drm/i915/gt: Yield the timeslice if caught waiting on a user semaphore
If we find ourselves waiting on a MI_SEMAPHORE_WAIT, either within the user batch or in our own preamble, the engine raises a GT_WAIT_ON_SEMAPHORE interrupt. We can unmask that interrupt and so respond to a semaphore wait by yielding the timeslice, if we have another context to yield to! The only real complication is that the interrupt is only generated for the start of the semaphore wait, and is asynchronous to our process_csb() -- that is, we may not have registered the timeslice before we see the interrupt. To ensure we don't miss a potential semaphore blocking forward progress (e.g. selftests/live_timeslice_preempt) we mark the interrupt and apply it to the next timeslice regardless of whether it was active at the time. v2: We use semaphores in preempt-to-busy, within the timeslicing implementation itself! Ergo, when we do insert a preemption due to an expired timeslice, the new context may start with the missed semaphore flagged by the retired context and be yielded, ad infinitum. To avoid this, read the context id at the time of the semaphore interrupt and only yield if that context is still active. Fixes: 8ee36e04 ("drm/i915/execlists: Minimalistic timeslicing") Signed-off-by: NChris Wilson <chris@chris-wilson.co.uk> Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com> Cc: Kenneth Graunke <kenneth@whitecape.org> Reviewed-by: NTvrtko Ursulin <tvrtko.ursulin@intel.com> Link: https://patchwork.freedesktop.org/patch/msgid/20200407130811.17321-1-chris@chris-wilson.co.uk (cherry picked from commit c4e8ba73) (cherry picked from commit cd60e4ac4738a6921592c4f7baf87f9a3499f0e2) Signed-off-by: NRodrigo Vivi <rodrigo.vivi@intel.com>
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