Re: [PATCH 2/3] drm/i915/gt: create workqueue dedicated to wake references

[Tvrtko Ursulin <tvrtko.ursulin@xxxxxxxxxxxxxxx>]

On 17/05/2023 12:18, Coelho, Luciano wrote:
> On Fri, 2023-05-12 at 13:16 +0100, Tvrtko Ursulin wrote:
> > On 12/05/2023 10:54, Coelho, Luciano wrote:
> > > On Fri, 2023-05-12 at 10:32 +0100, Tvrtko Ursulin wrote:
> > > > On 12/05/2023 10:10, Coelho, Luciano wrote:
> > > > > On Fri, 2023-05-12 at 10:04 +0100, Tvrtko Ursulin wrote:
> > > > > > On 11/05/2023 09:20, Luca Coelho wrote:
> > > > > > > Add a work queue in the intel_wakeref structure to be used exclusively
> > > > > > > by the wake reference mechanism.  This is needed in order to avoid
> > > > > > > using the system workqueue and relying on flush_scheduled_work().
> > > > > > >
> > > > > > > Cc: Tetsuo Handa <penguin-kernel@xxxxxxxxxxxxxxxxxxx>
> > > > > > > Cc: Tvrtko Ursulin <tvrtko.ursulin@xxxxxxxxx>
> > > > > > > Cc: Jani Nikula <jani.nikula@xxxxxxxxx>
> > > > > > > Cc: Ville Syrjälä <ville.syrjala@xxxxxxxxxxxxxxx>
> > > > > > > Signed-off-by: Luca Coelho <luciano.coelho@xxxxxxxxx>
> > > > > > > ---
> > > > > > >      drivers/gpu/drm/i915/gt/intel_engine_cs.c |  7 ++++++-
> > > > > > >      drivers/gpu/drm/i915/gt/intel_engine_pm.c | 15 ++++++++++++--
> > > > > > >      drivers/gpu/drm/i915/gt/intel_engine_pm.h |  3 ++-
> > > > > > >      drivers/gpu/drm/i915/gt/mock_engine.c     |  8 +++++++-
> > > > > > >      drivers/gpu/drm/i915/intel_wakeref.c      | 21 ++++++++++++++-----
> > > > > > >      drivers/gpu/drm/i915/intel_wakeref.h      | 25 +++++++++++++++--------
> > > > > > >      6 files changed, 60 insertions(+), 19 deletions(-)
> > > > > > >
> > > > > > > diff --git a/drivers/gpu/drm/i915/gt/intel_engine_cs.c b/drivers/gpu/drm/i915/gt/intel_engine_cs.c
> > > > > > > index 0aff5bb13c53..6505bfa70cd0 100644
> > > > > > > --- a/drivers/gpu/drm/i915/gt/intel_engine_cs.c
> > > > > > > +++ b/drivers/gpu/drm/i915/gt/intel_engine_cs.c
> > > > > > > @@ -1290,7 +1290,11 @@ static int engine_setup_common(struct intel_engine_cs *engine)
> > > > > > >      		goto err_cmd_parser;
> > > > > > >      
> > > > > > >      	intel_engine_init_execlists(engine);
> > > > > > > -	intel_engine_init__pm(engine);
> > > > > > > +
> > > > > > > +	err = intel_engine_init__pm(engine);
> > > > > > > +	if (err)
> > > > > > > +		goto err_cmd_parser;
> > > > > > > +
> > > > > > >      	intel_engine_init_retire(engine);
> > > > > > >      
> > > > > > >      	/* Use the whole device by default */
> > > > > > > @@ -1525,6 +1529,7 @@ void intel_engine_cleanup_common(struct intel_engine_cs *engine)
> > > > > > >      {
> > > > > > >      	GEM_BUG_ON(!list_empty(&engine->sched_engine->requests));
> > > > > > >      
> > > > > > > +	intel_engine_destroy__pm(engine);
> > > > > > >      	i915_sched_engine_put(engine->sched_engine);
> > > > > > >      	intel_breadcrumbs_put(engine->breadcrumbs);
> > > > > > >      
> > > > > > > diff --git a/drivers/gpu/drm/i915/gt/intel_engine_pm.c b/drivers/gpu/drm/i915/gt/intel_engine_pm.c
> > > > > > > index ee531a5c142c..859b62cf660f 100644
> > > > > > > --- a/drivers/gpu/drm/i915/gt/intel_engine_pm.c
> > > > > > > +++ b/drivers/gpu/drm/i915/gt/intel_engine_pm.c
> > > > > > > @@ -294,14 +294,25 @@ static const struct intel_wakeref_ops wf_ops = {
> > > > > > >      	.put = __engine_park,
> > > > > > >      };
> > > > > > >      
> > > > > > > -void intel_engine_init__pm(struct intel_engine_cs *engine)
> > > > > > > +int intel_engine_init__pm(struct intel_engine_cs *engine)
> > > > > > >      {
> > > > > > >      	struct intel_runtime_pm *rpm = engine->uncore->rpm;
> > > > > > > +	int err;
> > > > > > > +
> > > > > > > +	err = intel_wakeref_init(&engine->wakeref, rpm, &wf_ops);
> > > > > > > +	if (err)
> > > > > > > +		return err;
> > > > > > >      
> > > > > > > -	intel_wakeref_init(&engine->wakeref, rpm, &wf_ops);
> > > > > > >      	intel_engine_init_heartbeat(engine);
> > > > > > >      
> > > > > > >      	intel_gsc_idle_msg_enable(engine);
> > > > > > > +
> > > > > > > +	return 0;
> > > > > > > +}
> > > > > > > +
> > > > > > > +void intel_engine_destroy__pm(struct intel_engine_cs *engine)
> > > > > > > +{
> > > > > > > +	intel_wakeref_destroy(&engine->wakeref);
> > > > > > >      }
> > > > > > >      
> > > > > > >      /**
> > > > > > > diff --git a/drivers/gpu/drm/i915/gt/intel_engine_pm.h b/drivers/gpu/drm/i915/gt/intel_engine_pm.h
> > > > > > > index d68675925b79..e8568f7d10c6 100644
> > > > > > > --- a/drivers/gpu/drm/i915/gt/intel_engine_pm.h
> > > > > > > +++ b/drivers/gpu/drm/i915/gt/intel_engine_pm.h
> > > > > > > @@ -104,7 +104,8 @@ intel_engine_create_kernel_request(struct intel_engine_cs *engine)
> > > > > > >      	return rq;
> > > > > > >      }
> > > > > > >      
> > > > > > > -void intel_engine_init__pm(struct intel_engine_cs *engine);
> > > > > > > +int intel_engine_init__pm(struct intel_engine_cs *engine);
> > > > > > > +void intel_engine_destroy__pm(struct intel_engine_cs *engine);
> > > > > > >      
> > > > > > >      void intel_engine_reset_pinned_contexts(struct intel_engine_cs *engine);
> > > > > > >      
> > > > > > > diff --git a/drivers/gpu/drm/i915/gt/mock_engine.c b/drivers/gpu/drm/i915/gt/mock_engine.c
> > > > > > > index c0637bf799a3..0a3c702c21e2 100644
