Re: [PATCH RFC] sched_ext: Choose prev_cpu if idle and cache affine without WF_SYNC

[Libo Chen]

On 3/17/25 10:08, Tejun Heo wrote:
> Hello, Joel.
> 
> On Mon, Mar 17, 2025 at 04:28:02AM -0400, Joel Fernandes wrote:
>> Consider that the previous CPU is cache affined to the waker's CPU and
>> is idle. Currently, scx's default select function only selects the
>> previous CPU in this case if WF_SYNC request is also made to wakeup on the
>> waker's CPU.
>>
>> This means, without WF_SYNC, the previous CPU being cache affined to the
>> waker and is idle is not considered. This seems extreme. WF_SYNC is not
>> normally passed to the wakeup path outside of some IPC drivers but it is
>> very possible that the task is cache hot on previous CPU and shares
>> cache with the waker CPU. Lets avoid too many migrations and select the
>> previous CPU in such cases.
> 
> Hmm.. if !WF_SYNC:
> 
> 1. If smt, if prev_cpu's core is idle, pick it. If not, try to pick an idle
>    core in widening scopes.
> 
> 2. If no idle core is foudn, pick prev_cpu if idle. If not, search for an
>    idle CPU in widening scopes.
> 
> So, it is considering prev_cpu, right? I think it's preferring idle core a
> bit too much - it probably doesn't make sense to cross the NUMA boundary if
> there is an idle CPU in this node, at least.
> 
Hi Tejun,

Just as Peter said, this is whole wake affinity thing is highly workload
dependent. We have seen cases where even if there are idle cores in the
prev node, it's still beneficial to cross the NUMA boundary. Will it make
more sense to have the BPF scheduler implement/alter some of logic here so
it can fit to different workload?
 
> Isn't the cpus_share_cache() code block mostly about not doing
> waker-affining if prev_cpu of the wakee is close enough and idle, so
> waker-affining is likely to be worse?
> 
> Thanks.
>