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

[Steven Sistare <steven.sistare@oracle.com>]

On 2/2/2018 2:59 PM, Peter Zijlstra wrote:
> On Fri, Feb 02, 2018 at 11:53:40AM -0500, Steven Sistare wrote:
>> On 2/1/2018 7:33 AM, Peter Zijlstra wrote:
>>> On Mon, Jan 29, 2018 at 03:31:02PM -0800, subhra mazumdar wrote:
>>>> +	rcu_read_lock();
>>>> +	sd = rcu_dereference(per_cpu(sd_llc, this_cpu));
>>>> +	if (util) {
>>>> +		for_each_lower_domain(sd) {
>>>> +			if (sd->level == 0)
>>>> +				break;
>>>
>>> afaict you really only need this for the core, and here you're assuming
>>> everything below the LLC is cores. Would it not be much clearer if you
>>> introduce sd_core.
>>>
>>> As is, for_each_lower_domain includes the starting domain, sd->group
>>> then is the first core group for this cpu. But then you continue to the
>>> smt domain (on Intel, on other architectures there could be a cluster
>>> domain in between) and then you bail using that sd->level == 0 hack
>>> because otherwise things would go *bang*.
>>
>> Hi Peter,
>>
>> The code here and in smt_balance intentionally visits each level between
>> the llc and smt, including core-cluster on architectures that define it.
>> smt_balance thus has the chance to randomly pick a better cluster,
>> and then within that cluster randomly pick a better core.  It makes sense,
>> as resources are shared within a cluster, and choosing a less loaded cluster
>> should give better performance.  As you suggest in a few other places,
>> it would be nice to see performance results for this case.  We have
>> SPARC processors with core clusters.
>>
> 
> But then you get that atomic crud to contend on the cluster level, which
> is even worse than it contending on the core level.

True, but it can still be a net win if we make better scheduling decisions.
A saving grace is that the atomic counter is only updated if the cpu
makes a transition from idle to busy or vice versa.

We need data for this type of system, showing improvements for normal
workloads, and showing little downside for a high context switch rate
torture test.  

- Steve