• Chris Wilson's avatar
    drm/i915: Downgrade NEWCLIENT to non-preemptive · 68fc728b
    Chris Wilson authored
    Commit 1413b2bc ("drm/i915: Trim NEWCLIENT boosting") had the
    intended consequence of not allowing a sequence of work that merely
    crossed into a new engine the privilege to be promoted to NEWCLIENT
    status. It also had the unintended consequence of actually making
    NEWCLIENT effective on heavily oversubscribed transcode machines and
    impacting upon their throughput.
    
    If we consider a client packet composed of (rcsA, rcsB, vcs) and 30 of
    those clients, using the NEWCLIENT boost that will be scheduled as
    
    	rcsA x 30, (rcsB, vcs) x 30
    
    where as before it would have been
    
    	(rcsA, rcsB, vcs) x 30
    
    That is with NEWCLIENT only boosting the first request of each client,
    we would execute all rcsA requests prior to running on the vcs engines;
    acruing a lot of dead time as compared to the previous case where the
    vcs engine would be started in parallel to processing the second client.
    
    The previous patch has the effect of delaying submission until it is
    required by a third party (either the user with an explicit wait, or by
    another client/engine). We reduce the NEWCLIENT bump to a mere WAIT,
    which has the effect of removing its preemptive grant and reducing it to
    the same level as any other user interaction -- that it will not be
    promoted above the interengine dependencies, and so preventing NEWCLIENTS
    from starving other engines. This a large nerf to the rrul properties of
    the current NEWCLIENT, but it still does give prioritised submission to
    new requests from light workloads.
    
    References: b16c7651 ("drm/i915: Priority boost for new clients")
    Fixes: 1413b2bc ("drm/i915: Trim NEWCLIENT boosting") # customer impact
    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/20190515130052.4475-4-chris@chris-wilson.co.uk
    68fc728b
i915_request.c 44.8 KB