thoughts of looking at android fences

Message ID CAMbhsRQ67B+MLLp-YhYhegDrOWp9AYN8eAaZnjdTc-wuwnM9vA@mail.gmail.com
State New
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Commit Message

Colin Cross Nov. 2, 2013, 9:36 p.m.
On Wed, Oct 30, 2013 at 5:17 AM, Maarten Lankhorst
<maarten.lankhorst@canonical.com> wrote:
> op 24-10-13 14:13, Maarten Lankhorst schreef:
>> So I actually tried to implement it now. I killed all the deprecated members and assumed a linear timeline.
>> This means that syncpoints can only be added at the end, not in between. In particular it means sw_sync
>> might be slightly broken.
>>
>> I only tested it with a simple program I wrote called ufence.c, it's in drivers/staging/android/ufence.c in the following tree:
>>
>> http://cgit.freedesktop.org/~mlankhorst/linux
>>
>> the "rfc: convert android to fence api" has all the changes from my dma-fence proposal to what android would need,
>> it also converts the userspace fence api to use the dma-fence api.
>>
>> sync_pt is implemented as fence too. This meant not having to convert all of android right away, though I did make some changes.
>> I killed the deprecated members and made all the fence calls forward to the sync_timeline_ops. dup and compare are no longer used.
>>
>> I haven't given this a spin on a full android kernel, only with the components that are in mainline kernel under staging and my dumb test program.
>>
>> ~Maarten
>>
>> PS: The nomenclature is very confusing. I want to rename dma-fence to syncpoint, but I want some feedback from the android devs first. :)
>>
> Come on, any feedback? I want to move the discussion forward.
>
> ~Maarten

I experimented with it a little on a device that uses sync and came
across a few bugs:
1.  sync_timeline_signal needs to call __fence_signal on all signaled
points on the timeline, not just the first
2.  fence_add_callback doesn't always initialize cb.node
3.  sync_fence_wait should take ms
4.  sync_print_pt status printing was incorrect
5.  there is a deadlock:
   sync_print_obj takes obj->child_list_lock
   sync_print_pt
   fence_is_signaled
   fence_signal takes fence->lock == obj->child_list_lock
6.  freeing a timeline before all the fences holding points on that
timeline have timed out results in a crash

With the attached patch to fix these issues, our libsync and sync_test
give the same results as with our sync code.  I haven't tested against
the full Android framework yet.

The compare op and timeline ordering is critical to the efficiency of
sync points on Android.  The compare op is used when merging fences to
drop all but the latest point on the same timeline.  This is necessary
for example when the same buffer is submitted to the display on
multiple frames, like when there is a live wallpaper in the background
updating at 60 fps and a static screen of widgets on top of it.  The
static widget buffer is submitted on every frame, returning a new
fence each time.  The compositor merges the new fence with the fence
for the previous buffer, and because they are on the same timeline it
merges down to a single point.  I experimented with disabling the
merge optimization on a Nexus 10, and found that leaving the screen on
running a live wallpaper eventually resulted in 100k outstanding sync
points.

Comments

Maarten Lankhorst Nov. 3, 2013, 6:31 a.m. | #1
op 02-11-13 22:36, Colin Cross schreef:
> On Wed, Oct 30, 2013 at 5:17 AM, Maarten Lankhorst
> <maarten.lankhorst@canonical.com> wrote:
>> op 24-10-13 14:13, Maarten Lankhorst schreef:
>>> So I actually tried to implement it now. I killed all the deprecated members and assumed a linear timeline.
>>> This means that syncpoints can only be added at the end, not in between. In particular it means sw_sync
>>> might be slightly broken.
>>>
>>> I only tested it with a simple program I wrote called ufence.c, it's in drivers/staging/android/ufence.c in the following tree:
>>>
>>> http://cgit.freedesktop.org/~mlankhorst/linux
>>>
>>> the "rfc: convert android to fence api" has all the changes from my dma-fence proposal to what android would need,
>>> it also converts the userspace fence api to use the dma-fence api.
>>>
>>> sync_pt is implemented as fence too. This meant not having to convert all of android right away, though I did make some changes.
>>> I killed the deprecated members and made all the fence calls forward to the sync_timeline_ops. dup and compare are no longer used.
>>>
>>> I haven't given this a spin on a full android kernel, only with the components that are in mainline kernel under staging and my dumb test program.
>>>
>>> ~Maarten
>>>
>>> PS: The nomenclature is very confusing. I want to rename dma-fence to syncpoint, but I want some feedback from the android devs first. :)
>>>
>> Come on, any feedback? I want to move the discussion forward.
>>
>> ~Maarten
> I experimented with it a little on a device that uses sync and came
> across a few bugs:
> 1.  sync_timeline_signal needs to call __fence_signal on all signaled
> points on the timeline, not just the first
> 2.  fence_add_callback doesn't always initialize cb.node
> 3.  sync_fence_wait should take ms
> 4.  sync_print_pt status printing was incorrect
Well, in the normal case status may not be set, but fence may still be signaled. Any status should be set before signaling. In cases where non-android dma-fences are used.
I deliberately tried not to depend on any android stuff in the android fences, this way it could be used outside android too.

