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

[Joel Fernandes]

On 3/17/2025 6:30 PM, Andrea Righi wrote:
> On Mon, Mar 17, 2025 at 07:08:15AM -1000, 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.
>
> Yeah, we should probably be a bit more conservative by default and avoid
> jumping across nodes if there are still idle CPUs within the node.
>

Agreed. So maybe we check for fully idle cores *within the node* first, before
preferring idle SMTs *within the node* ? And then, as next step go looking at
other nodes. Would that be a reasonable middle ground?

> With the new scx_bpf_select_cpu_and() API [1] it'll be easier to enforce
> that while still using the built-in idle policy (since we can specify idle
> flags), but that doesn't preclude adjusting the default policy anyway, if
> it makes more sense.

Aren't you deprecating the usage of the default select function? If we are going
to be adjusting its behavior like my patch is doing, then we should probably not
also deprecate it.

thanks,

 - Joel