Re: [PATCH v3 05/22] sched: remove domain iterations in fork/exec/wake

[Alex Shi <alex.shi@intel.com>]

On 01/11/2013 06:07 PM, Morten Rasmussen wrote:
> On Fri, Jan 11, 2013 at 02:46:31AM +0000, Alex Shi wrote:
>> On 01/10/2013 02:21 AM, Morten Rasmussen wrote:
>>>>  		new_cpu = find_idlest_cpu(group, p, cpu);
>>>>> -
>>>>> -		/* Now try balancing at a lower domain level of new_cpu */
>>>>> -		cpu = new_cpu;
>>>>> -		weight = sd->span_weight;
>>>>> -		sd = NULL;
>>>>> -		for_each_domain(cpu, tmp) {
>>>>> -			if (weight <= tmp->span_weight)
>>>>> -				break;
>>>>> -			if (tmp->flags & sd_flag)
>>>>> -				sd = tmp;
>>>>> -		}
>>>>> -		/* while loop will break here if sd == NULL */
>>> I agree that this should be a major optimization. I just can't figure
>>> out why the existing recursive search for an idle cpu switches to the
>>> new cpu near the end and then starts a search for an idle cpu in the new
>>> cpu's domain. Is this to handle some exotic sched domain configurations?
>>> If so, they probably wouldn't work with your optimizations.
>>
>> I did not find odd configuration that asking for old logical.
>>
>> According to Documentation/scheduler/sched-domains.txt, Maybe never.
>> "A domain's span MUST be a superset of it child's span (this restriction
>> could be relaxed if the need arises), and a base domain for CPU i MUST
>> span at least i."  etc. etc.
> 
> The reason for my suspicion is the SD_OVERLAP flag, which has something
> to do overlapping sched domains. I haven't looked into what it does or
> how it works. I'm just wondering if this optimization will affect the
> use of that flag.

I didn't know any machine has this flag, but if just some cpu overlap,
not stay alone without any domain, the patch won't miss eligible cpu.
> 
> Morten
> 
>>
>>
>> -- 
>> Thanks Alex
>>
> 


-- 
Thanks
    Alex