Re: [PATCH 10/19] drm/i915/execlists: Assert there are no simple cycles in the dependencies

From: "Michał Winiarski" <michal.winiarski@intel.com>
To: Chris Wilson <chris@chris-wilson.co.uk>
Cc: intel-gfx@lists.freedesktop.org
Subject: Re: [PATCH 10/19] drm/i915/execlists: Assert there are no simple cycles in the dependencies
Date: Wed, 3 Jan 2018 12:13:40 +0100	[thread overview]
Message-ID: <20180103111340.3sc5ogabpikdnlwt@mwiniars-main.ger.corp.intel.com> (raw)
In-Reply-To: <20180102151235.3949-10-chris@chris-wilson.co.uk>

On Tue, Jan 02, 2018 at 03:12:26PM +0000, Chris Wilson wrote:
> The dependency chain must be an acyclic graph. This is checked by the
> swfence, but for sanity, also do a simple check that we do not corrupt
> our list iteration in execlists_schedule() by a shallow dependency
> cycle.
> 
> Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>

Reviewed-by: Michał Winiarski <michal.winiarski@intel.com>

-Michał

> ---
>  drivers/gpu/drm/i915/intel_lrc.c | 11 ++++++++---
>  1 file changed, 8 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/gpu/drm/i915/intel_lrc.c b/drivers/gpu/drm/i915/intel_lrc.c
> index 007aec9d95c9..8c9d6cef2482 100644
> --- a/drivers/gpu/drm/i915/intel_lrc.c
> +++ b/drivers/gpu/drm/i915/intel_lrc.c
> @@ -1006,7 +1006,8 @@ static void execlists_schedule(struct drm_i915_gem_request *request, int prio)
>  	stack.signaler = &request->priotree;
>  	list_add(&stack.dfs_link, &dfs);
>  
> -	/* Recursively bump all dependent priorities to match the new request.
> +	/*
> +	 * Recursively bump all dependent priorities to match the new request.
>  	 *
>  	 * A naive approach would be to use recursion:
>  	 * static void update_priorities(struct i915_priotree *pt, prio) {
> @@ -1026,12 +1027,15 @@ static void execlists_schedule(struct drm_i915_gem_request *request, int prio)
>  	list_for_each_entry_safe(dep, p, &dfs, dfs_link) {
>  		struct i915_priotree *pt = dep->signaler;
>  
> -		/* Within an engine, there can be no cycle, but we may
> +		/*
> +		 * Within an engine, there can be no cycle, but we may
>  		 * refer to the same dependency chain multiple times
>  		 * (redundant dependencies are not eliminated) and across
>  		 * engines.
>  		 */
>  		list_for_each_entry(p, &pt->signalers_list, signal_link) {
> +			GEM_BUG_ON(p == dep); /* no cycles! */
> +
>  			if (i915_gem_request_completed(priotree_to_request(p->signaler)))
>  				continue;
>  
> @@ -1043,7 +1047,8 @@ static void execlists_schedule(struct drm_i915_gem_request *request, int prio)
>  		list_safe_reset_next(dep, p, dfs_link);
>  	}
>  
> -	/* If we didn't need to bump any existing priorities, and we haven't
> +	/*
> +	 * If we didn't need to bump any existing priorities, and we haven't
>  	 * yet submitted this request (i.e. there is no potential race with
>  	 * execlists_submit_request()), we can set our own priority and skip
>  	 * acquiring the engine locks.
> -- 
> 2.15.1
> 
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