Re: [PATCH] irq_balance does not make sense with HT but single physical CPU

[Kai Bankett <kbankett@aol.com>]

Zwane Mwaikambo wrote:

> On Wed, 26 Nov 2003, Kai Bankett wrote:
> 
>> this patch should disable irq_balance threat in case of only one
>> installed physical cpu thats running in HyperThreading-mode (so reported
>> as 2 cpus).
>> I think it should make no sense to run irq_blanance in that special case
>> - please correct me if i´m wrong.
> 
> 
> +#ifdef CONFIG_X86_HT
> +   /* On Hyper-Threading CPUs - if only one physical installed
> +      balance does not make sense */
> +   if (cpu_has_ht && smp_num_siblings == 2 && num_online_cpus() == 2) {
> +       irqbalance_disabled = 1;
> +       return 0;
> +   }
> +#endif
> 
> Further down, i believe the following would have the same effect;
> 
>    /*
>     * Enable physical balance only if more than 1 physical processor
>     * is present
>     */
>    if (smp_num_siblings > 1 && !cpus_empty(tmp))
>        physical_balance = 1;
> 
> 
> tmp = cpu_online_map >> 2;
> 
> so we drop the first two processors (logical or physical) and only enable
> physical balance if there are still processors present in the map. Or are
> you observing something else?
> 

Ok - inserted an printk(smp_num_siblings) to have a look into 
smp_num_siblings at that point.

It says : smp_num_siblings = 2

But anyways if physical_balance is set to 1 that won´t prevent anything 
from running through/sleeping in the kernel_thread-loop.
The kernel_thread(balance_irq ...) later on will be started/will run not 
matter what physical_balance says.

Do there exist any cases where smp_siblings are created without 
HyperThreading ? (As far as I remember it´s only incremented/used on 
i386 hyperthreaded architectures - but not 100% sure)

-> At least the if has to look like :

...
if (smp_num_siblings > 2 && !cpus_empty(tm))
     physical_balance = 1;
...

(if - like in my case - smp_num_siblings == 2 on a single P4 system)

That one still does not solve the case in which the kernel_thread is not 
needed and only eats resources.
Of course - maybe the whole thing can be merged together. Not sure about 
that. But source will become more complex in that case - k.i.s.s. may be 
the better approach.

Thanks,

Kai

-- 
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Kai Bankett
Network Engineering

AOL Deutschland GmbH & Co. KG

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