> > > > > > > --- a/drivers/gpu/drm/i915/gt/mock_engine.c
> > > > > > > +++ b/drivers/gpu/drm/i915/gt/mock_engine.c
> > > > > > > @@ -336,6 +336,7 @@ static void mock_engine_release(struct intel_engine_cs *engine)
> > > > > > >      	intel_context_put(engine->kernel_context);
> > > > > > >      
> > > > > > >      	intel_engine_fini_retire(engine);
> > > > > > > +	intel_engine_destroy__pm(engine);
> > > > > > >      }
> > > > > > >      
> > > > > > >      struct intel_engine_cs *mock_engine(struct drm_i915_private *i915,
> > > > > > > @@ -393,6 +394,7 @@ struct intel_engine_cs *mock_engine(struct drm_i915_private *i915,
> > > > > > >      int mock_engine_init(struct intel_engine_cs *engine)
> > > > > > >      {
> > > > > > >      	struct intel_context *ce;
> > > > > > > +	int err;
> > > > > > >      
> > > > > > >      	INIT_LIST_HEAD(&engine->pinned_contexts_list);
> > > > > > >      
> > > > > > > @@ -402,7 +404,11 @@ int mock_engine_init(struct intel_engine_cs *engine)
> > > > > > >      	engine->sched_engine->private_data = engine;
> > > > > > >      
> > > > > > >      	intel_engine_init_execlists(engine);
> > > > > > > -	intel_engine_init__pm(engine);
> > > > > > > +
> > > > > > > +	err = intel_engine_init__pm(engine);
> > > > > > > +	if (err)
> > > > > > > +		return err;
> > > > > > > +
> > > > > > >      	intel_engine_init_retire(engine);
> > > > > > >      
> > > > > > >      	engine->breadcrumbs = intel_breadcrumbs_create(NULL);
> > > > > > > diff --git a/drivers/gpu/drm/i915/intel_wakeref.c b/drivers/gpu/drm/i915/intel_wakeref.c
> > > > > > > index dfd87d082218..6bae609e1312 100644
> > > > > > > --- a/drivers/gpu/drm/i915/intel_wakeref.c
> > > > > > > +++ b/drivers/gpu/drm/i915/intel_wakeref.c
> > > > > > > @@ -74,7 +74,7 @@ void __intel_wakeref_put_last(struct intel_wakeref *wf, unsigned long flags)
> > > > > > >      
> > > > > > >      	/* Assume we are not in process context and so cannot sleep. */
> > > > > > >      	if (flags & INTEL_WAKEREF_PUT_ASYNC || !mutex_trylock(&wf->mutex)) {
> > > > > > > -		mod_delayed_work(system_wq, &wf->work,
> > > > > > > +		mod_delayed_work(wf->wq, &wf->work,
> > > > > > >      				 FIELD_GET(INTEL_WAKEREF_PUT_DELAY, flags));
> > > > > > >      		return;
> > > > > > >      	}
> > > > > > > @@ -93,10 +93,10 @@ static void __intel_wakeref_put_work(struct work_struct *wrk)
> > > > > > >      	____intel_wakeref_put_last(wf);
> > > > > > >      }
> > > > > > >      
> > > > > > > -void __intel_wakeref_init(struct intel_wakeref *wf,
> > > > > > > -			  struct intel_runtime_pm *rpm,
> > > > > > > -			  const struct intel_wakeref_ops *ops,
> > > > > > > -			  struct intel_wakeref_lockclass *key)
> > > > > > > +int __intel_wakeref_init(struct intel_wakeref *wf,
> > > > > > > +			 struct intel_runtime_pm *rpm,
> > > > > > > +			 const struct intel_wakeref_ops *ops,
> > > > > > > +			 struct intel_wakeref_lockclass *key)
> > > > > > >      {
> > > > > > >      	wf->rpm = rpm;
> > > > > > >      	wf->ops = ops;
> > > > > > > @@ -105,9 +105,20 @@ void __intel_wakeref_init(struct intel_wakeref *wf,
> > > > > > >      	atomic_set(&wf->count, 0);
> > > > > > >      	wf->wakeref = 0;
> > > > > > >      
> > > > > > > +	wf->wq = alloc_workqueue("i1915-wakeref", 0, 0);
> > > > > >
> > > > > > Typo here -
> > > > >
> > > > > Oh, good catch! This is one of my "favorite" typos, for some reason.
> > > >
> > > > Yes, I had the same one. :) Patch 3/3 too.
> > > >
> > > > > >     I wanted to ask however - why does this particular wq
> > > > > > "deserves" to be dedicated and can't just use one of the
> > > > > > drm_i915_private ones?
> > > > >
> > > > > It's because there's no easy way to get access to the drm_i915_private
> > > > > structure from here.  And I don't think this work needs to be in sync
> > > > > with the rest of the works in i915.
> > > >
> > > > Yeah I don't think it needs to be synchronised either. Was just thinking
> > > > if we really need to be creating a bunch of separate workqueues (one per
> > > > engine) for not much use, or instead could just add a backpointer to
> > > > either intel_wakeref or intel_runtime_pm. Latter already has rpm->kdev
> > > > so could plausably be replaced with rpm->i915.
> > > >
> > > > Actually, looking at intel_runtime_pm_init_early, you could get to i915
> > > > via wf->rpm and container_of.
> > >
> > > Yeah, I considered that, but using container_of() can be problematic
> > > when we're not sure where exactly the element is coming from.  My worry
> > > was this:
> > >
> > > int intel_engine_init__pm(struct intel_engine_cs *engine)
> > > {
> > > 	struct intel_runtime_pm *rpm = engine->uncore->rpm;
> > > 	int err;
> > >
> > > 	err = intel_wakeref_init(&engine->wakeref, rpm, &wf_ops);
> > > [...]
> > > }
> > >
> > > In this case, we're getting to __intel_wakeref_init() with an *rpm that
> > > is coming from an intel_uncore structure and not from
> > > drm_i915_private...
> >
> > Right. Yes I agree that would be a flaky/questionable design, even if it
> > worked in practice. I'd just replace rpm->dev with rpm->i915 then. Not
> > feeling *that* strongly about it, but it just feels a waste to create a
> > bunch of workqueues for this.
>
> How many engines do we typically have at runtime?

