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

[Antonio Argenziano <antonio.argenziano@intel.com>]

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.

> 
> Maybe it is not disrupting higher priority clients but it is causing an 
> time unbound power drain.
> 
>> pretty much the same behavior of hangcheck IIRC so something we 
>> already accept.
> 
> You mean today hangcheck wouldn't notice a hanging batch in the same 
> scenario as above? If so it sounds like a huge gap we need to try and fix.

My understanding of it is that we only keep a record of what was running 
the last time hangcheck was run so it is possible to trick it into 
resetting when a preemption occurs but I could be missing something.

> 
>>>
>>> And finally there is one programming restriction which says:
>>>
>>>   * SW must not preempt the workload which has watchdog enabled. 
>>> Either it must:
>>>
>>> a) disable preemption for that workload completely, or
>>> b) disable the watchdog via mmio write before any write to ELSP
>>>
>>> This seems it contradiction with the statement that the counter gets 
>>> disabled on context switch and stays disabled.
>>>
>>> I did not spot anything like this in the series. So it would seem the 
>>> commit message is correct after all.
>>>
>>> It would be good if someone could re-read the bspec text on register 
>>> 0x2178 to double check what I wrote.
>>
>> The way I read it is that the restriction applies only to some 
>> platforms where the 'normal' description doesn't apply.
> 
> You are right. Are the listed parts in the field so the series would 
> have to handle this or we can ignore it?

I think there is something we need to handle e.g. BXT.

Antonio

> 
> Regards,
> 
> Tvrtko
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