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.
-Chris
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