Re: [Intel-gfx] [PATCH] i915/pmu: Wire GuC backend to per-client busyness

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

On 14/06/2022 17:32, Umesh Nerlige Ramappa wrote:
> On Tue, Jun 14, 2022 at 02:30:42PM +0100, Tvrtko Ursulin wrote:
>>
>> On 14/06/2022 01:46, Nerlige Ramappa, Umesh wrote:
>>> From: John Harrison <John.C.Harrison@Intel.com>
>>>
>>> GuC provides engine_id and last_switch_in ticks for an active context 
>>> in the
>>> pphwsp. The context image provides a 32 bit total ticks which is the 
>>> accumulated
>>> by the context (a.k.a. context[CTX_TIMESTAMP]). This information is 
>>> used to
>>> calculate the context busyness as follows:
>>>
>>> If the engine_id is valid, then busyness is the sum of accumulated 
>>> total ticks
>>> and active ticks. Active ticks is calculated with current gt time as 
>>> reference.
>>>
>>> If engine_id is invalid, busyness is equal to accumulated total ticks.
>>>
>>> Since KMD (CPU) retrieves busyness data from 2 sources - GPU and GuC, a
>>> potential race was highlighted in an earlier review that can lead to 
>>> double
>>> accounting of busyness. While the solution to this is a wip, busyness 
>>> is still
>>> usable for platforms running GuC submission.
>>>
>>> Signed-off-by: John Harrison <John.C.Harrison@Intel.com>
>>> Signed-off-by: Umesh Nerlige Ramappa <umesh.nerlige.ramappa@intel.com>
>>> ---
>>>  drivers/gpu/drm/i915/gt/intel_context.c       | 11 +++-
>>>  drivers/gpu/drm/i915/gt/intel_context.h       |  6 +-
>>>  drivers/gpu/drm/i915/gt/intel_context_types.h |  3 +
>>>  drivers/gpu/drm/i915/gt/uc/intel_guc_fwif.h   |  5 ++
>>>  .../gpu/drm/i915/gt/uc/intel_guc_submission.c | 55 ++++++++++++++++++-
>>>  drivers/gpu/drm/i915/i915_drm_client.c        |  6 +-
>>>  6 files changed, 75 insertions(+), 11 deletions(-)
>>>
>>> diff --git a/drivers/gpu/drm/i915/gt/intel_context.c 
>>> b/drivers/gpu/drm/i915/gt/intel_context.c
>>> index 4070cb5711d8..a49f313db911 100644
>>> --- a/drivers/gpu/drm/i915/gt/intel_context.c
>>> +++ b/drivers/gpu/drm/i915/gt/intel_context.c
>>> @@ -576,16 +576,23 @@ void intel_context_bind_parent_child(struct 
>>> intel_context *parent,
>>>      child->parallel.parent = parent;
>>>  }
>>> -u64 intel_context_get_total_runtime_ns(const struct intel_context *ce)
>>> +u64 intel_context_get_total_runtime_ns(struct intel_context *ce)
>>>  {
>>>      u64 total, active;
>>> +    if (ce->ops->update_stats)
>>> +        ce->ops->update_stats(ce);
>>> +
>>>      total = ce->stats.runtime.total;
>>>      if (ce->ops->flags & COPS_RUNTIME_CYCLES)
>>>          total *= ce->engine->gt->clock_period_ns;
>>>      active = READ_ONCE(ce->stats.active);
>>> -    if (active)
>>> +    /*
>>> +     * GuC backend returns the actual time the context was active, 
>>> so skip
>>> +     * the calculation here for GuC.
>>> +     */
>>> +    if (active && !intel_engine_uses_guc(ce->engine))
>>
>> What is the point of looking at ce->stats.active in GuC mode? I see 
>> that guc_context_update_stats/__guc_context_update_clks touches it, 
>> but I can't spot that there is a purpose to it. This is the only 
>> conditional reading it but it is short-circuited in GuC case.
>>
>> Also, since a GuC only vfunc (update_stats) has been added, I wonder 
>> why not just fork the whole runtime query (ce->get_total_runtime_ns). 
>> I think that would end up cleaner.
>>
>>>          active = intel_context_clock() - active;
>>>      return total + active;
> 
> In case of GuC the active is used directly here since the active updated 
> in update_stats is equal to the active time of the context already. I 
> will look into separate vfunc.

Ah right, I misread something. But yes, I think a separate vfunc will 
look cleaner. Another option (instead of vfunc) is a similar flag to 
control the express the flavour of active?

>>> diff --git a/drivers/gpu/drm/i915/gt/intel_context.h 
>>> b/drivers/gpu/drm/i915/gt/intel_context.h
>>> index b7d3214d2cdd..5fc7c19ab29b 100644
>>> --- a/drivers/gpu/drm/i915/gt/intel_context.h
>>> +++ b/drivers/gpu/drm/i915/gt/intel_context.h
>>> @@ -56,7 +56,7 @@ static inline bool intel_context_is_parent(struct 
>>> intel_context *ce)
>>>      return !!ce->parallel.number_children;
>>>  }
> 
> snip
> 
>>> +static void __guc_context_update_clks(struct intel_context *ce)
>>> +{
>>> +    struct intel_guc *guc = ce_to_guc(ce);
>>> +    struct intel_gt *gt = ce->engine->gt;
>>> +    u32 *pphwsp, last_switch, engine_id;
>>> +    u64 start_gt_clk = 0, active = 0;
>>
>> No need to init these two.
>>
>>> +    unsigned long flags;
>>> +    ktime_t unused;
>>> +
>>> +    spin_lock_irqsave(&guc->timestamp.lock, flags);
>>> +
>>> +    pphwsp = ((void *)ce->lrc_reg_state) - LRC_STATE_OFFSET;
>>> +    last_switch = READ_ONCE(pphwsp[PPHWSP_GUC_CONTEXT_USAGE_STAMP_LO]);
>>> +    engine_id = READ_ONCE(pphwsp[PPHWSP_GUC_CONTEXT_USAGE_ENGINE_ID]);
>>> +
>>> +    guc_update_pm_timestamp(guc, &unused);
>>> +
>>> +    if (engine_id != 0xffffffff && last_switch) {
>>> +        start_gt_clk = READ_ONCE(ce->stats.runtime.start_gt_clk);
>>> +        __extend_last_switch(guc, &start_gt_clk, last_switch);
>>> +        active = intel_gt_clock_interval_to_ns(gt, 
>>> guc->timestamp.gt_stamp - start_gt_clk);
>>> +        WRITE_ONCE(ce->stats.runtime.start_gt_clk, start_gt_clk);
>>> +        WRITE_ONCE(ce->stats.active, active);
>>> +    } else {
>>> +        lrc_update_runtime(ce);
>>
>> Why is this called from here? Presumably it was called already from 
>> guc_context_unpin if here code things context is not active. Or will 
>> be called shortly, once context save is done.
> 
> guc_context_unpin is only called in the path of ce->sched_disable. The 
> sched_disable is implemented in GuC (H2G message). Once the 
> corresponding G2H response is received, the context is actually 
> unpinned, eventually calling guc_context_unpin. Also the context may not 
> necessarily be disabled after each context exit.

So if I understand correctly, lrc runtime is only updated if someone is 
reading the busyness and not as part of normal context state transitions?

Regards,

Tvrtko