I'm trying to only set status on error. If there is a race with 2 threads calling fence_signal, one with error, one without, then the error will always be visible in fence->status that way.
But I guess this might be a corner case we wouldn't worry about normally...
> 5.  there is a deadlock:
>    sync_print_obj takes obj->child_list_lock
>    sync_print_pt
>    fence_is_signaled
>    fence_signal takes fence->lock == obj->child_list_lock
> 6.  freeing a timeline before all the fences holding points on that
> timeline have timed out results in a crash
To be honest, I was surprised this code mostly worked for the sw_fence test code I wrote originally. :)
I think I'll have to add a __fence_is_signaled for 5, which can be called with fence->lock held.


>
> With the attached patch to fix these issues, our libsync and sync_test
> give the same results as with our sync code.  I haven't tested against
> the full Android framework yet.
>
> The compare op and timeline ordering is critical to the efficiency of
> sync points on Android.  The compare op is used when merging fences to
> drop all but the latest point on the same timeline.  This is necessary
> for example when the same buffer is submitted to the display on
> multiple frames, like when there is a live wallpaper in the background
> updating at 60 fps and a static screen of widgets on top of it.  The
> static widget buffer is submitted on every frame, returning a new
> fence each time.  The compositor merges the new fence with the fence
> for the previous buffer, and because they are on the same timeline it
> merges down to a single point.  I experimented with disabling the
> merge optimization on a Nexus 10, and found that leaving the screen on
> running a live wallpaper eventually resulted in 100k outstanding sync
> points.
Yeah I've been looking into how to do that. It's very easy to optimize actually. The dma-fence code requires a context,
which is a number (even if it may become a pointer later, it could be seen as a number). If we order all fences based on number,
fences could have an order based on context number. Merging fences would simply become adding 2 ordered lists, and dropping
any duplicates and signaled points.

I left it out for the RFC I wanted to keep things as readable as possible. :)

~Maarten

Patch hide | download patch | download mbox

From e8f89ae535e227490d3116a03f2be5dc780e5be9 Mon Sep 17 00:00:00 2001
From: Colin Cross <ccross@android.com>
Date: Fri, 1 Nov 2013 19:26:54 -0700
Subject: [PATCH] dma fence fixes

Change-Id: I9308b51c23e762fde95106db301cf6aedd8845f5
---
 drivers/staging/android/sync.c       | 27 +++++++++++++++++++++------
 drivers/staging/android/sync_debug.c | 10 ++++------
 include/linux/fence.h                |  4 ++--
 3 files changed, 27 insertions(+), 14 deletions(-)

diff --git a/drivers/staging/android/sync.c b/drivers/staging/android/sync.c
index 110a9e99cb71..672020d5f566 100644
--- a/drivers/staging/android/sync.c
+++ b/drivers/staging/android/sync.c
@@ -74,6 +74,16 @@  static void sync_timeline_free(struct kref *kref)
 	kfree(obj);
 }
 
+static void sync_timeline_get(struct sync_timeline *obj)
+{
+	kref_get(&obj->kref);
+}
+
+static void sync_timeline_put(struct sync_timeline *obj)
+{
+	kref_put(&obj->kref, sync_timeline_free);
+}
+
 void sync_timeline_destroy(struct sync_timeline *obj)
 {
 	obj->destroyed = true;
@@ -83,8 +93,8 @@  void sync_timeline_destroy(struct sync_timeline *obj)
 	 * that their parent is going away.
 	 */
 
-	if (!kref_put(&obj->kref, sync_timeline_free))
-		sync_timeline_signal(obj);
+	sync_timeline_signal(obj);
+	sync_timeline_put(obj);
 }
 EXPORT_SYMBOL(sync_timeline_destroy);
 
