Re: [PATCH v2 05/23] sched/cache: Assign preferred LLC ID to processes

[Tim Chen]

On Tue, 2025-12-09 at 13:11 +0100, Peter Zijlstra wrote:
> On Wed, Dec 03, 2025 at 03:07:24PM -0800, Tim Chen wrote:
> > With cache-aware scheduling enabled, each task is assigned a
> > preferred LLC ID. This allows quick identification of the LLC domain
> > where the task prefers to run, similar to numa_preferred_nid in
> > NUMA balancing.
> 
> > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> > index 0a3918269906..10cec83f65d5 100644
> > --- a/kernel/sched/fair.c
> > +++ b/kernel/sched/fair.c
> > @@ -1300,6 +1300,7 @@ void account_mm_sched(struct rq *rq, struct task_struct *p, s64 delta_exec)
> >  	struct mm_struct *mm = p->mm;
> >  	struct mm_sched *pcpu_sched;
> >  	unsigned long epoch;
> > +	int mm_sched_llc = -1;
> >  
> >  	if (!sched_cache_enabled())
> >  		return;
> > @@ -1330,6 +1331,23 @@ void account_mm_sched(struct rq *rq, struct task_struct *p, s64 delta_exec)
> >  		if (mm->mm_sched_cpu != -1)
> >  			mm->mm_sched_cpu = -1;
> >  	}
> > +
> > +	if (mm->mm_sched_cpu != -1) {
> > +		mm_sched_llc = llc_id(mm->mm_sched_cpu);
> > +
> > +#ifdef CONFIG_NUMA_BALANCING
> > +		/*
> > +		 * Don't assign preferred LLC if it
> > +		 * conflicts with NUMA balancing.
> > +		 */
> > +		if (p->numa_preferred_nid >= 0 &&
> > +		    cpu_to_node(mm->mm_sched_cpu) != p->numa_preferred_nid)
> > +			mm_sched_llc = -1;
> > +#endif
> > +	}
> > +
> > +	if (p->preferred_llc != mm_sched_llc)
> > +		p->preferred_llc = mm_sched_llc;
> >  }
> 
> This can of course still happen when sched_setnuma() gets called. I'm
> thinking it is not much of an issue because we expect this thing to get
> called fairly regularly -- at a higher rate than sched_setnuma() at
> least -- and thus the conflict only exists for a short period of time?
> 
> If so, that would make for a good comment.

Sure.  Will do.

> 
> Additionally, we could of course search for the busiest LLC inside the
> node, instead of setting -1. Again, that could live as a comment for
> future work.

A potential issue with scanning only the preferred node of a single task 
is that tasks within the same process may have different preferred nodes.
For example, task 1 may prefer one node, while tasks 2…n prefer another. 
If we base the busiest-LLC scan solely on task 1’s preference, we may 
ignore the preferences of tasks 2…n. Consequently, constraining 
the preferred LLC according to task 1’s node can interfere with 
NUMA balancing for the rest of the process. This problem does not 
arise when all tasks being aggregated belong to the same numa_group, 
since they will share the same preferred node.

Tim