On Wed, 2021-10-20 at 13:34 -0700, John Harrison wrote: > On 10/11/2021 10:57, Matthew Brost wrote: > > perf_parallel_engines is micro benchmark to test i915 request > > scheduling. The test creates a thread per physical engine and > > submits > > NOP requests and waits the requests to complete in a loop. In > > execlists > > mode this works perfectly fine as powerful CPU has enough cores to > > feed > > each engine and process the CSBs. With GuC submission the uC gets > > overwhelmed as all threads feed into a single CTB channel and the > > GuC > > gets bombarded with CSBs as contexts are immediately switched in > > and out > > on the engines due to the zero runtime of the requests. When the > > GuC is > > overwhelmed scheduling of contexts is unfair due to the nature of > > the > > GuC scheduling algorithm. This behavior is understood and deemed > > acceptable as this micro benchmark isn't close to real world use > > case. > > Increasing the timeout of wait period for requests to complete. > > This > > makes the test understand that is ok for contexts to get starved in > > this > > scenario. > > > > A future patch / cleanup may just delete these micro benchmark > > tests as > > they basically mean nothing. We care about real workloads not made > > up > > ones. > > > > Signed-off-by: Matthew Brost <matthew.brost@xxxxxxxxx> > Reviewed-by: John Harrison <John.C.Harrison@xxxxxxxxx> Also Reviewed-by: Thomas Hellström <thomas.hellstrom@xxxxxxxxxxxxxxx> I think one important thing to keep in mind here is that this selftest actually *did* find a flaw, Albeit it upon analysis turned out not to be significant. But given that, user-space clients like, for example, gem_exec_suspend seems to be able to trigger similar delays as well at least to some extend with a huge amount of small requests submitted from user-space we shold probably verify at some point that this isn't exploitable by a malicious client starving other clients on the same system. /Thomas > > > --- > > drivers/gpu/drm/i915/selftests/i915_request.c | 4 ++-- > > 1 file changed, 2 insertions(+), 2 deletions(-) > > > > diff --git a/drivers/gpu/drm/i915/selftests/i915_request.c > > b/drivers/gpu/drm/i915/selftests/i915_request.c > > index d67710d10615..6496671a113c 100644 > > --- a/drivers/gpu/drm/i915/selftests/i915_request.c > > +++ b/drivers/gpu/drm/i915/selftests/i915_request.c > > @@ -2805,7 +2805,7 @@ static int p_sync0(void *arg) > > i915_request_add(rq); > > > > err = 0; > > - if (i915_request_wait(rq, 0, HZ / 5) < 0) > > + if (i915_request_wait(rq, 0, HZ) < 0) > > err = -ETIME; > > i915_request_put(rq); > > if (err) > > @@ -2876,7 +2876,7 @@ static int p_sync1(void *arg) > > i915_request_add(rq); > > > > err = 0; > > - if (prev && i915_request_wait(prev, 0, HZ / 5) < 0) > > + if (prev && i915_request_wait(prev, 0, HZ) < 0) > > err = -ETIME; > > i915_request_put(prev); > > prev = rq; >
