Re: [PATCH] drm/i915: Track number of pending freed objects

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

On 19/02/2018 16:19, Chris Wilson wrote:
> During igt, we frequently call into the driver to reset both HW and
> driver state (idling the device, waiting for it to become idle and
> freeing off old objects) to ensure that we start each test/subtest/pass
> from known state. This process incurs an RCU barrier or two to ensure
> that any such pending frees are indeed flushed before we return.
> However, unconditionally waiting on the RCU barrier adds needless delay
> to many callers, which adds up to several seconds when repeated thousands
> of times. We can skip the rcu_barrier() if by tracking how many outstanding
> frees we have, we know there are none.

To be pedantic it is not skipping the rcu_barrier, but skipping the 
drain altogether.

So theoretically there is a tiny difference in behaviour where today 
drain would wait for all frees currently executing, where after the 
patch it will ignore these and only process the ones which got to the 
end of the function.

Perhaps it atomic_inc was at the very top of i915_gem_free_object it 
would be closer to today. But such suggestions feel extremely iffy.

Nosing around the code base suggest the change is completely fine. Only 
potentially relevant site which might care about the subtle difference 
is i915_gem_freeze_late, which actually doesnt care since everything has 
been frozen at that point. So all frees have presumably exited and 
incremented the new counter.

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

Regards,

Tvrtko

> 
> The same path is used along suspend, where we may be able to save the
> unconditional RCU barrier.
> 
> Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
> Cc: Mika Kuoppala <mika.kuoppala@linux.intel.com>
> Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
> ---
>   drivers/gpu/drm/i915/i915_drv.h | 8 ++++++++
>   drivers/gpu/drm/i915/i915_gem.c | 4 ++++
>   2 files changed, 12 insertions(+)
> 
> diff --git a/drivers/gpu/drm/i915/i915_drv.h b/drivers/gpu/drm/i915/i915_drv.h
> index 6b215c411338..76bfe909168c 100644
> --- a/drivers/gpu/drm/i915/i915_drv.h
> +++ b/drivers/gpu/drm/i915/i915_drv.h
> @@ -1095,6 +1095,11 @@ struct i915_gem_mm {
>   	struct llist_head free_list;
>   	struct work_struct free_work;
>   	spinlock_t free_lock;
> +	/**
> +	 * Count of objects pending destructions. Used to skip needlessly
> +	 * waiting on an RCU barrier if no objects are waiting to be freed.
> +	 */
> +	atomic_t free_count;
>   
>   	/**
>   	 * Small stash of WC pages
> @@ -3134,6 +3139,9 @@ void i915_gem_free_object(struct drm_gem_object *obj);
>   
>   static inline void i915_gem_drain_freed_objects(struct drm_i915_private *i915)
>   {
> +	if (!atomic_read(&i915->mm.free_count))
> +		return;
> +
>   	/* A single pass should suffice to release all the freed objects (along
>   	 * most call paths) , but be a little more paranoid in that freeing
>   	 * the objects does take a little amount of time, during which the rcu
> diff --git a/drivers/gpu/drm/i915/i915_gem.c b/drivers/gpu/drm/i915/i915_gem.c
> index f530cd247724..f2dcf09f4ebc 100644
> --- a/drivers/gpu/drm/i915/i915_gem.c
> +++ b/drivers/gpu/drm/i915/i915_gem.c
> @@ -4758,6 +4758,9 @@ static void __i915_gem_free_objects(struct drm_i915_private *i915,
>   		kfree(obj->bit_17);
>   		i915_gem_object_free(obj);
>   
> +		GEM_BUG_ON(!atomic_read(&i915->mm.free_count));
> +		atomic_dec(&i915->mm.free_count);
> +
>   		if (on)
>   			cond_resched();
>   	}
> @@ -4846,6 +4849,7 @@ void i915_gem_free_object(struct drm_gem_object *gem_obj)
>   	 * i915_gem_busy_ioctl(). For the corresponding synchronized
>   	 * lookup see i915_gem_object_lookup_rcu().
>   	 */
> +	atomic_inc(&to_i915(obj->base.dev)->mm.free_count);
>   	call_rcu(&obj->rcu, __i915_gem_free_object_rcu);
>   }
>   
> 
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