Re: [Intel-gfx] [PATCH 1/2] drm/i915: Disable semaphore inter-engine sync without timeslicing

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

On 21/05/2020 11:17, Chris Wilson wrote:
> Quoting Chris Wilson (2020-05-21 10:42:26)
>> Quoting Tvrtko Ursulin (2020-05-21 10:10:10)
>>>
>>> On 21/05/2020 09:53, Chris Wilson wrote:
>>>> Since the remove of the no-semaphore boosting, we rely on timeslicing to
>>>> reorder past inter-dependency hogs across the engines. However, we
>>>> require preemption to support timeslicing into user payloads, and not all
>>>> machine support preemption so we do not universally enable timeslicing
>>>> even when it would preempt our own inter-engine semaphores.
>>>>
>>>> Testcase: igt/gem_exec_schedule/semaphore-codependency # bdw/bsw
>>>> Fixes: 18e4af04d218 ("drm/i915: Drop no-semaphore boosting")
>>>> Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
>>>> Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
>>>> Cc: Mika Kuoppala <mika.kuoppala@linux.intel.com>
>>>> ---
>>>>    drivers/gpu/drm/i915/gem/i915_gem_context.c | 4 ++--
>>>>    1 file changed, 2 insertions(+), 2 deletions(-)
>>>>
>>>> diff --git a/drivers/gpu/drm/i915/gem/i915_gem_context.c b/drivers/gpu/drm/i915/gem/i915_gem_context.c
>>>> index 900ea8b7fc8f..f5d59d18cd5b 100644
>>>> --- a/drivers/gpu/drm/i915/gem/i915_gem_context.c
>>>> +++ b/drivers/gpu/drm/i915/gem/i915_gem_context.c
>>>> @@ -230,7 +230,7 @@ static void intel_context_set_gem(struct intel_context *ce,
>>>>                ce->timeline = intel_timeline_get(ctx->timeline);
>>>>    
>>>>        if (ctx->sched.priority >= I915_PRIORITY_NORMAL &&
>>>> -         intel_engine_has_semaphores(ce->engine))
>>>> +         intel_engine_has_timeslices(ce->engine))
>>>>                __set_bit(CONTEXT_USE_SEMAPHORES, &ce->flags);
>>>>    }
>>>>    
>>>> @@ -1969,7 +1969,7 @@ static int __apply_priority(struct intel_context *ce, void *arg)
>>>>    {
>>>>        struct i915_gem_context *ctx = arg;
>>>>    
>>>> -     if (!intel_engine_has_semaphores(ce->engine))
>>>> +     if (!intel_engine_has_timeslices(ce->engine))
>>>>                return 0;
>>>>    
>>>>        if (ctx->sched.priority >= I915_PRIORITY_NORMAL)
>>>>
>>>
>>> __i915_request_await_execution is okay to keep using semaphores?
>>
>> I think so. Using semaphores there still benefits from synchronising
>> with a master in ELSP[1]. The danger is that it does increase the
>> hangcheck possibility for the bond request, such that a slow request
>> before the master would result in us declaring the bond hung. The
>> question is whether that is worse than executing the bond before the
>> master.
>>
>> I should be able to write a test to demonstrate the hang in the bond.
>> For example, if we do something like:
>>
>> on master engine:
>>          submit spin
>>          submit master -> submit fence -> submit bond
>>          for(;;)
>>                  submit high priority spin
>>                  terminate previous spin
>>          
>> Hmm. But without preemption... master will execute before we get a
>> chance to submit the high priority spinners. So this will not actually
>> hang.
>>
>> Ok, so this is safer than it seems :)
> 
> Even more so, since we do preempt the semaphore for the hangcheck.

Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>

Regards,

Tvrtko

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