On Fri, 16 Sep 2022 19:33:27 +0300 Ville Syrjala <ville.syrjala@xxxxxxxxxxxxxxx> wrote: > From: Ville Syrjälä <ville.syrjala@xxxxxxxxxxxxxxx> > > I've talked about making blocking commits lockless a few > times in the past, so here's finally an attempt at it. > The main benefit I see from this is that TEST_ONLY commits > no longer getting blocked on the mutexes by parallel blocking > commits. > > I have a small test here that spools up two threads, > one does just TEST_ONLY commits in a loop, the other > does either blocking or non-blocking page flips. Results > came out as follows on a snb machine here: > > test-only-vs-non-blocking: > -85319 TEST_ONLY commits in 2000000 usecs, 23 usecs / commit > +87144 TEST_ONLY commits in 2000006 usecs, 22 usecs / commit > > test-only-vs-blocking: > -219 TEST_ONLY commits in 2001768 usecs, 9140 usecs / commit > +82442 TEST_ONLY commits in 2000011 usecs, 24 usecs / commit > > Now, I have no idea if anyone actually cares about lack > of parallelism due to locked blocking commits or not. Hence > Cc'd some compositor folks as well. I guess this is more of > an RFC at this point. > > Also curious to see if CI goes up in smoke or not... Hi Ville, thanks for thinking about this. If I understand correctly, the issue you are solving here happens only when a blocking commit is underway while TEST_ONLY commits are done. This can only happen if userspace does the blocking commits from one thread, while another thread is doing TEST_ONLY probing on the same DRM device. It is inconsequential whether the two threads target distinct CRTCs or same CRTCs. If so, this is not a problem for Weston for two reasons: - Weston is fundamentally single-threaded, so if it does use a blocking commit, it's not going to do anything else at the same time. - Weston practically always uses non-blocking commits. I cannot imagine those two facts to change. Ah, but there is a case: KMS leasing! With leasing you have two processes poking distinct CRTCs on the same device at the same time. Even if Weston never blocks, an arbitrary leasing client might, and I presume that would then stall Weston's TEST_ONLY commits. I believe working on optimising this could be useful for KMS leasing use cases, assuming lessees do blocking commits. I don't know if any do. Thanks, pq > > Cc: Daniel Vetter <daniel.vetter@xxxxxxxx> > Cc: Maarten Lankhorst <maarten.lankhorst@xxxxxxxxxxxxxxx> > Cc: Rob Clark <robdclark@xxxxxxxxx> > Cc: Simon Ser <contact@xxxxxxxxxxx> > Cc: Pekka Paalanen <pekka.paalanen@xxxxxxxxxxxxx> > Cc: Jonas Ådahl <jadahl@xxxxxxxxx> > > Ville Syrjälä (4): > drm/atomic: Treat a nonblocking commit following a blocking commit as > blocking commit > drm/i915: Don't reuse commit_work for the cleanup > drm/atomic: Allow lockless blocking commits > drm/i915: Make blocking commits lockless > > drivers/gpu/drm/drm_atomic.c | 32 +++++++++++++++++-- > drivers/gpu/drm/drm_atomic_helper.c | 19 +++++++---- > drivers/gpu/drm/drm_atomic_uapi.c | 11 +++++-- > drivers/gpu/drm/i915/display/intel_display.c | 15 +++------ > .../drm/i915/display/intel_display_types.h | 1 + > include/drm/drm_atomic.h | 8 +++++ > 6 files changed, 64 insertions(+), 22 deletions(-) >
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