• Chris Wilson's avatar
    drm/i915: Make the semaphore saturation mask global · 44d89409
    Chris Wilson authored
    The idea behind keeping the saturation mask local to a context backfired
    spectacularly. The premise with the local mask was that we would be more
    proactive in attempting to use semaphores after each time the context
    idled, and that all new contexts would attempt to use semaphores
    ignoring the current state of the system. This turns out to be horribly
    optimistic. If the system state is still oversaturated and the existing
    workloads have all stopped using semaphores, the new workloads would
    attempt to use semaphores and be deprioritised behind real work. The
    new contexts would not switch off using semaphores until their initial
    batch of low priority work had completed. Given sufficient backload load
    of equal user priority, this would completely starve the new work of any
    GPU time.
    
    To compensate, remove the local tracking in favour of keeping it as
    global state on the engine -- once the system is saturated and
    semaphores are disabled, everyone stops attempting to use semaphores
    until the system is idle again. One of the reason for preferring local
    context tracking was that it worked with virtual engines, so for
    switching to global state we could either do a complete check of all the
    virtual siblings or simply disable semaphores for those requests. This
    takes the simpler approach of disabling semaphores on virtual engines.
    
    The downside is that the decision that the engine is saturated is a
    local measure -- we are only checking whether or not this context was
    scheduled in a timely fashion, it may be legitimately delayed due to user
    priorities. We still have the same dilemma though, that we do not want
    to employ the semaphore poll unless it will be used.
    
    v2: Explain why we need to assume the worst wrt virtual engines.
    
    Fixes: ca6e56f6 ("drm/i915: Disable semaphore busywaits on saturated systems")
    Signed-off-by: default avatarChris Wilson <chris@chris-wilson.co.uk>
    Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
    Cc: Dmitry Rogozhkin <dmitry.v.rogozhkin@intel.com>
    Cc: Dmitry Ermilov <dmitry.ermilov@intel.com>
    Reviewed-by: default avatarTvrtko Ursulin <tvrtko.ursulin@intel.com>
    Link: https://patchwork.freedesktop.org/patch/msgid/20190618074153.16055-8-chris@chris-wilson.co.uk
    44d89409
intel_engine_pm.c 4.28 KB