Messages in this thread | | | From | Valentin Schneider <> | Subject | Re: [RFC PATCH v1 1/1] sched/fair: select idle cpu from idle cpumask in sched domain | Date | Mon, 14 Sep 2020 12:26:47 +0100 |
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On 14/09/20 12:08, Qais Yousef wrote: > On 09/14/20 11:31, Valentin Schneider wrote: >> >> On 12/09/20 00:04, Li, Aubrey wrote: >> >>> +++ b/include/linux/sched/topology.h >> >>> @@ -65,8 +65,21 @@ struct sched_domain_shared { >> >>> atomic_t ref; >> >>> atomic_t nr_busy_cpus; >> >>> int has_idle_cores; >> >>> + /* >> >>> + * Span of all idle CPUs in this domain. >> >>> + * >> >>> + * NOTE: this field is variable length. (Allocated dynamically >> >>> + * by attaching extra space to the end of the structure, >> >>> + * depending on how many CPUs the kernel has booted up with) >> >>> + */ >> >>> + unsigned long idle_cpus_span[]; >> >> >> >> Can't you use cpumask_var_t and zalloc_cpumask_var() instead? >> > >> > I can use the existing free code. Do we have a problem of this? >> > >> >> Nah, flexible array members are the preferred approach here; this also > > Is this your opinion or a rule written somewhere I missed?
I don't think there's a written rule, but AIUI it is preferred by at least Peter:
https://lore.kernel.org/linux-pm/20180612125930.GP12217@hirez.programming.kicks-ass.net/ https://lore.kernel.org/lkml/20180619110734.GO2458@hirez.programming.kicks-ass.net/
And my opinion is that, if you can, having fewer separate allocation is better.
> >> means we don't let CONFIG_CPUMASK_OFFSTACK dictate where this gets >> allocated. >> >> See struct numa_group, struct sched_group, struct sched_domain, struct >> em_perf_domain... > > struct root_domain, struct cpupri_vec, struct generic_pm_domain, > struct irq_common_data.. > > Use cpumask_var_t. > > Both approach look correct to me, so no objection in principle. cpumask_var_t > looks neater IMO and will be necessary once more than one cpumask are required > in a struct. >
You're right in that cpumask_var_t becomes necessary when you need more than one mask. For those that use it despite requiring only one mask (cpupri stuff, struct nohz too), I'm not sure.
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