Re: [RFC PATCH v4 07/28] sched: Add helper function to decide whether to allow cache aware scheduling

[Chen, Yu C]

On 10/1/2025 9:17 PM, Peter Zijlstra wrote:
> On Sat, Aug 09, 2025 at 01:03:10PM +0800, Chen Yu wrote:
> > From: Tim Chen <tim.c.chen@xxxxxxxxxxxxxxx>
> >
> > Cache-aware scheduling is designed to aggregate threads into their
> > preferred LLC, either via the task wake up path or the load balancing
> > path. One side effect is that when the preferred LLC is saturated,
> > more threads will continue to be stacked on it, degrading the workload's
> > latency. A strategy is needed to prevent this aggregation from going too
> > far such that the preferred LLC is too overloaded.
>
> So one of the ideas was to extend the preferred llc number to a mask.
> Update the preferred mask with (nr_threads / llc_size) bits, indicating
> the that many top llc as sorted by occupancy.

Having more than one preferred LLC helps prevent aggregation from going
too far on a single preferred LLC.

One question would be: if one LLC cannot hold all the threads of a process,
does a second preferred LLC help in this use case? Currently, this patch
gives up task aggregation and falls back to legacy load balancing if the
preferred LLC is overloaded. If we place threads across two preferred LLCs,
these threads might encounter cross-LLC latency anyway - so we may as 
well let
legacy load balancing spread them out IMO.

Another issue that Patch 7 tries to address is avoiding task
bouncing between preferred LLCs and non-preferred LLCs. If we
introduce a preferred LLC priority list, logic to prevent task
bouncing between different preferred LLCs might be needed in
load balancing, which could become complicated. Currently, we
mainly implement cache-aware scheduling in load balancing rather
than during task wakeup, because the wakeup path conflicts with
the load balance path and causes task migration bouncing.


thanks,
Chenyu