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<div class="moz-cite-prefix">On 02/08/2019 09:10, Koenig, Christian
wrote:<br>
</div>
<blockquote type="cite"
cite="mid:e2a1839e-1ee1-4ecb-9b18-af338046c0f1@email.android.com">
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<div class="gmail_quote">Am 02.08.2019 07:38 schrieb Lionel
Landwerlin <a class="moz-txt-link-rfc2396E" href="mailto:lionel.g.landwerlin@intel.com"><lionel.g.landwerlin@intel.com></a>:<br
type="attribution">
<blockquote class="quote" style="margin:0 0 0
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<div>
<div>On 02/08/2019 08:21, Koenig, Christian wrote:<br>
</div>
<blockquote>
<div dir="auto">
<div><br>
<div><br>
<div class="elided-text">Am 02.08.2019 07:17
schrieb Lionel Landwerlin <a
href="mailto:lionel.g.landwerlin@intel.com"
moz-do-not-send="true">
<lionel.g.landwerlin@intel.com></a>:<br
type="attribution">
<blockquote style="margin:0 0 0
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<div>On 02/08/2019 08:08, Koenig,
Christian wrote:<br>
</div>
<blockquote>
<div dir="auto">Hi Lionel,
<div dir="auto"><br>
</div>
<div dir="auto">Well that looks more
like your test case is buggy.</div>
<div dir="auto"><br>
</div>
<div dir="auto">According to the
code the ctx1 queue always waits
for sem1 and ctx2 queue always
waits for sem2.</div>
</div>
</blockquote>
<p><br>
</p>
<p>That's supposed to be the same
underlying syncobj because it's
exported from one VkDevice as opaque
FD from sem1 and imported into sem2.<br>
</p>
</div>
</blockquote>
</div>
</div>
</div>
<div dir="auto"><br>
</div>
<div dir="auto">Well than that's still buggy and
won't synchronize at all.</div>
<div dir="auto"><br>
</div>
<div dir="auto">When ctx1 waits for a semaphore
and then signals the same semaphore there is no
guarantee that ctx2 will run in between jobs.</div>
<div dir="auto"><br>
</div>
<div dir="auto">It's perfectly valid in this case
to first run all jobs from ctx1 and then all
jobs from ctx2.</div>
</div>
</blockquote>
<p><br>
</p>
<p>That's not really how I see the semaphores working.</p>
<p>The spec describe VkSemaphore as an interface to an
internal payload opaque to the application.</p>
<p><br>
</p>
<p>When ctx1 waits on the semaphore, it waits on the
payload put there by the previous iteration.</p>
</div>
</blockquote>
</div>
</div>
</div>
<div dir="auto"><br>
</div>
<div dir="auto">And who says that it's not waiting for it's own
previous payload?</div>
</div>
</blockquote>
<p><br>
</p>
<p>That's was I understood from you previous comment : "there is no
guarantee that ctx2 will run in between jobs"</p>
<p><br>
</p>
<blockquote type="cite"
cite="mid:e2a1839e-1ee1-4ecb-9b18-af338046c0f1@email.android.com">
<div dir="auto">
<div dir="auto"><br>
</div>
<div dir="auto">See if the payload is a counter this won't work
either. Keep in mind that this has the semantic of a
semaphore. Whoever grabs the semaphore first wins and can run,
everybody else has to wait.</div>
</div>
</blockquote>
<p><br>
</p>
<p>What performs the "grab" here?</p>
<p>I thought that would be vkQueueSubmit().</p>
<p>Since that occuring from a single application thread, that should
then be ordered in execution of ctx1,ctx2,ctx1,...<br>
</p>
<p><br>
</p>
<p>Thanks for your time on this,</p>
<p><br>
</p>
<p>-Lionel<br>
</p>
<p><br>
</p>
<blockquote type="cite"
cite="mid:e2a1839e-1ee1-4ecb-9b18-af338046c0f1@email.android.com">
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<div dir="auto"><br>
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<p>Then it proceeds to signal it by replacing the
internal payload.</p>
</div>
</blockquote>
</div>
</div>
</div>
<div dir="auto"><br>
</div>
<div dir="auto">That's an implementation detail of our sync
objects, but I don't think that this behavior is part of the
Vulkan specification.</div>
<div dir="auto"><br>
</div>
<div dir="auto">Regards,</div>
<div dir="auto">Christian.</div>
<div dir="auto">
<div class="gmail_extra">
