Re: [PATCH] sched/fair: avoid creating misfits during cache-aware balancing

[Tim Chen]

On Mon, 2026-08-31 at 10:41 +0200, Peter Zijlstra wrote:
> On Tue, Aug 25, 2026 at 10:41:12AM -0700, Tim Chen wrote:
> > Cache-aware load balancing biases tasks toward their preferred LLC. On
> > asymmetric CPU capacity systems (e.g. big.LITTLE) the destination LLC may
> > contain CPUs that are too small to run the task. Pulling the task there
> > turns it into a misfit, trading a cache-locality gain for a capacity loss
> > that's more detrimental to performance.
> > 
> > Guard both cache-aware migration entry points against this:
> > 
> >  - can_migrate_llc_task(): forbid the LLC migration when the task fits its
> >    source CPU but would not fit the destination CPU.
> >  - alb_break_llc(): veto the active balance under the same condition so the
> >    runnable task is not pushed onto a CPU that cannot accommodate it.
> > 
> > Both checks are gated with checks for hybrid processors, so symmetric
> > systems are unaffected. Tasks that already do not fit their source CPU
> > are left to the existing LLC policy, since the move cannot make their
> > fitness worse (this also preserves misfit up-migration to bigger CPUs).
> > 
> > Additionally, if there are misfit tasks found in the load balancing
> > classification phase, prioritize misfit task migrations
> > over LLC load aggregation on asymmetric systems. A better fitting
> > CPU will boost performance more than better cache locality.
> > 
> > Reviewed-by: Ricardo Neri <ricardo.neri-calderon@xxxxxxxxxxxxxxx>
> > Tested-by: Ricardo Neri <ricardo.neri-calderon@xxxxxxxxxxxxxxx>
> > Reviewed-by: Chen Yu <yu.c.chen@xxxxxxxxx>
> 
> Tim sends patch, Tim adds SoB, yes?

Yeah, I forgot to add my SoB.  Added that in a followed up email.

> 
> Also, we start $subject with capital after subsystem: part.

Sorry about that.  Let me know if you prefer me send an updated
patch with those corrected.

Thanks.

Tim