Re: drm/i915: Watchdog timeout: IRQ handler for gen8+

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

On 11/01/2019 21:28, John Harrison wrote:
> 
> On 1/11/2019 09:31, Antonio Argenziano wrote:
>>
>> On 11/01/19 00:22, Tvrtko Ursulin wrote:
>>>
>>> On 11/01/2019 00:47, Antonio Argenziano wrote:
>>>> On 07/01/19 08:58, Tvrtko Ursulin wrote:
>>>>> On 07/01/2019 13:57, Chris Wilson wrote:
>>>>>> Quoting Tvrtko Ursulin (2019-01-07 13:43:29)
>>>>>>>
>>>>>>> On 07/01/2019 11:58, Tvrtko Ursulin wrote:
>>>>>>>
>>>>>>> [snip]
>>>>>>>
>>>>>>>>> Note about future interaction with preemption: Preemption could 
>>>>>>>>> happen
>>>>>>>>> in a command sequence prior to watchdog counter getting disabled,
>>>>>>>>> resulting in watchdog being triggered following preemption 
>>>>>>>>> (e.g. when
>>>>>>>>> watchdog had been enabled in the low priority batch). The 
>>>>>>>>> driver will
>>>>>>>>> need to explicitly disable the watchdog counter as part of the
>>>>>>>>> preemption sequence.
>>>>>>>>
>>>>>>>> Does the series take care of preemption?
>>>>>>>
>>>>>>> I did not find that it does.
>>>>>>
>>>>>> Oh. I hoped that the watchdog was saved as part of the context... 
>>>>>> Then
>>>>>> despite preemption, the timeout would resume from where we left 
>>>>>> off as
>>>>>> soon as it was back on the gpu.
>>>>>>
>>>>>> If the timeout remaining was context saved it would be much 
>>>>>> simpler (at
>>>>>> least on first glance), please say it is.
>>>>>
>>>>> I made my comments going only by the text from the commit message 
>>>>> and the absence of any preemption special handling.
>>>>>
>>>>> Having read the spec, the situation seems like this:
>>>>>
>>>>>   * Watchdog control and threshold register are context saved and 
>>>>> restored.
>>>>>
>>>>>   * On a context switch watchdog counter is reset to zero and 
>>>>> automatically disabled until enabled by a context restore or 
>>>>> explicitly.
>>>>>
>>>>> So it sounds the commit message could be wrong that special 
>>>>> handling is needed from this direction. But read till the end on 
>>>>> the restriction listed.
>>>>>
>>>>>   * Watchdog counter is reset to zero and is not accumulated across 
>>>>> multiple submission of the same context (due preemption).
>>>>>
>>>>> I read this as - after preemption contexts gets a new full timeout 
>>>>> allocation. Or in other words, if a context is preempted N times, 
>>>>> it's cumulative watchdog timeout will be N * set value.
>>>>>
>>>>> This could be theoretically exploitable to bypass the timeout. If a 
>>>>> client sets up two contexts with prio -1 and -2, and keeps 
>>>>> submitting periodical no-op batches against prio -1 context, while 
>>>>> prio -2 is it's own hog, then prio -2 context defeats the watchdog 
>>>>> timer. I think.. would appreciate is someone challenged this 
>>>>> conclusion.
>>>>
>>>> I think you are right that is a possibility but, is that a problem? 
>>>> The client can just not set the threshold to bypass the timeout. 
>>>> Also because you need the hanging batch to be simply preemptible, 
>>>> you cannot disrupt any work from another client that is higher 
>>>> priority. This is 
>>>
>>> But I think higher priority client can have the same effect on the 
>>> lower priority purely by accident, no?
>>>
>>> As a real world example, user kicks off an background transcoding 
>>> job, which happens to use prio -2, and uses the watchdog timer.
>>>
>>> At the same time user watches a video from a player of normal 
>>> priority. This causes periodic, say 24Hz, preemption events, due 
>>> frame decoding activity on the same engine as the transcoding client.
>>>
>>> Does this defeat the watchdog timer for the former is the question? 
>>> Then the questions of can we do something about it and whether it 
>>> really isn't a problem?
>>
>> I guess it depends if you consider that timeout as the maximum 
>> lifespan a workload can have or max contiguous active time.
> 
> I believe the intended purpose of the watchdog is to prevent broken 
> bitstreams hanging the transcoder/player. That is, it is a form of error 
> detection used by the media driver to handle bad user input. So if there 
> is a way for the watchdog to be extended indefinitely under normal 
> situations, that would be a problem. It means the transcoder will not 
> detect the broken input data in a timely manner and effectively hang 
> rather than skip over to the next packet. And note that broken input 
> data can be caused by something as innocent as a dropped packet due to 
> high network contention. No need for any malicious activity at all.

My understanding of the intended purpose is the same. And it would be a 
very useful feature.

Chris mentioned the other day that until hardware is fixed to context 
save/restore the watchdog counter this could simply be implemented using 
timers. And I have to say I agree. Shouldn't be too hard to prototype it 
using  hrtimers - start on context in, stop on context out and kick 
forward on user interrupts. More or less.

Then if the cost of these hrtimer manipulations wouldn't show in 
profiles significantly we would have a solution. At least in execlists 
mode. :) But in parallel we could file a feature request to fix the 
hardware implementation and then could just switch the timer "backend" 
from hrtimers to GPU.

Regards,

Tvrtko
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