<div class="gmail_quote">
<blockquote class="quote" style="margin:0 0 0
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<p><br>
</p>
<p>ctx2 then waits on that and replaces the payload
again with the new internal synchronization object.</p>
<p><br>
</p>
<p>The internal payload is a dma fence in our case and
signaling just replaces a dma fence by another or
puts one where there was none before.</p>
<p>So we should have created a dependecy link between
all the submissions and then should be executed in
the order of QueueSubmit() calls.<br>
</p>
<p><br>
</p>
<p>-Lionel<br>
</p>
<p><br>
</p>
<blockquote>
<div dir="auto">
<div dir="auto"><br>
</div>
<div dir="auto">It only prevents running both at
the same time and as far as I can see that still
works even with threaded submission.</div>
<div dir="auto"><br>
</div>
<div dir="auto">You need at least two semaphores
for a tandem submission.</div>
<div dir="auto"><br>
</div>
<div dir="auto">Regards,</div>
<div dir="auto">Christian.</div>
<div dir="auto">
<div>
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</p>
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<div dir="auto">
<div dir="auto"><br>
</div>
<div dir="auto">This way there can't
be any Synchronisation between the
two.</div>
<div dir="auto"><br>
</div>
<div dir="auto">Regards,</div>
<div dir="auto">Christian.</div>
</div>
<div><br>
<div class="elided-text">Am
02.08.2019 06:55 schrieb Lionel
Landwerlin <a
href="mailto:lionel.g.landwerlin@intel.com"
moz-do-not-send="true">
<lionel.g.landwerlin@intel.com></a>:<br type="attribution">
</div>
</div>
<div>
<div>Hey Christian,</div>
<div><br>
</div>
<div>The problem boils down to the
fact that we don't immediately
create dma fences when calling
vkQueueSubmit().</div>
<div>This is delayed to a thread.</div>
<div><br>
</div>
<div>From a single application
thread, you can QueueSubmit() to 2
queues from 2 different devices.</div>
<div>Each QueueSubmit to one queue
has a dependency on the previous
QueueSubmit on the other queue
through an exported/imported
semaphore.</div>
<div><br>
</div>
<div>From the API point of view the
state of the semaphore should be
changed after each QueueSubmit().</div>
<div>The problem is that it's not
because of the thread and because
you might have those 2 submission
threads tied to different
VkDevice/VkInstance or even
different applications
(synchronizing themselves outside
the vulkan API).</div>
<div><br>
</div>
<div>Hope that makes sense.</div>
<div>It's not really easy to explain
by mail, the best explanation is
probably reading the test :
<a
href="https://gitlab.freedesktop.org/mesa/crucible/blob/master/src/tests/func/sync/semaphore-fd.c#L788"
moz-do-not-send="true">
https://gitlab.freedesktop.org/mesa/crucible/blob/master/src/tests/func/sync/semaphore-fd.c#L788</a></div>
<div><br>
</div>
<div>Like David mentioned you're not
running into that issue right now,
because you only dispatch to the
thread under specific conditions.</div>
<div>But I could build a case to
force that and likely run into the
same issue.<br>
</div>
<div><br>
</div>
<div>-Lionel<br>
</div>
<div><br>
</div>
<div>On 02/08/2019 07:33, Koenig,
Christian wrote:<br>
</div>
<blockquote>
<div>
<div dir="auto">Hi Lionel,
<div dir="auto"><br>
</div>
<div dir="auto">Well could you
describe once more what the
problem is?</div>
<div dir="auto"><br>
</div>
<div dir="auto">Cause I don't
fully understand why a
rather normal tandem
submission with two
semaphores should fail in
any way.</div>
<div dir="auto"><br>
</div>
<div dir="auto">Regards,</div>
<div dir="auto">Christian.</div>
</div>
<div><br>
<div>Am 02.08.2019 06:28
schrieb Lionel Landwerlin <a
href="mailto:lionel.g.landwerlin@intel.com" moz-do-not-send="true">
<lionel.g.landwerlin@intel.com></a>:<br type="attribution">
</div>
</div>
</div>
<font size="2"><span
style="font-size:11pt">
<div>There aren't CTS tests
covering the issue I was
mentioning.<br>
But we could add them.<br>
