Re: [PATCH 3/5] drm/i915/execlists: Direct submit onto idle engines

[Chris Wilson <chris@chris-wilson.co.uk>]

Quoting Tvrtko Ursulin (2018-05-11 09:25:00)
> 
> On 10/05/2018 18:40, Chris Wilson wrote:
> > Quoting Tvrtko Ursulin (2018-05-10 18:26:31)
> >>
> >> On 10/05/2018 17:25, Chris Wilson wrote:
> >>> Quoting Tvrtko Ursulin (2018-05-10 17:09:14)
> >>>>
> >>>> On 09/05/2018 15:27, Chris Wilson wrote:
> >>>>> Bypass using the tasklet to submit the first request to HW, as the
> >>>>> tasklet may be deferred unto ksoftirqd and at a minimum will add in
> >>>>> excess of 10us (and maybe tens of milliseconds) to our execution
> >>>>> latency. This latency reduction is most notable when execution flows
> >>>>> between engines.
> >>>>>
> >>>>> v2: Beware handling preemption completion from the direct submit path as
> >>>>> well.
> >>>>> v3: Make the abuse clear and track our extra state inside i915_tasklet.
> >>>>>
> >>>>> Suggested-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
> >>>>> Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
> >>>>> Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
> >>>>> ---
> >>>>>     drivers/gpu/drm/i915/i915_tasklet.h         | 24 +++++++
> >>>>>     drivers/gpu/drm/i915/intel_guc_submission.c | 10 ++-
> >>>>>     drivers/gpu/drm/i915/intel_lrc.c            | 71 +++++++++++++++++----
> >>>>>     3 files changed, 89 insertions(+), 16 deletions(-)
> >>>>>
> >>>>> diff --git a/drivers/gpu/drm/i915/i915_tasklet.h b/drivers/gpu/drm/i915/i915_tasklet.h
> >>>>> index 42b002b88edb..99e2fa2241ba 100644
> >>>>> --- a/drivers/gpu/drm/i915/i915_tasklet.h
> >>>>> +++ b/drivers/gpu/drm/i915/i915_tasklet.h
> >>>>> @@ -8,8 +8,11 @@
> >>>>>     #define _I915_TASKLET_H_
> >>>>>     
> >>>>>     #include <linux/atomic.h>
> >>>>> +#include <linux/bitops.h>
> >>>>>     #include <linux/interrupt.h>
> >>>>>     
> >>>>> +#include "i915_gem.h"
> >>>>> +
> >>>>>     /**
> >>>>>      * struct i915_tasklet - wrapper around tasklet_struct
> >>>>>      *
> >>>>> @@ -19,6 +22,8 @@
> >>>>>      */
> >>>>>     struct i915_tasklet {
> >>>>>         struct tasklet_struct base;
> >>>>> +     unsigned long flags;
> >>>>> +#define I915_TASKLET_DIRECT_SUBMIT BIT(0)
> >>>>
> >>>> I would suggest a more generic name for the bit since i915_tasklet is
> >>>> generic-ish. For instance simply I915_TASKLET_DIRECT would signify the
> >>>> callback has been invoked directly and not (necessarily) from softirq
> >>>> context. Then it is for each user to know what that means for them
> >>>> specifically.
> >>>
> >>> Problem is we have two direct invocations, only one is special. It
> >>> really wants to be something like I915_TASKLET_ENGINE_IS_LOCKED - you can
> >>> see why I didn't propose that.
> >>
> >> TBC...
> >>
> >>>>> -static void __submit_queue(struct intel_engine_cs *engine, int prio)
> >>>>> +static void __wakeup_queue(struct intel_engine_cs *engine, int prio)
> >>>>>     {
> >>>>>         engine->execlists.queue_priority = prio;
> >>>>> +}
> >>>>
> >>>> Why is this called wakeup? Plans to add something in it later?
> >>>
> >>> Yes. It's called wakeup because it's setting the value that the dequeue
> >>> wakes up at. First name was kick_queue, but it doesn't kick either.
> >>>
> >>> The later side-effect involves controlling timers.
> >>>
> >>> __restart_queue()?
> >>
> >> __update_queue_priority? :)
> > 
> > It doesn't just update the priority...
> > 
> > Now a choice between restart_queue and update_queue.
> 
> Update sounds better match to me.
> 
> > 
> >>>>> +static void __schedule_queue(struct intel_engine_cs *engine)
> >>>>> +{
> >>>>>         i915_tasklet_schedule(&engine->execlists.tasklet);
> >>>>>     }
> >>>>>     
> >>>>> +static bool __direct_submit(struct intel_engine_execlists *const execlists)
> >>>>> +{
> >>>>> +     struct i915_tasklet * const t = &execlists->tasklet;
> >>>>> +
> >>>>> +     if (!tasklet_trylock(&t->base))
> >>>>> +             return false;
> >>>>> +
> >>>>> +     t->flags |= I915_TASKLET_DIRECT_SUBMIT;
> >>>>> +     i915_tasklet_run(t);
> >>>>> +     t->flags &= ~I915_TASKLET_DIRECT_SUBMIT;
> >>>>> +
> >>>>> +     tasklet_unlock(&t->base);
> >>>>
> >>>> Feels like this whole sequence belongs to i915_tasklet since it touches
> >>>> the internals. Maybe i915_tasklet_try_run, or i915_tasklet_run_or_schedule?
> >>>
> >>> Keep reading the series and you'll see just why this is so special and
> >>> confined to execlists.
> >>
> >> ... TBC here.
> >>
> >> Having peeked ahead, it feels a bit not generic enough as it is, a bit
> >> too hacky.
> >>
> >> Would it work to pass context together with the invocation. Like:
> >>
> >> i915_tasklet_try(..., I915_TASKLET_SUBMIT_IDLE);
> >> i915_tasklet_try(..., I915_TASKLET_SUBMIT_IRQ);
> >>
> >> i915_tasklet.flags field namespace would then be owned by the caller
> >> completely. And the tasklet func itself would have more context on what
> >> to do.
> > 
> > That doesn't apply very well to the use case either. It's not the
> > tasklet being called from irq/process that's significant but whether we
> > are calling it with the engine/data locked.
> 
> That's why I am proposing to allow a generic mechanism to pass in a 
> "token" to the API, which API will pass down to the user if invoking the 
> tasklet directly.
> 
> The user then decides how to interpret the token.
> 
> I915_TASKLET_SUBMIT_IDLE would mean "I know this is the path with 
> timeline lock already taken".
> 
> I915_TASKLET_SUBMIT_IRQ token would mean "I need to take the lock and I 
> need an early abort if tasklet is disabled".

I don't see a reason to extend it to a generic mechanism yet.
direct-submit-onto-idle is the special case. I've 3 users for the normal
case (just calling into the tasklet directly), one for wanting to pass
along private state after claiming the tasklet for itself, and one that
does unspeakable things that doesn't match any of the above ;)
-Chris
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