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

[Valentin Schneider]

On 30/10/2019 17:24, Dietmar Eggemann wrote:
> On 30.10.19 15:39, Phil Auld wrote:
>> Hi Vincent,
>>
>> On Mon, Oct 28, 2019 at 02:03:15PM +0100 Vincent Guittot wrote:
> 
> [...]
> 
>>>> When you say slow versus fast wakeup paths what do you mean? I'm still
>>>> learning my way around all this code.
>>>
>>> When task wakes up, we can decide to
>>> - speedup the wakeup and shorten the list of cpus and compare only
>>> prev_cpu vs this_cpu (in fact the group of cpu that share their
>>> respective LLC). That's the fast wakeup path that is used most of the
>>> time during a wakeup
>>> - or start to find the idlest CPU of the system and scan all domains.
>>> That's the slow path that is used for new tasks or when a task wakes
>>> up a lot of other tasks at the same time
> 
> [...]
> 
> Is the latter related to wake_wide()? If yes, is the SD_BALANCE_WAKE
> flag set on the sched domains on your machines? IMHO, otherwise those
> wakeups are not forced into the slowpath (if (unlikely(sd))?
> 
> I had this discussion the other day with Valentin S. on #sched and we
> were not sure how SD_BALANCE_WAKE is set on sched domains on
> !SD_ASYM_CPUCAPACITY systems.
> 

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.