Re: [PATCH] sched, fair: Allow a small degree of load imbalance between SD_NUMA domains v2

[Valentin Schneider <valentin.schneider@arm.com>]

On 20/12/2019 14:22, Mel Gorman wrote:
>> Now, I have to say I'm not sold on the idle_cpus thing, I'd much rather use
>> the number of runnable tasks. We are setting up a threshold for how far we
>> are willing to ignore imbalances; if we have overloaded CPUs we *really*
>> should try to solve this. Number of tasks is the safer option IMO: when we
>> do have one task per CPU, it'll be the same as if we had used idle_cpus, and
>> when we don't have one task per CPU we'll load-balance more often that we
>> would have with idle_cpus.
>>
> 
> I couldn't convince myself to really push back hard on the sum_nr_runnable
> versus idle_cpus.  If the local group has spare capacity and the busiest
> group has multiple tasks stacked on CPUs then it's most likely due to
> CPU affinity.

Not necessarily, for instance wakeup balancing (select_idle_sibling()) could
end up packing stuff within a node if said node spans more than one LLC
domain, which IIRC is the case on some AMD chips.

Or, still with the same LLC < node topology, you could start with the node
being completely utilized, then some tasks on some LLC domains terminate but
there's an LLC that still has a bunch of tasks running, and then you're left
with an imbalance between LLC domains that the wakeup balance cannot solve.

> In that case, there is no guarantee tasks can move to the
> local group either. In that case, the difference between sum_nr_running
> and idle_cpus is almost moot.  There may be common use cases where the
> distinction really matters but right now, I'm at the point where I think
> such a change could be a separate patch with the use case included and
> supporting data on why it must be sum_nr_running.  Right now, I feel it's
> mostly a cosmetic issue given the context and intent of the patch.
> 

Let me spin it this way: do we need to push this ignoring of the imbalance
as far as possible, or are we okay with it only happening when there's just a
few tasks running? The latter is achieved with sum_nr_running and is the
safer option IMO.