On Fri, Feb 10, 2023 at 5:07 AM Tvrtko Ursulin <tvrtko.ursu...@linux.intel.com> wrote: > > From: Tvrtko Ursulin <tvrtko.ursu...@intel.com> > > In i915 we have this concept of "wait boosting" where we give a priority boost > for instance to fences which are actively waited upon from userspace. This has > it's pros and cons and can certainly be discussed at lenght. However fact is > some workloads really like it. > > Problem is that with the arrival of drm syncobj and a new userspace waiting > entry point it added, the waitboost mechanism was bypassed. Hence I cooked up > this mini series really (really) quickly to see if some discussion can be had. > > It adds a concept of "wait count" to dma fence, which is incremented for every > explicit dma_fence_enable_sw_signaling and dma_fence_add_wait_callback (like > dma_fence_add_callback but from explicit/userspace wait paths).
I was thinking about a similar thing, but in the context of dma_fence (or rather sync_file) fd poll()ing. How does the kernel differentiate between "housekeeping" poll()ers that don't want to trigger boost but simply know when to do cleanup, and waiters who are waiting with some urgency. I think we could use EPOLLPRI for this purpose. Not sure how that translates to waits via the syncobj. But I think we want to let userspace give some hint about urgent vs housekeeping waits. Also, on a related topic: https://lwn.net/Articles/868468/ BR, -R > Individual drivers can then inspect this via dma_fence_wait_count() and decide > to wait boost the waits on such fences. > > Again, quickly put together and smoke tested only - no guarantees whatsoever > and > I will rely on interested parties to test and report if it even works or how > well. > > v2: > * Small fixups based on CI feedback: > * Handle decrement correctly for already signalled case while adding > callback. > * Remove i915 assert which was making sure struct i915_request does not > grow. > * Split out the i915 patch into three separate functional changes. > > Tvrtko Ursulin (5): > dma-fence: Track explicit waiters > drm/syncobj: Mark syncobj waits as external waiters > drm/i915: Waitboost external waits > drm/i915: Mark waits as explicit > drm/i915: Wait boost requests waited upon by others > > drivers/dma-buf/dma-fence.c | 102 ++++++++++++++++------ > drivers/gpu/drm/drm_syncobj.c | 6 +- > drivers/gpu/drm/i915/gt/intel_engine_pm.c | 1 - > drivers/gpu/drm/i915/i915_request.c | 13 ++- > include/linux/dma-fence.h | 14 +++ > 5 files changed, 101 insertions(+), 35 deletions(-) > > -- > 2.34.1 >
