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SubjectRe: [PATCH v8 4/6] cpuset: Make generate_sched_domains() recognize isolated_cpus
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On 05/23/2018 01:34 PM, Patrick Bellasi wrote:
> Hi Waiman,
>
> On 17-May 16:55, Waiman Long wrote:
>
> [...]
>
>> @@ -672,13 +672,14 @@ static int generate_sched_domains(cpumask_var_t **domains,
>> int ndoms = 0; /* number of sched domains in result */
>> int nslot; /* next empty doms[] struct cpumask slot */
>> struct cgroup_subsys_state *pos_css;
>> + bool root_load_balance = is_sched_load_balance(&top_cpuset);
>>
>> doms = NULL;
>> dattr = NULL;
>> csa = NULL;
>>
>> /* Special case for the 99% of systems with one, full, sched domain */
>> - if (is_sched_load_balance(&top_cpuset)) {
>> + if (root_load_balance && !top_cpuset.isolation_count) {
> Perhaps I'm missing something but, it seems to me that, when the two
> conditions above are true, then we are going to destroy and rebuild
> the exact same scheduling domains.
>
> IOW, on 99% of systems where:
>
> is_sched_load_balance(&top_cpuset)
> top_cpuset.isolation_count = 0
>
> since boot time and forever, then every time we update a value for
> cpuset.cpus we keep rebuilding the same SDs.
>
> It's not strictly related to this patch, the same already happens in
> mainline based just on the first condition, but since you are extending
> that optimization, perhaps you can tell me where I'm possibly wrong or
> which cases I'm not considering.
>
> I'm interested mainly because on Android systems those conditions
> are always true and we see SDs rebuilds every time we write
> something in cpuset.cpus, which ultimately accounts for almost all the
> 6-7[ms] time required for the write to return, depending on the CPU
> frequency.
>
> Cheers Patrick
>
Yes, that is true. I will look into how to further optimize this. Thanks
for the suggestion.

-Longman

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