<br>
I don't have all the details
regarding your
implementation but even with
<br>
the "semaphore thread", I
could see it running into
the same issues.<br>
What if a mix of binary
& timeline semaphores
are handed to
vkQueueSubmit()?<br>
<br>
For example with queueA
& queueB from 2
different VkDevice :<br>
vkQueueSubmit(queueA,
signal semA);<br>
vkQueueSubmit(queueA,
wait on [semA,
timelineSemB]); with <br>
timelineSemB triggering a
wait before signal.<br>
vkQueueSubmit(queueB,
signal semA);<br>
<br>
<br>
-Lionel<br>
<br>
On 02/08/2019 06:18, Zhou,
David(ChunMing) wrote:<br>
> Hi Lionel,<br>
><br>
> By the Queue thread is
a heavy thread, which is
always resident in driver
during application running,
our guys don't like that. So
we switch to Semaphore
Thread, only when
waitBeforeSignal of timeline
happens, we spawn a thread
to handle that wait. So we
don't have your this issue.<br>
> By the way, I already
pass all your CTS cases for
now. I suggest you to switch
to Semaphore Thread instead
of Queue Thread as well. It
works very well.<br>
><br>
> -David<br>
><br>
> -----Original
Message-----<br>
> From: Lionel Landwerlin
<a
href="mailto:lionel.g.landwerlin@intel.com"
moz-do-not-send="true"><lionel.g.landwerlin@intel.com></a><br>
> Sent: Friday, August 2,
2019 4:52 AM<br>
> To: dri-devel <a
href="mailto:dri-devel@lists.freedesktop.org"
moz-do-not-send="true"><dri-devel@lists.freedesktop.org></a>;
Koenig, Christian
<a
href="mailto:Christian.Koenig@amd.com"
moz-do-not-send="true"><Christian.Koenig@amd.com></a>;
Zhou, David(ChunMing)
<a
href="mailto:David1.Zhou@amd.com"
moz-do-not-send="true"><David1.Zhou@amd.com></a>;
Jason Ekstrand <a
href="mailto:jason@jlekstrand.net"
moz-do-not-send="true">
<jason@jlekstrand.net></a><br>
> Subject: Threaded
submission & semaphore
sharing<br>
><br>
> Hi Christian, David,<br>
><br>
> Sorry to report this so
late in the process, but I
think we found an issue not
directly related to syncobj
timelines themselves but
with a side effect of the
threaded submissions.<br>
><br>
> Essentially we're
failing a test in crucible :<br>
>
func.sync.semaphore-fd.opaque-fd<br>
> This test create a
single binary semaphore,
shares it between 2
VkDevice/VkQueue.<br>
> Then in a loop it
proceeds to submit workload
alternating between the 2
VkQueue with one submit
depending on the other.<br>
> It does so by waiting
on the VkSemaphore signaled
in the previous iteration
and resignaling it.<br>
><br>
> The problem for us is
that once things are
dispatched to the submission
thread, the ordering of the
submission is lost.<br>
> Because we have 2
devices and they both have
their own submission thread.<br>
><br>
> Jason suggested that we
reestablish the ordering by
having semaphores/syncobjs
carry an additional uint64_t
payload.<br>
> This 64bit integer
would represent be an
identifier that submission
threads will
WAIT_FOR_AVAILABLE on.<br>
><br>
> The scenario would look
like this :<br>
> -
vkQueueSubmit(queueA, signal
on semA);<br>
> - in the
caller thread, this would
increment the syncobj
additional u64 payload and
return it to userspace.<br>
> - at some
point the submission thread
of queueA submits the
workload and signal the
syncobj of semA with value
returned in the caller
thread of vkQueueSubmit().<br>
> -
vkQueueSubmit(queueB, wait
on semA);<br>
> - in the
caller thread, this would
read the syncobj additional<br>
> u64 payload<br>
> - at some
point the submission thread
of queueB will try to submit
the work, but first it will
WAIT_FOR_AVAILABLE the u64
value returned in the step
above<br>
><br>
> Because we want the
binary semaphores to be
shared across processes and
would like this to remain a
single FD, the simplest
location to store this
additional u64 payload would
be the DRM syncobj.<br>
> It would need an
additional ioctl to read
& increment the value.<br>
><br>
> What do you think?<br>
><br>
> -Lionel<br>
<br>
<br>
</div>
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