Am 03.09.2018 um 12:07 schrieb Chunming Zhou:
在 2018/9/3 16:50, Christian König 写道:
Am 03.09.2018 um 06:13 schrieb Chunming Zhou:
在 2018/8/30 19:32, Christian König 写道:
[SNIP]
+
+struct drm_syncobj_wait_pt {
+ struct drm_syncobj_stub_fence base;
+ u64 value;
+ struct rb_node node;
+};
+struct drm_syncobj_signal_pt {
+ struct drm_syncobj_stub_fence base;
+ struct dma_fence *signal_fence;
+ struct dma_fence *pre_pt_base;
+ struct dma_fence_cb signal_cb;
+ struct dma_fence_cb pre_pt_cb;
+ struct drm_syncobj *syncobj;
+ u64 value;
+ struct list_head list;
+};
What are those two structures good for
They are used to record wait ops points and signal ops points.
For timeline, they are connected by timeline value, works like:
a. signal pt increase timeline value to signal_pt value when
signal_pt is signaled. signal_pt is depending on previous pt
fence and itself signal fence from signal ops.
b. wait pt tree checks if timeline value over itself value.
For normal, works like:
a. signal pt increase 1 for syncobj_timeline->signal_point
every time when signal ops is performed.
b. when wait ops is comming, wait pt will fetch above last
signal pt value as its wait point. wait pt will be only signaled
by equal point value signal_pt.
and why is the stub fence their base?
Good question, I tried to kzalloc for them as well when I debug
them, I encountered a problem:
I lookup/find wait_pt or signal_pt successfully, but when I
tried to use them, they are freed sometimes, and results in NULL
point.
and generally, when lookup them, we often need their stub fence
as well, and in the meantime, their lifecycle are same.
Above reason, I make stub fence as their base.
That sounds like you only did this because you messed up the
lifecycle.
Additional to that I don't think you correctly considered the
lifecycle of the waits and the sync object itself. E.g. blocking
in drm_syncobj_timeline_fini() until all waits are done is not a
good idea.
What you should do instead is to create a fence_array object with
all the fence we need to wait for when a wait point is requested.
Yeah, this is our initial discussion result, but when I tried to
do that, I found that cannot meet the advance signal requirement:
a. We need to consider the wait and signal pt value are not
one-to-one match, it's difficult to find dependent point, at
least, there is some overhead.
As far as I can see that is independent of using a fence array
here. See you can either use a ring buffer or an rb-tree, but when
you want to wait for a specific point we need to condense the not
yet signaled fences into an array.
again, need to find the range of where the specific point in, we
should close to timeline semantics, I also refered the sw_sync.c
timeline, usally wait_pt is signalled by timeline point. And I agree
we can implement it with several methods, but I don't think there
are basical differences.
The problem is that with your current approach you need the sync_obj
alive for the synchronization to work. That is most likely not a good
idea.
Indeed, I will fix that. How abount only creating fence array for
every wait pt when syncobj release? when syncobj release, wait pt must
have waited the signal opertation, then we can easily condense fences
for every wait pt. And meantime, we can take timeline based wait pt
advantage.
That could work, but I don't see how you want to replace the already
issued fence with a fence_array when the sync object is destroyed.
Additional to that I would rather prefer a consistent handling, e.g.
without extra rarely used code paths.
Additional to that you enable signaling without a need from the
waiting side. That is rather bad for implementations which need that
optimization.
Do you mean increasing timeline based on signal fence is not better?
only update timeline value when requested by a wait pt?
Yes, exactly.
This way, we will not update timeline value immidiately and cannot
free signal pt immidiately, and we also need to consider it to CPU
query and wait.
That is actually the better coding style. We usually try to avoid doing
things in interrupt handlers as much as possible.
How about this idea:
1. Each signaling point is a fence implementation with an rb node.
2. Each node keeps a reference to the last previously inserted node.
3. Each node is referenced by the sync object itself.
4. Before each signal/wait operation we do a garbage collection and
remove the first node from the tree as long as it is signaled.
5. When enable_signaling is requested for a node we cascade that to the
left using rb_prev.
This ensures that signaling is enabled for the current fence as
well as all previous fences.
6. A wait just looks into the tree for the signal point lower or equal
of the requested sequence number.
7. When the sync object is released we use
rbtree_postorder_for_each_entry_safe() and drop the extra reference to
each node, but never call rb_erase!
This way the rb_tree stays in memory, but without a root (e.g. the
sync object). It only destructs itself when the looked up references to
the nodes are dropped.
Well that is quite a bunch of logic, but I think that should work fine.
Regards,
Christian.
Thanks,
David Zhou
I suggest to either condense all the fences you need to wait for in
an array during the wait operation, or reference count the sync_obj
and only enable the signaling on the fences when requested by a wait.
b. because we allowed "wait-before-signal", if
"wait-before-signal" happens, there isn't signal fence which can
be used to create fence array.
Well, again we DON'T support wait-before-signal here. I will
certainly NAK any implementation which tries to do this until we
haven't figured out all the resource management constraints and I
still don't see how we can do this.
yes, we don't support real wait-before-signal in patch now, just a
fake wait-before-signal, which still wait on CS submission until
signal operation coming through wait_event, which is the conclusion
we disscussed before.
Well in this case we should call it wait-for-signal and not
wait-before-signal :)
So timeline value is good to resolve that.
Otherwise somebody can easily construct a situation where
timeline sync obj A waits on B and B waits on A.
Same situation also can happens with fence-array, we only can see
this is a programming bug with incorrect use.
No, fence-array is initialized only once with a static list of
fences. This way it is impossible to add the fence-array to itself
for example.
E.g. you can't build circle dependencies with that.
we already wait for signal operation event, so never build circle
dependencies with that. The theory is same.
Yeah, ok that is certainly true.
Regards,
Christian.
Better use rbtree_postorder_for_each_entry_safe() for this.
From the comments, seems we cannot use it here, we need erase
node here.
Comments:
* Note, however, that it cannot handle other modifications that
re-order the
* rbtree it is iterating over. This includes calling rb_erase()
on @pos, as
* rb_erase() may rebalance the tree, causing us to miss some
nodes.
*/
Yeah, but your current implementation has the same problem :)
You need to use something like "while (node = rb_first(...))"
instead if you want to destruct the whole tree.
OK, got it, I will change it in v4.
Thanks,
David Zhou
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