Re: [PATCH 18/46] drm/i915: Replace global_seqno with a hangcheck heartbeat seqno

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

On 11/02/2019 12:44, Chris Wilson wrote:
> Quoting Tvrtko Ursulin (2019-02-11 12:40:07)
>>
>> On 06/02/2019 13:03, Chris Wilson wrote:
>>> To determine whether an engine has 'stuck', we simply check whether or
>>> not is still on the same seqno for several seconds. To keep this simple
>>> mechanism intact over the loss of a global seqno, we can simply add a
>>> new global heartbeat seqno instead. As we cannot know the sequence in
>>> which requests will then be completed, we use a primitive random number
>>> generator instead (with a cycle long enough to not matter over an
>>> interval of a few thousand requests between hangcheck samples).
>>
>> We couldn't keep the global seqno just for hangcheck puposes? I mean as
>> long as it is unique, which would be guaranteed by obtaining an
>> increment on every submission to hw and storing it in atomic_t
>> i915->hangcheck_global_seqno / rq->hangcheck_global_seqno, hangcheck
>> does not care about the order of execution, no?
> 
> s/global_seqno/hangcheck_seqno/ ?

Yes sure, I was just trying to express the idea that a "globally" unique 
number is all that I thought we need. Like:

     rq->hangcheck_seqno = atomic_inc_return(&i915->hangcheck_seqno);

Did I get that right then? That we don't really need the pseudo random 
number solution? We could even avoid calling it a seqno if desired. 
rq->unique, wait.. we possibly had this name for something in the past..

> (a) the goal is to kill off global_seqno entirely so we are all sure
> there is no such seqno or ordering anymore
> (b) this is a temporary patch and we kill off hangcheck_seqno, just as
> soon as I can submit requests without struct_mutex

The heartbeat request solution? Is that better than the hangcheck seqno?

Regards,

Tvrtko
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