Re: [PATCH 12/37] drm/i915: Priority boost for new clients

[Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>]

On 29/06/2018 11:51, Chris Wilson wrote:
> Quoting Tvrtko Ursulin (2018-06-29 11:36:50)
>>
>> On 29/06/2018 11:09, Chris Wilson wrote:
>>> Quoting Tvrtko Ursulin (2018-06-29 11:04:36)
>>>>
>>>> On 29/06/2018 08:53, Chris Wilson wrote:
>>>>> Taken from an idea used for FQ_CODEL, we give new request flows a
>>>>> priority boost. These flows are likely to correspond with interactive
>>>>> tasks and so be more latency sensitive than the long queues. As soon as
>>>>> the client has more than one request in the queue, further requests are
>>>>> not boosted and it settles down into ordinary steady state behaviour.
>>>>> Such small kicks dramatically help combat the starvation issue, by
>>>>> allowing each client the opportunity to run even when the system is
>>>>> under heavy throughput load (within the constraints of the user
>>>>> selected priority).
>>>>
>>>> Any effect on non-micro benchmarks to mention in the commit message as
>>>> the selling point?
>>>
>>> Desktop interactivity, subjective.
>>> wsim showed a major impact
>>>    
>>>>> Testcase: igt/benchmarks/rrul
>>>>> Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
>>>>> Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
>>>>> Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
>>>>> ---
>>>>>     drivers/gpu/drm/i915/i915_request.c   | 16 ++++++++++++++--
>>>>>     drivers/gpu/drm/i915/i915_scheduler.h |  4 +++-
>>>>>     2 files changed, 17 insertions(+), 3 deletions(-)
>>>>>
>>>>> diff --git a/drivers/gpu/drm/i915/i915_request.c b/drivers/gpu/drm/i915/i915_request.c
>>>>> index 14bf0be6f994..2d7a785dd88c 100644
>>>>> --- a/drivers/gpu/drm/i915/i915_request.c
>>>>> +++ b/drivers/gpu/drm/i915/i915_request.c
>>>>> @@ -1052,8 +1052,20 @@ void i915_request_add(struct i915_request *request)
>>>>>          */
>>>>>         local_bh_disable();
>>>>>         rcu_read_lock(); /* RCU serialisation for set-wedged protection */
>>>>> -     if (engine->schedule)
>>>>> -             engine->schedule(request, &request->gem_context->sched);
>>>>> +     if (engine->schedule) {
>>>>> +             struct i915_sched_attr attr = request->gem_context->sched;
>>>>> +
>>>>> +             /*
>>>>> +              * Boost priorities to new clients (new request flows).
>>>>> +              *
>>>>> +              * Allow interactive/synchronous clients to jump ahead of
>>>>> +              * the bulk clients. (FQ_CODEL)
>>>>> +              */
>>>>> +             if (!prev || i915_request_completed(prev))
>>>>> +                     attr.priority |= I915_PRIORITY_NEWCLIENT;
>>>>
>>>> Now a "lucky" client can always get higher priority an keep preempting
>>>> everyone else by just timing it's submissions right. So I have big
>>>> reservations on this one.
>>>
>>> Lucky being someone who is _idle_, everyone else being steady state. You
>>> can't keep getting lucky and stealing the show.
>>
>> Why couldn't it? All it is needed is to send a new execbuf after the
>> previous has completed.
>>
>> 1. First ctx A eb -> priority boost
>> 2. Other people get back in and start executing
>> 3. Another ctx A eb -> first has completed -> another priority boost ->
>> work from 2) is preempted
>> 4. Goto 2.
> 
> So you have one client spinning, it's going to win most races and starve
> the system, simply by owning struct_mutex. We give the other starving
> steady-state clients an opportunity to submit ahead of the spinner when
> they come to resubmit.

What do you mean by spinning and owning struct_mutex?

I was thinking for example the client A sending a 1ms batch and client B 
executing a 10ms batch.

If client A keeps submitting its batch at 1ms intervals when will client 
B complete?

Regards,

Tvrtko
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