linux-kernel - Re: linux-next: new scheduler messages span: 0-15 (max cpu

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Message-ID: <7f2d2e7c-f3a4-7ffb-9e48-96866d681714@arm.com>
Date:   Mon, 19 Sep 2016 14:58:54 +0100
From:   Dietmar Eggemann <dietmar.eggemann@....com>
To:     Peter Zijlstra <peterz@...radead.org>,
        Christian Borntraeger <borntraeger@...ibm.com>
Cc:     Ingo Molnar <mingo@...nel.org>, Tejun Heo <tj@...nel.org>,
        linux-kernel@...r.kernel.org
Subject: Re: linux-next: new scheduler messages span: 0-15 (max cpu_capacity =
 589) when starting KVM guests

On 19/09/16 14:40, Peter Zijlstra wrote:
> On Mon, Sep 19, 2016 at 03:19:11PM +0200, Christian Borntraeger wrote:
>> Dietmar, Ingo, Tejun,
>>
>> since commit cd92bfd3b8cb0ec2ee825e55a3aee704cd55aea9
>>    sched/core: Store maximum per-CPU capacity in root domain
>>
>> I get tons of messages from the scheduler like
>> [..]
>> span: 0-15 (max cpu_capacity = 589)
>> span: 0-15 (max cpu_capacity = 589)
>> span: 0-15 (max cpu_capacity = 589)
>> span: 0-15 (max cpu_capacity = 589)
>> [..]
>>
> 
> Oh, oops ;-)
> 
> Something like the below ought to cure I think.

Haven't tested it in kvm guests with libvirt env.

This message makes sense for asymmetric compute capacities (ARM
big.LITTLE) for a setup where cpu_capacity = 1024 (a logical cpu w/o
SMT) can't be assumed for the big cpus.

I also tells you that you run in an SMT env. (2 hw threads hence 589)
but this is probably less important.

Guarding it w/ sched_debug_enabled makes sense for this.

> ---
>  kernel/sched/core.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index f5f7b3cdf0be..fdc9e311fd29 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -6990,7 +6990,7 @@ static int build_sched_domains(const struct cpumask *cpu_map,
>  	}
>  	rcu_read_unlock();
>  
> -	if (rq) {
> +	if (rq && sched_debug_enabled) {
>  		pr_info("span: %*pbl (max cpu_capacity = %lu)\n",
>  			cpumask_pr_args(cpu_map), rq->rd->max_cpu_capacity);
>  	}
>