Re: [RFC 6/9] drm/i915: Delay the freeing of requests until retire time

John Harrison
2015-10-28T13:00:23Z

On 23/07/2015 15:25, Tvrtko Ursulin wrote:
>
> Hi,
>
> On 07/17/2015 03:31 PM, John.C.Harrison@Intel.com wrote:
>> From: John Harrison <John.C.Harrison@Intel.com>
>>
>> The request structure is reference counted. When the count reached
>> zero, the request was immediately freed and all associated objects
>> were unrefereced/unallocated. This meant that the driver mutex lock
>> must be held at the point where the count reaches zero. This was fine
>> while all references were held internally to the driver. However, the
>> plan is to allow the underlying fence object (and hence the request
>> itself) to be returned to other drivers and to userland. External
>> users cannot be expected to acquire a driver private mutex lock.
>>
>> Rather than attempt to disentangle the request structure from the
>> driver mutex lock, the decsion was to defer the free code until a
>> later (safer) point. Hence this patch changes the unreference callback
>> to merely move the request onto a delayed free list. The driver's
>> retire worker thread will then process the list and actually call the
>> free function on the requests.
>>
>> [new patch in series]
>>
>> For: VIZ-5190
>> Signed-off-by: John Harrison <John.C.Harrison@Intel.com>
>> ---
>> drivers/gpu/drm/i915/i915_drv.h | 22 +++---------------
>> drivers/gpu/drm/i915/i915_gem.c | 41
>> +++++++++++++++++++++++++++++----
>> drivers/gpu/drm/i915/intel_display.c | 2 +-
>> drivers/gpu/drm/i915/intel_lrc.c | 2 ++
>> drivers/gpu/drm/i915/intel_pm.c | 2 +-
>> drivers/gpu/drm/i915/intel_ringbuffer.c | 2 ++
>> drivers/gpu/drm/i915/intel_ringbuffer.h | 4 ++++
>> 7 files changed, 50 insertions(+), 25 deletions(-)
>>
>> diff --git a/drivers/gpu/drm/i915/i915_drv.h
>> b/drivers/gpu/drm/i915/i915_drv.h
>> index 88a4746..61c3db2 100644
>> --- a/drivers/gpu/drm/i915/i915_drv.h
>> +++ b/drivers/gpu/drm/i915/i915_drv.h
>> @@ -2161,14 +2161,9 @@ void i915_gem_track_fb(struct
>> drm_i915_gem_object *old,
>> * initial reference taken using kref_init
>> */
>> struct drm_i915_gem_request {
>> - /**
>> - * Underlying object for implementing the signal/wait stuff.
>> - * NB: Never return this fence object to user land! It is unsafe to
>> - * let anything outside of the i915 driver get hold of the fence
>> - * object as the clean up when decrementing the reference count
>> - * requires holding the driver mutex lock.
>> - */
>> + /** Underlying object for implementing the signal/wait stuff. */
>> struct fence fence;
>> + struct list_head delay_free_list;
>
> Maybe call this delay_free_link to continue the established convention.
>
>>
>> /** On Which ring this request was generated */
>> struct drm_i915_private *i915;
>> @@ -2281,21 +2276,10 @@ i915_gem_request_reference(struct
>> drm_i915_gem_request *req)
>> static inline void
>> i915_gem_request_unreference(struct drm_i915_gem_request *req)
>> {
>> - WARN_ON(!mutex_is_locked(&req->ring->dev->struct_mutex));
>> - fence_put(&req->fence);
>> -}
>> -
>> -static inline void
>> -i915_gem_request_unreference__unlocked(struct drm_i915_gem_request
>> *req)
>> -{
>> - struct drm_device *dev;
>> -
>> if (!req)
>> return;
>>
>> - dev = req->ring->dev;
>> - if (kref_put_mutex(&req->fence.refcount, fence_release,
>> &dev->struct_mutex))
>> - mutex_unlock(&dev->struct_mutex);
>> + fence_put(&req->fence);
>> }
>>
>> static inline void i915_gem_request_assign(struct
>> drm_i915_gem_request **pdst,
>> diff --git a/drivers/gpu/drm/i915/i915_gem.c
>> b/drivers/gpu/drm/i915/i915_gem.c
>> index af79716..482835a 100644
>> --- a/drivers/gpu/drm/i915/i915_gem.c
>> +++ b/drivers/gpu/drm/i915/i915_gem.c
>> @@ -2616,10 +2616,27 @@ static void i915_set_reset_status(struct
>> drm_i915_private *dev_priv,
>> }
>> }
>>
>> -static void i915_gem_request_free(struct fence *req_fence)
>> +static void i915_gem_request_release(struct fence *req_fence)
>> {
>> struct drm_i915_gem_request *req = container_of(req_fence,
>> typeof(*req), fence);
>> + struct intel_engine_cs *ring = req->ring;
>> + struct drm_i915_private *dev_priv = to_i915(ring->dev);
>> + unsigned long flags;
>> +
>> + /*
>> + * Need to add the request to a deferred dereference list to be
>> + * processed at a mutex lock safe time.
>> + */
>> + spin_lock_irqsave(&ring->delayed_free_lock, flags);
>
> At the moment there is no request unreferencing from irq handlers
> right? Unless (or until) you plan to add that you could use simple
> spin_lock here. (And in the i915_gem_retire_requests_ring.)

