[Intel-gfx] [PATCH 2/3] drm/i915/gt: Make the heartbeat play nice with long pre-emption timeouts

[Tvrtko Ursulin]

On 03/03/2022 19:09, John Harrison wrote:

>> Actions:
>>
>> 1)
>> Get a number from compute/OpenCL people for what they say is minimum 
>> preempt timeout for default out of the box Linux desktop experience.
> That would be the one that has been agreed upon by both linux software 
> arch and all UMD teams and has been in use for the past year or more in 
> the internal tree.

What has been used in the internal tree is irrelevant when UMD ack is needed for changes which affect upstream shipping platforms like Tigerlake.

>> This does not mean them running some tests and can't be bothered to 
>> setup up the machine for the extreme use cases, but workloads average 
>> users can realistically be expected to run.
>>
>> Say for instance some image manipulation software which is OpenCL 
>> accelerated or similar. How long unpreemptable sections are expected 
>> there. Or similar. I am not familiar what all OpenCL accelerated use 
>> cases there are on Linux.
>>
>> And this number should be purely about minimum preempt timeout, not 
>> considering heartbeats. This is because preempt timeout may kick in 
>> sooner than stopped heartbeat if the user workload is low priority.
>>
> And driver is simply hosed in the intervening six months or more that it 
> takes for the right people to find the time to do this.

What is hosed? Driver currently contains a patch which was acked by the compute UMD to disable preemption. If it takes six months for compute UMD to give us a different number which works for the open source stack and typical use cases then what can we do.

> Right now, it is broken. This patch set improves things. Actual numbers 
> can be refined later as/when some random use case that we hadn't 
> previously thought of pops up. But not fixing the basic problem at all 
> until we have an absolutely perfect for all parties solution is 
> pointless. Not least because there is no perfect solution. No matter 
> what number you pick it is going to be wrong for someone.
> 
> 2.5s, 7.5s, X.Ys, I really don't care. 2.5s is a number you seem to have 
> picked out of the air totally at random, or maybe based on it being the 
> heartbeat period (except that you keep arguing that basing tP on tH is 
> wrong). 7.5s is a number that has been in active use for a lot of 
> testing for quite some time - KMD CI, UMD CI, E2E, etc. But either way, 
> the initial number is almost irrelevant as long as it is not zero. So 
> can we please just get something merged now as a starting point?
> 
> 
>> 2)
>> Commit message should explain the effect on the worst case time until 
>> engine reset.
>>
>> 3)
>> OpenCL/compute should ack the change publicly as well since they acked 
>> the disabling of preemption.
> This patch set has already been publicly acked by the compute team. See 
> the 'acked-by' tag.

I can't find the reply which contained the ack on the mailing list - do you have a msg-id or an archive link?

Also, ack needs to be against the the fixed timeout patch and not one dependent on the heartbeat interval.

>> 4)
>> I really want overflows_type in the first patch.
> In the final GuC assignment? Only if it is a BUG_ON. If we get a failure 
> there it is an internal driver error and cannot be corrected for. It is 
> too late for any plausible range check action.

If you can find a test which exercises setting insane values to the relevant timeouts and so would hit the problem in our CI then BUG_ON is fine. Otherwise I think BUG_ON is too anti-social and prefer drm_warn or drm_WARN_ON. I don't think adding a test is strictly necessary, if we don't already have one, given how unlikely this is too be hit, but if you insist on a BUG_ON instead of a flavour of a warn then I think we need one so we can catch in CI 100% of the time.
  
> And if you mean in the the actual helper function with the rest of the 
> clamping then you are bleeding internal GuC API structure details into 
> non-GuC code. Plus the test would be right next to the 'if (size < 

In my other reply I exactly described that would be a downside and that I prefer checks at the assignment sites.

Also regarding this comment in the relevant patch:

+	/*
+	 * NB: The GuC API only supports 32bit values. However, the limit is further
+	 * reduced due to internal calculations which would otherwise overflow.
+	 */

I would suggest clarifying this as "The GuC API only supports timeouts up to U32_MAX micro-seconds. However, ...". Given the function at hand deals in milliseconds explicitly calling out that additional scaling factor makes sense I think.

Big picture - it's really still very simple. Public ack for a fixed number and a warn on is not really a lot to ask.

Regards,

Tvrtko