Am 27.01.25 um 17:33 schrieb Lucas Stach:
Hi Christian,
Am Montag, dem 27.01.2025 um 17:14 +0100 schrieb Christian König:
Am 27.01.25 um 17:02 schrieb Lucas Stach:
This effectively reverts 0fea2ed61e7f ("drm/amdgpu: Remove call to
reservation_object_test_signaled_rcu before wait"), as the premise of
that commit is wrong. dma_resv_wait_timeout() without a timeout will
not turn into a wait-free dma_resv_test_signaled, but will wait a
jiffy for unsignaled fences, which is not at all what userspace expects
when it calls GEM_WAIT_IDLE with a timeout of 0.
Marek pinged me about that strange behavior as well. That sounds like a
separate bug in dma_resv_wait_timeout() to me.
I don't see why the function should be waiting with a timeout of 0 and
we have multiple cases where that is assumed.
What should happen is that dma_resv_wait_timeout() should return 1 when
all fences are signaled even when the timeout is 0.
My educated guess is that this behavior is somehow broken and instead we
wait for at least 1 jiffies.
This here is probably just the tip of the iceberg.
dma_resv_wait_timeout() explicitly sets timeout to a single jiffy when
it is entered with timeout == 0. This behavior was introduced with your
commit 06a66b5c77ed ("reservation: revert "wait only with non-zero
timeout specified (v3)" v2"), which seems to fix a real bug.
Ah, yes! I see where the problem is now.
Between the two behaviors I think it is wrong for callers of
dma_resv_wait_timeout() to assume that the call is necessarily wait-
free in case of timeout == 0. If you want wait-free but are able to
deal with false positive unsignaled returns use dma_resv_test_signaled,
otherwise use dma_resv_wait_timeout to get accurate answers with a
possible wait.
Well dma_resv_wait_timeout() with a zero timeout is indeed supposed to
return immediately. Anything else would be quite a difference to all
other kernel functions which take a jiffies timeout.
But the difference between dma_resv_test_signaled() and
dma_resv_wait_timeout() with a zero timeout is that the later should
still make sure to enable signaling!
A couple of people have pinged me about that as well and we also
discussed the desired behavior on a bug report with Faith and a few others.
Going to take a look, thanks for pointing this out.
Regards,
Christian.
Regards,
Lucas
Regards,
Christian.
Signed-off-by: Lucas Stach <l.stach@xxxxxxxxxxxxxx>
---
This is most likely the correct kernel-side solution for the unexpected
slowdown worked around with in userspace with this Mesa series:
https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/32877
---
drivers/gpu/drm/amd/amdgpu/amdgpu_gem.c | 9 +++++++--
1 file changed, 7 insertions(+), 2 deletions(-)
diff --git a/drivers/gpu/drm/amd/amdgpu/amdgpu_gem.c b/drivers/gpu/drm/amd/amdgpu/amdgpu_gem.c
index 1a5df8b94661..75b5d5149911 100644
--- a/drivers/gpu/drm/amd/amdgpu/amdgpu_gem.c
+++ b/drivers/gpu/drm/amd/amdgpu/amdgpu_gem.c
@@ -567,8 +567,13 @@ int amdgpu_gem_wait_idle_ioctl(struct drm_device *dev, void *data,
return -ENOENT;
robj = gem_to_amdgpu_bo(gobj);
- ret = dma_resv_wait_timeout(robj->tbo.base.resv, DMA_RESV_USAGE_READ,
- true, timeout);
+ if (timeout == 0)
+ ret = dma_resv_test_signaled(robj->tbo.base.resv,
+ DMA_RESV_USAGE_READ);
+ else
+ ret = dma_resv_wait_timeout(robj->tbo.base.resv,
+ DMA_RESV_USAGE_READ,
+ true, timeout);
/* ret == 0 means not signaled,
* ret > 0 means signaled
