linux-kernel - Re: [PATCH v4 03/10] sched/topology: Provide cfs_overload

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Date:   Fri, 7 Dec 2018 17:35:37 -0500
From:   Steven Sistare <steven.sistare@...cle.com>
To:     Valentin Schneider <valentin.schneider@....com>, mingo@...hat.com,
        peterz@...radead.org
Cc:     subhra.mazumdar@...cle.com, dhaval.giani@...cle.com,
        daniel.m.jordan@...cle.com, pavel.tatashin@...rosoft.com,
        matt@...eblueprint.co.uk, umgwanakikbuti@...il.com,
        riel@...hat.com, jbacik@...com, juri.lelli@...hat.com,
        vincent.guittot@...aro.org, quentin.perret@....com,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4 03/10] sched/topology: Provide cfs_overload_cpus bitmap

On 12/7/2018 3:20 PM, Valentin Schneider wrote:
> Hi Steve,
> 
> On 06/12/2018 21:28, Steve Sistare wrote:
> [...]
>> @@ -1621,7 +1626,22 @@ static void __sdt_free(const struct cpumask *cpu_map)
>>  
>>  static int sd_llc_alloc(struct sched_domain *sd)
>>  {
>> -	/* Allocate sd->shared data here. Empty for now. */
>> +	struct sched_domain_shared *sds = sd->shared;
>> +	struct cpumask *span = sched_domain_span(sd);
>> +	int nid = cpu_to_node(cpumask_first(span));
>> +	int flags = __GFP_ZERO | GFP_KERNEL;
>> +	struct sparsemask *mask;
>> +
>> +	/*
>> +	 * Allocate the bitmap if not already allocated.  This is called for
>> +	 * every CPU in the LLC but only allocates once per sd_llc_shared.
>> +	 */
>> +	if (!sds->cfs_overload_cpus) {
>> +		mask = sparsemask_alloc_node(nr_cpu_ids, 3, flags, nid);
>                                              ^^^^^^^^^^ ^^^
>                                                  (1)    (2)
> 
> (1): Is this necessary? Wouldn't cpumask_weight(span) suffice?

weight does not work because the ids are not consecutive.  As a future 
optimization I have a patch that computes a local CPU id within the LLC
and uses that as the bitmap index, and that uses weight.  For this first
series I am keeping things simple.

> (2): Could this get a name somewhere? In v3 this was
> SPARSEMASK_DENSITY_DEFAULT, I think it'd make sense to have it in
> sched/sparsemask.c

Darn, suspected someone would like this identifier after I deleted it.

- Steve

>> +		if (!mask)
>> +			return 1;
>> +		sds->cfs_overload_cpus = mask;
>> +	}
>>  
>>  	return 0;
>>  }
>> @@ -1633,7 +1653,8 @@ static void sd_llc_free(struct sched_domain *sd)
>>  	if (!sds)
>>  		return;
>>  
>> -	/* Free data here. Empty for now. */
>> +	sparsemask_free(sds->cfs_overload_cpus);
>> +	sds->cfs_overload_cpus = NULL;
>>  }
>>  
>>  static int sd_llc_alloc_all(const struct cpumask *cpu_map, struct s_data *d)
>>