On 15/07/2020 15:47, Chris Wilson wrote:
Quoting Chris Wilson (2020-07-15 15:47:15)
Quoting Tvrtko Ursulin (2020-07-15 13:26:23)
On 15/07/2020 13:06, Tvrtko Ursulin wrote:
On 15/07/2020 11:50, Chris Wilson wrote:
Currently, we use i915_request_completed() directly in
i915_request_wait() and follow up with a manual invocation of
dma_fence_signal(). This appears to cause a large number of contentions
on i915_request.lock as when the process is woken up after the fence is
signaled by an interrupt, we will then try and call dma_fence_signal()
ourselves while the signaler is still holding the lock.
dma_fence_is_signaled() has the benefit of checking the
DMA_FENCE_FLAG_SIGNALED_BIT prior to calling dma_fence_signal() and so
avoids most of that contention.
Signed-off-by: Chris Wilson <chris@xxxxxxxxxxxxxxxxxx>
Cc: Matthew Auld <matthew.auld@xxxxxxxxx>
Cc: Tvrtko Ursulin <tvrtko.ursulin@xxxxxxxxx>
---
drivers/gpu/drm/i915/i915_request.c | 12 ++++--------
1 file changed, 4 insertions(+), 8 deletions(-)
diff --git a/drivers/gpu/drm/i915/i915_request.c
b/drivers/gpu/drm/i915/i915_request.c
index 0b2fe55e6194..bb4eb1a8780e 100644
--- a/drivers/gpu/drm/i915/i915_request.c
+++ b/drivers/gpu/drm/i915/i915_request.c
@@ -1640,7 +1640,7 @@ static bool busywait_stop(unsigned long timeout,
unsigned int cpu)
return this_cpu != cpu;
}
-static bool __i915_spin_request(const struct i915_request * const rq,
int state)
+static bool __i915_spin_request(struct i915_request * const rq, int
state)
{
unsigned long timeout_ns;
unsigned int cpu;
@@ -1673,7 +1673,7 @@ static bool __i915_spin_request(const struct
i915_request * const rq, int state)
timeout_ns = READ_ONCE(rq->engine->props.max_busywait_duration_ns);
timeout_ns += local_clock_ns(&cpu);
do {
- if (i915_request_completed(rq))
+ if (dma_fence_is_signaled(&rq->fence))
return true;
if (signal_pending_state(state, current))
@@ -1766,10 +1766,8 @@ long i915_request_wait(struct i915_request *rq,
* duration, which we currently lack.
*/
if (IS_ACTIVE(CONFIG_DRM_I915_MAX_REQUEST_BUSYWAIT) &&
- __i915_spin_request(rq, state)) {
- dma_fence_signal(&rq->fence);
+ __i915_spin_request(rq, state))
goto out;
- }
/*
* This client is about to stall waiting for the GPU. In many cases
@@ -1796,10 +1794,8 @@ long i915_request_wait(struct i915_request *rq,
for (;;) {
set_current_state(state);
- if (i915_request_completed(rq)) {
- dma_fence_signal(&rq->fence);
+ if (dma_fence_is_signaled(&rq->fence))
break;
- }
intel_engine_flush_submission(rq->engine);
In other words putting some latency back into the waiters, which is
probably okay, since sync waits is not our primary model.
I have a slight concern about the remaining value of busy spinning if
i915_request_completed check is removed from there as well. Of course it
doesn't make sense to have different completion criteria between the
two.. We could wait a bit longer if real check in busyspin said request
is actually completed, just not signal it but wait for the breadcrumbs
to do it.
What a load of nonsense.. :)
Okay, I think the only real question is i915_request_completed vs
dma_fence_signaled in __i915_spin_request. Do we want to burn CPU cycles
waiting on GPU and breadcrumb irq work, or just the GPU.
dma_fence_is_signaled() {
if (test_bit(SIGNALED_BIT))
return true;
if (i915_request_completed()) {
dma_fence_signal();
return true;
}
return false;
}
with the indirection. So the question is whether the indirection is
worth the extra test bit. Just purely looking at the i915_request.lock
contention suggests that it probably is. For the spinner, burning a few
extra CPU cycles for *vfunc is not an issue, it's the wakeup latency,
and since we are calling dma_fence_signal() upon wakeup we do take the
spinlock without checking for an early return from the SIGNALED_BIT.
So I think it's a net positive. The alternative was to write
if (i915_request_completed()) {
if (!i915_request_is_signaled())
dma_fence_signal();
break;
}
but
if (dma_fence_is_signaled())
break;
does appear simpler, if only by virtue of hiding the details in an
inline.
Or a patch to add test_bit() to dma_fence_signal, but this looked
better.
Right I missed dma_fence_is_signaled calls i915_request_completed.
In this case the remaining question is do we care about wait ioctl
potentially returning before the hypothetical sync fence for the same
request is signaled? This seems like a slight change in user observable
behaviour.
Regards,
Tvrtko
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