Re: [RESEND RFC PATCH V3] sched: Improve scalability of select_idle_sibling using SMT balance

[Steven Sistare]

On 2/2/2018 12:17 PM, Peter Zijlstra wrote:
> On Fri, Feb 02, 2018 at 11:53:40AM -0500, Steven Sistare wrote:
>>>> +static int select_idle_smt(struct task_struct *p, struct sched_group *sg)
>>>>  {
>>>> +	int i, rand_index, rand_cpu;
>>>> +	int this_cpu = smp_processor_id();
>>>>  
>>>> +	rand_index = CPU_PSEUDO_RANDOM(this_cpu) % sg->group_weight;
>>>> +	rand_cpu = sg->cp_array[rand_index];
>>>
>>> Right, so yuck.. I know why you need that, but that extra array and
>>> dereference is the reason I never went there.
>>>
>>> How much difference does it really make vs the 'normal' wrapping search
>>> from last CPU ?
>>>
>>> This really should be a separate patch with separate performance numbers
>>> on.
>>
>> For the benefit of other readers, if we always search and choose starting from
>> the first CPU in a core, then later searches will often need to traverse the first
>> N busy CPU's to find the first idle CPU.  Choosing a random starting point avoids
>> such bias.  It is probably a win for processors with 4 to 8 CPUs per core, and
>> a slight but hopefully negligible loss for 2 CPUs per core, and I agree we need
>> to see performance data for this as a separate patch to decide.  We have SPARC
>> systems with 8 CPUs per core.
> 
> Which is why the current code already doesn't start from the first cpu
> in the mask. We start at whatever CPU the task ran last on, which is
> effectively 'random' if the system is busy.
> 
> So how is a per-cpu rotor better than that?

The current code is:
        for_each_cpu(cpu, cpu_smt_mask(target)) {

For an 8-cpu/core processor, 8 values of target map to the same cpu_smt_mask.
8 different tasks will traverse the mask in the same order.

- Steve