Re: [PATCH 54/60] xen/sched: add minimalistic idle scheduler for free cpus

[Juergen Gross <jgross@suse.com>]

On 28/05/2019 13:47, Jan Beulich wrote:
>>>> On 28.05.19 at 12:33, <jgross@suse.com> wrote:
>> Instead of having a full blown scheduler running for the free cpus
>> add a very minimalistic scheduler for that purpose only ever scheduling
>> the related idle vcpu. This has the big advantage of not needing any
>> per-cpu, per-domain or per-scheduling unit data for free cpus and in
>> turn simplifying moving cpus to and from cpupools a lot.
> 
> And the null scheduler is not minimalistic enough?

The main disadvantage of the null scheduler are the need for private
data (which has to be allocated/freed on scheduler switch), its not
yet perfect stability, and its "complexity" (e.g. 900 lines vs. 50).

> 
>> --- a/xen/arch/arm/smpboot.c
>> +++ b/xen/arch/arm/smpboot.c
>> @@ -350,8 +350,6 @@ void start_secondary(unsigned long boot_phys_offset,
>>  
>>      setup_cpu_sibling_map(cpuid);
>>  
>> -    scheduler_percpu_init(cpuid);
>> -
>>      /* Run local notifiers */
>>      notify_cpu_starting(cpuid);
>>      /*
>> --- a/xen/arch/x86/smpboot.c
>> +++ b/xen/arch/x86/smpboot.c
>> @@ -382,8 +382,6 @@ void start_secondary(void *unused)
>>  
>>      set_cpu_sibling_map(cpu);
>>  
>> -    scheduler_percpu_init(cpu);
>> -
>>      init_percpu_time();
>>  
>>      setup_secondary_APIC_clock();
> 
> Seeing you revert here what an earlier patch in the series has added,
> I don't suppose re-ordering could avoid this code churn?

This would require another major shuffling of patches. I'd like to avoid
that.


Juergen

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