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Chris Wilson authored
In commit b7404c7e ("drm/i915: Bump ready tasks ahead of busywaits"), I tried cutting a corner in order to not install a signal for each of our dependencies, and only listened to requests on which we were intending to busywait. The compromise that was made was that instead of then being able to promote the request with a full NOSEMAPHORE like its non-busywaiting brethren, as we had not ensured we had cleared the semaphore chain, we settled for only using the NEWCLIENT boost. With an over saturated system with multiple NEWCLIENTS in flight at any time, this was found to be an inadequate promotion and left us with a much poorer scheduling order than prior to using semaphores. The outcome of this patch, is that all requests have NOSEMAPHORE priority when they have no dependencies and are ready to run and not busywait, restoring the pre-semaphore ordering on saturated systems. We can demonstrate the effect of poor scheduling order by oversaturating the system using gem_wsim on a system with multiple vcs engines (i.e running the same workloads across more clients than required for peak throughput, e.g. media_load_balance_17i7.wsim -c4 -b context): x v5.1 (normalized) + tip * fix +------------------------------------------------------------------------+ | x | | x | | x | | x | | %x | | %%x | | %%x | | %%x | | %%x | | %%x | | %%x | | %%x | | %%x | | %%x | | %%x | | %#x | | %#x | | %#x | | %#x | | %#x | | + %#xx | | + %#xx | | + %%#xx | | + %%#xx | | + %%#xx | | + %%#xx | | + %%##x | | +++ %%##x | | +++ %%##x | | +++ %%##x | | ++++ %%##x | | ++++ %%##x | | ++++ %%##xx | | ++++ %###xx | | ++++ %###xx | | ++++ %###xx | | ++++ %###xx | | ++++ + %#O#xx | | ++++ + %#O#xx | | ++++++ + %#O#xx | | ++++++++++ %OOOxxx| | ++++++++++ + %#OOO#xx| | + ++++++++++++ ++ +++++ + ++ @@OOOO#xx| | |A_| | ||__________M_______A____________________| | | |A_| | +------------------------------------------------------------------------+ N Min Max Median Avg Stddev x 120 0.99456 1.00628 0.999985 1.0001545 0.0024387139 + 120 0.873021 1.00037 0.884134 0.90148752 0.039190862 Difference at 99.5% confidence -0.098667 +/- 0.0110762 -9.86517% +/- 1.10745% (Student's t, pooled s = 0.0277657) % 120 0.990207 1.00165 0.9970265 0.99699748 0.0021024 Difference at 99.5% confidence -0.003157 +/- 0.000908245 -0.315651% +/- 0.0908105% (Student's t, pooled s = 0.00227678) Fixes: b7404c7e ("drm/i915: Bump ready tasks ahead of busywaits") Signed-off-by:
Chris 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:
Tvrtko Ursulin <tvrtko.ursulin@intel.com> Link: https://patchwork.freedesktop.org/patch/msgid/20190515130052.4475-2-chris@chris-wilson.co.uk
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