Currently not that many I think. There were plans for like 18 per tile, 
but I don't think it will be happening.

> I was looking into getting the i915 struct via rpm->kdev and
> container_of(), but it's not trivial either.  This is how we initialize
> rpm->kdev:
>
> void intel_runtime_pm_init_early(struct intel_runtime_pm *rpm)
> {
> 	struct drm_i915_private *i915 =
> 			container_of(rpm, struct drm_i915_private, runtime_pm);
> 	struct pci_dev *pdev = to_pci_dev(i915->drm.dev);
> 	struct device *kdev = &pdev->dev;
>
> 	rpm->kdev = kdev;
> [...]
> }
>
> So, rpm->kdev actually points to the device structure from inside a
> pci_dev.  i915->drm.dev points to the same device structure, but we
> lose the pointer to the element inside i915, so there's no way to trace
> it back.  We keep only the pointer to the element inside pci_dev.
>
> Or did you mean that we should change the kdev pointer to an i915
> pointer in intel_runtime_pm and derive kdev when needed? This would
> cause some code shuffle.  And, currently, the wakeref code doesn't have
> any references to drm_i915_private at all (or even to drm for that
> matter)...

Yes, that's what I meant. Struct intel_runtime_pm is only intimately 
tied to i915 via lmem_userfault_list so I think conceptually it is fine. 
But I haven't look at the level of churn it would require.

> I don't know.  We could go either way.  I think it depends mostly on
> how many engines instances we typically have, so we can weigh runtime
> resources vs code complexity...

Or we just go with Tetsuo's approach of a global i915_wq. For me that 
one is completely fine.

Regards,

Tvrtko