Re: [RFC 11/14] drm/i915: Engine busy time tracking

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

On 18/07/2017 16:19, Chris Wilson wrote:
> Quoting Tvrtko Ursulin (2017-07-18 15:36:15)
>> From: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
>>
>> Track total time requests have been executing on the hardware.
>>
>> To make this cheap it is hidden behind a static branch with the
>> intention that it is only enabled when there is a consumer
>> listening. This means that in the default off case the total
>> cost of the tracking is just a few no-op instructions on the
>> fast paths.
> 
>> +static inline void intel_engine_context_in(struct intel_engine_cs *engine)
>> +{
>> +       if (static_branch_unlikely(&i915_engine_stats_key)) {
>> +               unsigned long flags;
>> +
>> +               spin_lock_irqsave(&engine->stats.lock, flags);
> 
> What's the purpose of this lock? RMW is ordered by virtue of the tasklet
> (only one cpu can be doing the rmw at any time). Did I miss another
> user?

Hm it should be a plain spin_lock and a _bh variant on the external API.

But the purpose is to allow atomic sampling of accumulated busy time 
plus the current engine status.

>> +               if (engine->stats.ref++ == 0)
>> +                       engine->stats.start = ktime_get_real_ns();
> 
> Use ktime_get_raw() and leave the conversion to ns to the caller.

You mean both store in ktime_t and don't fetch the wall time but 
monotonic? Do you say this because perf pmu perhaps needs monotonic or 
for some other reason?

> What is the cost of a ktime nowadays?

I don't see it in the profiles, so not much I think.

Regards,

Tvrtko

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