On 2021-04-13 3:10 a.m., Christian König wrote:
Am 12.04.21 um 22:01 schrieb Andrey Grodzovsky:
On 2021-04-12 3:18 p.m., Christian König wrote:
Am 12.04.21 um 21:12 schrieb Andrey Grodzovsky:
[SNIP]
So what's the right approach ? How we guarantee that when running
amdgpu_fence_driver_force_completion we will signal all the HW
fences and not racing against some more fences insertion into
that array ?
Well I would still say the best approach would be to insert this
between the front end and the backend and not rely on signaling
fences while holding the device srcu.
My question is, even now, when we run
amdgpu_fence_driver_fini_hw->amdgpu_fence_wait_empty or
amdgpu_fence_driver_fini_hw->amdgpu_fence_driver_force_completion,
what there prevents a race with another fence being at the same
time emitted and inserted into the fence array ? Looks like nothing.
Each ring can only be used by one thread at the same time, this
includes emitting fences as well as other stuff.
During GPU reset we make sure nobody writes to the rings by stopping
the scheduler and taking the GPU reset lock (so that nobody else can
start the scheduler again).
What about direct submissions not through scheduler -
amdgpu_job_submit_direct, I don't see how this is protected.
Those only happen during startup and GPU reset.
Ok, but then looks like I am missing something, see the following steps
in amdgpu_pci_remove -
1) Use disable_irq API function to stop and flush all in flight HW
interrupts handlers
2) Grab the reset lock and stop all the schedulers
After above 2 steps the HW fences array is idle, no more insertions and
no more extractions from the array
3) Run one time amdgpu_fence_process to signal all current HW fences
4) Set drm_dev_unplug (will 'flush' all in flight IOCTLs), release the
GPU reset lock and go on with the rest of the sequence (cancel timers,
work items e.t.c)
What's problematic in this sequence ?
Andrey
BTW: Could it be that the device SRCU protects more than one
device and we deadlock because of this?
I haven't actually experienced any deadlock until now but, yes,
drm_unplug_srcu is defined as static in drm_drv.c and so in the
presence of multiple devices from same or different drivers we in
fact are dependent on all their critical sections i guess.
Shit, yeah the devil is a squirrel. So for A+I laptops we actually
need to sync that up with Daniel and the rest of the i915 guys.
IIRC we could actually have an amdgpu device in a docking station
which needs hotplug and the driver might depend on waiting for the
i915 driver as well.
Can't we propose a patch to make drm_unplug_srcu per drm_device ? I
don't see why it has to be global and not per device thing.
I'm really wondering the same thing for quite a while now.
Adding Daniel as well, maybe he knows why the drm_unplug_srcu is global.
Regards,
Christian.
Andrey
Christian.
Andrey
Christian.
Andrey
Andrey
Christian.
/* Past this point no more fence are submitted to HW ring
and hence we can safely call force signal on all that are
currently there.
* Any subsequently created HW fences will be returned
signaled with an error code right away
*/
for_each_ring(adev)
amdgpu_fence_process(ring)
drm_dev_unplug(dev);
Stop schedulers
cancel_sync(all timers and queued works);
hw_fini
unmap_mmio
}
Andrey
Alternatively grabbing the reset write side and stopping
and then restarting the scheduler could work as well.
Christian.
I didn't get the above and I don't see why I need to
reuse the GPU reset rw_lock. I rely on the SRCU unplug
flag for unplug. Also, not clear to me why are we
focusing on the scheduler threads, any code patch to
generate HW fences should be covered, so any code leading
to amdgpu_fence_emit needs to be taken into account such
as, direct IB submissions, VM flushes e.t.c
You need to work together with the reset lock anyway,
cause a hotplug could run at the same time as a reset.
For going my way indeed now I see now that I have to take
reset write side lock during HW fences signalling in order
to protect against scheduler/HW fences detachment and
reattachment during schedulers stop/restart. But if we go
with your approach then calling drm_dev_unplug and scoping
amdgpu_job_timeout with drm_dev_enter/exit should be enough
to prevent any concurrent GPU resets during unplug. In fact
I already do it anyway -
https://nam11.safelinks.protection.outlook.com/?url=https:%2F%2Fcgit.freedesktop.org%2F~agrodzov%2Flinux%2Fcommit%2F%3Fh%3Ddrm-misc-next%26id%3Def0ea4dd29ef44d2649c5eda16c8f4869acc36b1&data=04%7C01%7Candrey.grodzovsky%40amd.com%7Cc7fc6cb505c34aedfe6d08d8fe4b3947%7C3dd8961fe4884e608e11a82d994e183d%7C0%7C0%7C637538946324857369%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C1000&sdata=64362PRC8xTgR2Uj2R256bMegVm8YWq1KI%2BAjzeYXv4%3D&reserved=0
Yes, good point as well.
Christian.
Andrey
Christian.
Andrey
Christian.
Andrey
Andrey
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