On Thu, Jul 22, 2021 at 1:42 AM Christian König <ckoenig.leichtzumerken@xxxxxxxxx> wrote: > > Am 21.07.21 um 21:03 schrieb Daniel Vetter: > > On Wed, Jul 21, 2021 at 09:34:43AM -0700, Rob Clark wrote: > >> On Wed, Jul 21, 2021 at 12:59 AM Daniel Vetter <daniel@xxxxxxxx> wrote: > >>> On Wed, Jul 21, 2021 at 12:32 AM Rob Clark <robdclark@xxxxxxxxx> wrote: > >>>> On Tue, Jul 20, 2021 at 1:55 PM Daniel Vetter <daniel@xxxxxxxx> wrote: > >>>>> On Tue, Jul 20, 2021 at 8:26 PM Rob Clark <robdclark@xxxxxxxxx> wrote: > >>>>>> On Tue, Jul 20, 2021 at 11:03 AM Christian König > >>>>>> <ckoenig.leichtzumerken@xxxxxxxxx> wrote: > >>>>>>> Hi Rob, > >>>>>>> > >>>>>>> Am 20.07.21 um 17:07 schrieb Rob Clark: > >>>>>>>> From: Rob Clark <robdclark@xxxxxxxxxxxx> > >>>>>>>> > >>>>>>>> Somehow we had neither ->wait() nor dma_fence_signal() calls, and no > >>>>>>>> one noticed. Oops. > >>>>>>> > >>>>>>> I'm not sure if that is a good idea. > >>>>>>> > >>>>>>> The dma_fence->wait() callback is pretty much deprecated and should not > >>>>>>> be used any more. > >>>>>>> > >>>>>>> What exactly do you need that for? > >>>>>> Well, the alternative is to track the set of fences which have > >>>>>> signalling enabled, and then figure out which ones to signal, which > >>>>>> seems like a lot more work, vs just re-purposing the wait > >>>>>> implementation we already have for non-dma_fence cases ;-) > >>>>>> > >>>>>> Why is the ->wait() callback (pretty much) deprecated? > >>>>> Because if you need it that means for your driver dma_fence_add_cb is > >>>>> broken, which means a _lot_ of things don't work. Like dma_buf poll > >>>>> (compositors have patches to start using that), and I think > >>>>> drm/scheduler also becomes rather unhappy. > >>>> I'm starting to page back in how this works.. fence cb's aren't broken > >>>> (which is also why dma_fence_wait() was not completely broken), > >>>> because in retire_submits() we call > >>>> dma_fence_is_signaled(submit->hw_fence). > >>>> > >>>> But the reason that the custom wait function cleans up a tiny bit of > >>>> jank is that the wait_queue_head_t gets signaled earlier, before we > >>>> start iterating the submits and doing all that retire_submit() stuff > >>>> (unpin/unref bo's, etc). I suppose I could just split things up to > >>>> call dma_fence_signal() earlier, and *then* do the retire_submits() > >>>> stuff. > >>> Yeah reducing the latency there sounds like a good idea. > >>> -Daniel > >>> > >> Hmm, no, turns out that isn't the problem.. or, well, it is probably a > >> good idea to call drm_fence_signal() earlier. But it seems like > >> waking up from wait_event_* is faster than wake_up_state(wait->task, > >> TASK_NORMAL). I suppose the wake_up_state() approach still needs for > >> the scheduler to get around to schedule the runnable task. > > As far as I know wake_up_state() tries to run the thread on the CPU it > was scheduled last, while wait_event_* makes the thread run on the CPU > who issues the wake by default. > > And yes I've also noticed this already and it was one of the reason why > I suggested to use a wait_queue instead of the hand wired dma_fence_wait > implementation. > > >> > >> So for now, I'm going back to my own wait function (plus earlier > >> drm_fence_signal()) > >> > >> Before removing dma_fence_opps::wait(), I guess we want to re-think > >> dma_fence_default_wait().. but I think that would require a > >> dma_fence_context base class (rather than just a raw integer). > > Uh that's not great ... can't we fix this instead of papering over it in > > drivers? Aside from maybe different wakeup flags it all is supposed to > > work exactly the same underneath, and whether using a wait queue or not > > really shouldn't matter. > > Well it would have been nicer if we used the existing infrastructure > instead of re-inventing stuff for dma_fence, but that chance is long gone. > > And you don't need a dma_fence_context base class, but rather just a > flag in the dma_fence_ops if you want to change the behavior. Hmm, I was thinking dma_fence_context to have a place for the wait_queue_head, but I guess that could also be per-dma_fence