I don't believe there is an unreference at IRQ time at this precise
moment. However, there certainly have been in various other iterations
of the code (including one on the display side that has since
disappeared due to changes by others completely unrelated to this work).
So I would be nervous about not making it IRQ compatible. It seems like
a bug waiting to happen.

>> + list_add_tail(&req->delay_free_list, &ring->delayed_free_list);
>> + spin_unlock_irqrestore(&ring->delayed_free_lock, flags);
>> +
>> + queue_delayed_work(dev_priv->wq, &dev_priv->mm.retire_work, 0);
>
> Have you decided to re-use the retire worker just for convenience of
> for some other reason as well?

It seemed like the logical place to do this. It is a periodic function
that cleans up requests. Converting the unreference into a free isn't a
hugely time critical thing so adding an entire dedicated work handler
seems like overkill. Plus, retire requests is usually the place that
releases the last reference so it makes sense to do the free at the end
of that code.

>
> I found it a bit unexpected and though dedicated request free worker
> would be cleaner, but I don't know, not a strong opinion.
>
>> +}
>> +
>> +static void i915_gem_request_free(struct drm_i915_gem_request *req)
>> +{
>> struct intel_context *ctx = req->ctx;
>>
>> BUG_ON(!mutex_is_locked(&req->ring->dev->struct_mutex));
>> @@ -2696,7 +2713,7 @@ static const struct fence_ops
>> i915_gem_request_fops = {
>> .enable_signaling = i915_gem_request_enable_signaling,
>> .signaled = i915_gem_request_is_completed,
>> .wait = fence_default_wait,
>> - .release = i915_gem_request_free,
>> + .release = i915_gem_request_release,
>> .fence_value_str = i915_fence_value_str,
>> .timeline_value_str = i915_fence_timeline_value_str,
>> };
>> @@ -2992,6 +3009,21 @@ i915_gem_retire_requests_ring(struct
>> intel_engine_cs *ring)
>> i915_gem_request_assign(&ring->trace_irq_req, NULL);
>> }
>>
>> + while (!list_empty(&ring->delayed_free_list)) {
>> + struct drm_i915_gem_request *request;
>> + unsigned long flags;
>> +
>> + request = list_first_entry(&ring->delayed_free_list,
>> + struct drm_i915_gem_request,
>> + delay_free_list);
>
> Need a spinlock to sample list head here. Then maybe move it on a
> temporary list and do the freeing afterwards.

Not necessary. The only other usage of the list is to add to it. So this
code can't pull an entry that gets removed beneath its feet. Either the
list empty test will return true and nothing further happens or there is
definitely a node on the list and list_first_entry() will return
something sane. The spinlock is only required when actually deleting
that node.

>
> Regards,
>
> Tvrtko