Re: [PATCH v4 00/10] sched/fair: rework the CFS load balance

[Valentin Schneider]

On 30/10/2019 18:19, Phil Auld wrote:
>> Well from the code nobody but us (asymmetric capacity systems) set
>> SD_BALANCE_WAKE. I was however curious if there were some folks who set it
>> with out of tree code for some reason.
>>
>> As Dietmar said, not having SD_BALANCE_WAKE means you'll never go through
>> the slow path on wakeups, because there is no domain with SD_BALANCE_WAKE for
>> the domain loop to find. Depending on your topology you most likely will
>> go through it on fork or exec though.
>>
>> IOW wake_wide() is not really widening the wakeup scan on wakeups using
>> mainline topology code (disregarding asymmetric capacity systems), which
>> sounds a bit... off.
> 
> Thanks. It's not currently set. I'll set it and re-run to see if it makes
> a difference. 
> 

Note that it might do more harm than good, it's not set in the default
topology because it's too aggressive, see 

  182a85f8a119 ("sched: Disable wakeup balancing")

> 
> However, I'm not sure why it would be making a difference for only the cgroup
> case. If this is causing issues I'd expect it to effect both runs. 
> 
> In general I think these threads want to wake up the last cpu they were on.
> And given there are fewer cpu bound tasks that CPUs that wake cpu should,
> more often than not, be idle. 
> 
> 
> Cheers,
> Phil
> 
> 
>