[Intel-gfx] [PATCH 3/9] drm/i915/gt: Unlock engine-pm after queuing the kernel context switch

[Chris Wilson]

Quoting Tvrtko Ursulin (2019-11-20 12:40:13)
> 
> On 20/11/2019 12:07, Chris Wilson wrote:
> > Quoting Tvrtko Ursulin (2019-11-20 11:58:27)
> >>
> >> On 20/11/2019 09:32, Chris Wilson wrote:
> >>> In commit a79ca656b648 ("drm/i915: Push the wakeref->count deferral to
> >>> the backend"), I erroneously concluded that we last modify the engine
> >>> inside __i915_request_commit() meaning that we could enable concurrent
> >>> submission for userspace as we enqueued this request. However, this
> >>> falls into a trap with other users of the engine->kernel_context waking
> >>> up and submitting their request before the idle-switch is queued, with
> >>> the result that the kernel_context is executed out-of-sequence most
> >>> likely upsetting the GPU and certainly ourselves when we try to retire
> >>> the out-of-sequence requests.
> >>>
> >>> As such we need to hold onto the effective engine->kernel_context mutex
> >>> lock (via the engine pm mutex proxy) until we have finish queuing the
> >>> request to the engine.
> >>>
> >>> v2: Serialise against concurrent intel_gt_retire_requests()
> >>> v3: Describe the hairy locking scheme with intel_gt_retire_requests()
> >>> for future reference.
> >>> v4: Combine timeline->lock and engine pm release; it's hairy.
> >>>
> >>> Fixes: a79ca656b648 ("drm/i915: Push the wakeref->count deferral to the backend")
> >>> Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
> >>> Cc: Mika Kuoppala <mika.kuoppala at linux.intel.com>
> >>> Cc: Tvrtko Ursulin <tvrtko.ursulin at intel.com>
> >>> ---
> >>>    drivers/gpu/drm/i915/gt/intel_engine_pm.c | 47 +++++++++++++++++++----
> >>>    1 file changed, 40 insertions(+), 7 deletions(-)
> >>>
> >>> diff --git a/drivers/gpu/drm/i915/gt/intel_engine_pm.c b/drivers/gpu/drm/i915/gt/intel_engine_pm.c
> >>> index 3c0f490ff2c7..1f517357a268 100644
> >>> --- a/drivers/gpu/drm/i915/gt/intel_engine_pm.c
> >>> +++ b/drivers/gpu/drm/i915/gt/intel_engine_pm.c
> >>> @@ -73,8 +73,25 @@ static inline void __timeline_mark_unlock(struct intel_context *ce,
> >>>    
> >>>    #endif /* !IS_ENABLED(CONFIG_LOCKDEP) */
> >>>    
> >>> +static void
> >>> +__intel_timeline_enter_and_pm_release(struct intel_timeline *tl,
> >>> +                                   struct intel_engine_cs *engine)
> >>> +{
> >>> +     struct intel_gt_timelines *timelines = &engine->gt->timelines;
> >>> +
> >>> +     spin_lock(&timelines->lock);
> >>> +
> >>> +     if (!atomic_fetch_inc(&tl->active_count))
> >>> +             list_add_tail(&tl->link, &timelines->active_list);
> >>
> >> Hmm with these new part it maybe matches/answers my question from
> >> "drm/i915/gt: Close race between engine_park and
> >> intel_gt_retire_requests". I think at least. Since it now adds a second
> >> place timeline can enter the active_list.
> >>
> >> But no, where does the intel_timeline_enter race come from? Can't be
> >> userspace submission since they are blocked on wf->lock.
> > 
> > The race is not just with intel_timeline_enter, but with
> > intel_gt_retire_requests.
> > 
> > As soon as we are on that list, we may be retired. If we are retired
> > before adjusting the engine->wakeref.count, we are b0rked.
> > 
> > As soon as we adjust the engine->wakeref.count, another submission may
> > call intel_timeline_enter, and again may even retire this request. The
> > enter itself is perfectly fine, but we need to serialise against the
> > retires.
> 
> I think the two patches combined work, I am just not sure two 
> atomic_fetch_inc()->list_add() are needed now that you re-ordered 
> __i915_requests_queue and __intel_wakeref_defer_park - that's the part 
> which is confusing me. But it also doesn't harm...

I tried to get away with not, but the selftests hammer very heavily on
the engine->kernel_context so we do encounter the scenarios where we are
using the kernel_context to park on one cpu while submitting a new
request on another.

I would have got away with it but for these pesky selftests.
-Chris