linux-kernel - Re: [RFC PATCH 3/4] sched/topology: remove smt

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Date:   Tue, 4 Sep 2018 01:24:24 -0700
From:   Srikar Dronamraju <srikar@...ux.vnet.ibm.com>
To:     Vincent Guittot <vincent.guittot@...aro.org>
Cc:     peterz@...radead.org, mingo@...hat.com,
        linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH 3/4] sched/topology: remove smt_gain

> Cc: Peter Zijlstra <peterz@...radead.org>
> Cc: Ingo Molnar <mingo@...hat.com>
> Cc: linux-kernel@...r.kernel.org (open list)
> Signed-off-by: Vincent Guittot <vincent.guittot@...aro.org>
> diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
> index 4a2e8ca..b1715b8 100644
> --- a/kernel/sched/sched.h
> +++ b/kernel/sched/sched.h
> @@ -1758,9 +1758,6 @@ unsigned long arch_scale_freq_capacity(int cpu)
>  static __always_inline
>  unsigned long arch_scale_cpu_capacity(struct sched_domain *sd, int cpu)
>  {
> -	if (sd && (sd->flags & SD_SHARE_CPUCAPACITY) && (sd->span_weight > 1))
> -		return sd->smt_gain / sd->span_weight;
> -
>  	return SCHED_CAPACITY_SCALE;

Without this change, the capacity_orig of an SMT would have been based
on the number of threads.
For example on SMT2, capacity_orig would have been 589 and
for SMT 8, capacity_orig would have been 148.

However after this change, capacity_orig of each SMT thread would be
1024. For example SMT 8 core capacity_orig would now be 8192.

smt_gain was suppose to make a multi threaded core was slightly more
powerful than a single threaded core. I suspect if that sometimes hurt
us when doing load balance between 2 cores i.e at MC or DIE sched
domain. Even with 2 threads running on a core, the core might look
lightly loaded 2048/8192. Hence might dissuade movement to a idle core.

I always wonder why arch_scale_cpu_capacity() is called with NULL
sched_domain, in scale_rt_capacity(). This way capacity might actually
be more than the capacity_orig. I am always under an impression that
capacity_orig > capacity.  Or am I misunderstanding that?

-- 
Thanks and Regards
Srikar Dronamraju