@@ -98,9 +108,7 @@  void sync_timeline_signal(struct sync_timeline *obj)
 
 	spin_lock_irqsave(&obj->child_list_lock, flags);
 	list_for_each_entry_safe(pt, next, &obj->active_list_head, active_list) {
-		if (!pt->base.ops->signaled(&pt->base))
-			break;
-		else {
+		if (pt->base.ops->signaled(&pt->base)) {
 			__fence_signal(&pt->base);
 			list_del(&pt->active_list);
 		}
@@ -122,6 +130,7 @@  struct sync_pt *sync_pt_create(struct sync_timeline *obj, int size)
 		return NULL;
 
 	spin_lock_irqsave(&obj->child_list_lock, flags);
+	sync_timeline_get(obj);
 	__fence_init(&pt->base, &android_fence_ops, &obj->child_list_lock, obj->context, ++obj->value);
 	list_add_tail(&pt->child_list, &obj->child_list_head);
 	INIT_LIST_HEAD(&pt->active_list);
@@ -186,6 +195,7 @@  struct sync_fence *sync_fence_create(const char *name, struct sync_pt *pt)
 	fence_get(&pt->base);
 	fence->cbs[0].sync_pt = &pt->base;
 	fence->cbs[0].fence = fence;
+	INIT_LIST_HEAD(&fence->cbs[0].cb.node);
 	if (fence_add_callback(&pt->base, &fence->cbs[0].cb, fence_check_cb_func))
 		atomic_dec(&fence->status);
 
@@ -245,6 +255,7 @@  struct sync_fence *sync_fence_merge(const char *name,
 		fence_get(pt);
 		fence->cbs[i].sync_pt = pt;
 		fence->cbs[i].fence = fence;
+		INIT_LIST_HEAD(&fence->cbs[i].cb.node);
 		if (fence_add_callback(pt, &fence->cbs[i].cb, fence_check_cb_func))
 			atomic_dec(&fence->status);
 	}
@@ -255,7 +266,8 @@  struct sync_fence *sync_fence_merge(const char *name,
 		fence_get(pt);
 		fence->cbs[a->num_fences + i].sync_pt = pt;
 		fence->cbs[a->num_fences + i].fence = fence;
-		if (fence_add_callback(pt, &fence->cbs[i].cb, fence_check_cb_func))
+		INIT_LIST_HEAD(&fence->cbs[a->num_fences + i].cb.node);
+		if (fence_add_callback(pt, &fence->cbs[a->num_fences + i].cb, fence_check_cb_func))
 			atomic_dec(&fence->status);
 	}
 
@@ -325,6 +337,8 @@  int sync_fence_wait(struct sync_fence *fence, long timeout)
 
 	if (timeout < 0)
 		timeout = MAX_SCHEDULE_TIMEOUT;
+	else
+		timeout = msecs_to_jiffies(timeout);
 
 	trace_sync_wait(fence, 1);
 	for (i = 0; i < fence->num_fences; ++i)
@@ -383,6 +397,7 @@  static void android_fence_release(struct fence *fence)
 	if (parent->ops->free_pt)
 		parent->ops->free_pt(pt);
 
+	sync_timeline_put(parent);
 	kfree(pt);
 }
 
diff --git a/drivers/staging/android/sync_debug.c b/drivers/staging/android/sync_debug.c
index 55ad34085f2f..8671fbb7d143 100644
--- a/drivers/staging/android/sync_debug.c
+++ b/drivers/staging/android/sync_debug.c
@@ -82,18 +82,16 @@  static const char *sync_status_str(int status)
 
 static void sync_print_pt(struct seq_file *s, struct sync_pt *pt, bool fence)
 {
-	int status = 0;
+	int status = 1;
 	struct sync_timeline *parent = sync_pt_parent(pt);
-	if (fence_is_signaled(&pt->base)) {
+	if (fence_is_signaled(&pt->base))
 		status = pt->base.status;
-		if (!status)
-			status = 1;
-	}
+
 	seq_printf(s, "  %s%spt %s",
 		   fence ? parent->name : "",
 		   fence ? "_" : "",
 		   sync_status_str(status));
-	if (status) {
+	if (!status) {
 		struct timeval tv = ktime_to_timeval(pt->base.timestamp);
 		seq_printf(s, "@%ld.%06ld", tv.tv_sec, tv.tv_usec);
 	}
diff --git a/include/linux/fence.h b/include/linux/fence.h
index 2beb3b0ff2a3..e1b2fc6e8007 100644
--- a/include/linux/fence.h
+++ b/include/linux/fence.h
@@ -257,10 +257,10 @@  fence_is_signaled(struct fence *fence)
 	if (test_bit(FENCE_FLAG_SIGNALED_BIT, &fence->flags))
 		return true;
 
-	if (fence->ops->signaled && fence->ops->signaled(fence)) {
+	/*if (fence->ops->signaled && fence->ops->signaled(fence)) {
 		fence_signal(fence);
 		return true;
-	}
+	}*/
 
 	return false;
 }
-- 
1.8.4.1