linux-kernel - Re: [PATCH v2] sched/debug: Add new tracepoint to track cpu

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Date:   Fri, 28 Aug 2020 19:10:41 +0200
From:   Dietmar Eggemann <dietmar.eggemann@....com>
To:     Qais Yousef <qais.yousef@....com>, vincent.donnefort@....com
Cc:     mingo@...hat.com, peterz@...radead.org, vincent.guittot@...aro.org,
        linux-kernel@...r.kernel.org, valentin.schneider@....com
Subject: Re: [PATCH v2] sched/debug: Add new tracepoint to track cpu_capacity

On 28/08/2020 12:27, Qais Yousef wrote:
> On 08/28/20 10:00, vincent.donnefort@....com wrote:
>> From: Vincent Donnefort <vincent.donnefort@....com>
>>
>> rq->cpu_capacity is a key element in several scheduler parts, such as EAS
>> task placement and load balancing. Tracking this value enables testing
>> and/or debugging by a toolkit.
>>
>> Signed-off-by: Vincent Donnefort <vincent.donnefort@....com>
>>
>> diff --git a/include/linux/sched.h b/include/linux/sched.h
> 
> [...]
> 
>> +int sched_trace_rq_cpu_capacity(struct rq *rq)
>> +{
>> +	return rq ?
>> +#ifdef CONFIG_SMP
>> +		rq->cpu_capacity
>> +#else
>> +		SCHED_CAPACITY_SCALE
>> +#endif
>> +		: -1;
>> +}
>> +EXPORT_SYMBOL_GPL(sched_trace_rq_cpu_capacity);
>> +
> 
> The placement of this #ifdef looks odd to me. But FWIW
> 
> Reviewed-by: Qais Yousef <qais.yousef@....com>

Returning -1 for cpu_capacity? It makes sense for sched_trace_rq_cpu()
but for cpu_capacity?

Can you remind me why we have all these helper functions like
sched_trace_rq_cpu_capacity?

In case we would let the extra code (which transforms trace points into
trace events) know the internals of struct rq we could handle those
things in the TRACE_EVENT and/or the register_trace_##name(void
(*probe)(data_proto), void *data) thing.
We always said when the internal things will change this extra code will
break. So that's not an issue.