linux-kernel - Re: [PATCH 5/5] sched/fair: Merge select_idle

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Date:   Wed, 20 Jan 2021 14:00:18 +0530
From:   Gautham R Shenoy <ego@...ux.vnet.ibm.com>
To:     Mel Gorman <mgorman@...hsingularity.net>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Li Aubrey <aubrey.li@...ux.intel.com>,
        Qais Yousef <qais.yousef@....com>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 5/5] sched/fair: Merge select_idle_core/cpu()

Hello Mel, Peter,

On Tue, Jan 19, 2021 at 11:22:11AM +0000, Mel Gorman wrote:
> From: Peter Zijlstra (Intel) <peterz@...radead.org>
> 
> Both select_idle_core() and select_idle_cpu() do a loop over the same
> cpumask. Observe that by clearing the already visited CPUs, we can
> fold the iteration and iterate a core at a time.
> 
> All we need to do is remember any non-idle CPU we encountered while
> scanning for an idle core. This way we'll only iterate every CPU once.
> 
> Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
> Signed-off-by: Mel Gorman <mgorman@...hsingularity.net>
> ---
>  kernel/sched/fair.c | 101 ++++++++++++++++++++++++++------------------
>  1 file changed, 61 insertions(+), 40 deletions(-)
> 
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 12e08da90024..822851b39b65 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c

[..snip..]


> @@ -6157,18 +6169,31 @@ static int select_idle_cpu(struct task_struct *p, struct sched_domain *sd, int t
>  	}
> 
>  	for_each_cpu_wrap(cpu, cpus, target) {
> -		if (!--nr)
> -			return -1;
> -		if (available_idle_cpu(cpu) || sched_idle_cpu(cpu))
> -			break;
> +		if (smt) {
> +			i = select_idle_core(p, cpu, cpus, &idle_cpu);
> +			if ((unsigned int)i < nr_cpumask_bits)
> +				return i;
> +
> +		} else {
> +			if (!--nr)
> +				return -1;
> +			i = __select_idle_cpu(cpu);
> +			if ((unsigned int)i < nr_cpumask_bits) {
> +				idle_cpu = i;
> +				break;
> +			}
> +		}
>  	}
> 
> -	if (sched_feat(SIS_PROP)) {
> +	if (smt)
> +		set_idle_cores(this, false);

Shouldn't we set_idle_cores(false) only if this was the last idle
core in the LLC ? 

--
Thanks and Regards
gautham.