, void *vm)
>  static struct i915_address_space *
>  __set_ppgtt(struct i915_gem_context *ctx, struct i915_address_space *vm)
>  {
> -	struct i915_address_space *old = i915_gem_context_vm(ctx);
> +	struct i915_address_space *old;
>  
> +	old = rcu_replace_pointer(ctx->vm,
> +				  i915_vm_open(vm),
> +				  lockdep_is_held(&ctx->mutex));
>  	GEM_BUG_ON(old && i915_vm_is_4lvl(vm) != i915_vm_is_4lvl(old));
>  
> -	rcu_assign_pointer(ctx->vm, i915_vm_open(vm));
>  	context_apply_all(ctx, __apply_ppgtt, vm);
>  
>  	return old;
> @@ -1069,30 +1096,6 @@ static void cb_retire(struct i915_active *base)
>  	kfree(cb);
>  }
>  
> -static inline struct i915_gem_engines *
> -__context_engines_await(const struct i915_gem_context *ctx)
> -{
> -	struct i915_gem_engines *engines;
> -
> -	rcu_read_lock();
> -	do {
> -		engines = rcu_dereference(ctx->engines);
> -		if (unlikely(!engines))
> -			break;
> -
> -		if (unlikely(!i915_sw_fence_await(&engines->fence)))
> -			continue;
> -
> -		if (likely(engines == rcu_access_pointer(ctx->engines)))
> -			break;
> -
> -		i915_sw_fence_complete(&engines->fence);
> -	} while (1);
> -	rcu_read_unlock();
> -
> -	return engines;
> -}
> -
>  I915_SELFTEST_DECLARE(static intel_engine_mask_t context_barrier_inject_fault);
>  static int context_barrier_task(struct i915_gem_context *ctx,
>  				intel_engine_mask_t engines,
> -- 
> 2.20.1
>
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