Re: [PATCH 01/19] sched/fair: Add infrastructure for cache-aware load balancing

[Chen, Yu C]

On 10/15/2025 7:54 PM, Peter Zijlstra wrote:
> On Wed, Oct 15, 2025 at 12:42:48AM +0530, Madadi Vineeth Reddy wrote:
> > > +static void get_scan_cpumasks(cpumask_var_t cpus, int cache_cpu,
> > > +			      int pref_nid, int curr_cpu)
> > > +{
> > > +#ifdef CONFIG_NUMA_BALANCING
> > > +	/* First honor the task's preferred node. */
> > > +	if (pref_nid != NUMA_NO_NODE)
> > > +		cpumask_or(cpus, cpus, cpumask_of_node(pref_nid));
> > > +#endif
> > > +
> > > +	/* Next honor the task's cache CPU if it is not included. */
> > > +	if (cache_cpu != -1 && !cpumask_test_cpu(cache_cpu, cpus))
> > > +		cpumask_or(cpus, cpus,
> > > +			   cpumask_of_node(cpu_to_node(cache_cpu)));
> > > +
> > > +	/*
> > > +	 * Lastly make sure that the task's current running node is
> > > +	 * considered.
> > > +	 */
> > > +	if (!cpumask_test_cpu(curr_cpu, cpus))
> > > +		cpumask_or(cpus, cpus, cpumask_of_node(cpu_to_node(curr_cpu)));
> > > +}
> > > +
> > > +static void __no_profile task_cache_work(struct callback_head *work)
> > > +{
> > > +	struct task_struct *p = current;
> > > +	struct mm_struct *mm = p->mm;
> > > +	unsigned long m_a_occ = 0;
> > > +	unsigned long curr_m_a_occ = 0;
> > > +	int cpu, m_a_cpu = -1, cache_cpu,
> > > +	    pref_nid = NUMA_NO_NODE, curr_cpu;
> > > +	cpumask_var_t cpus;
> > > +
> > > +	WARN_ON_ONCE(work != &p->cache_work);
> > > +
> > > +	work->next = work;
> > > +
> > > +	if (p->flags & PF_EXITING)
> > > +		return;
> > > +
> > > +	if (!zalloc_cpumask_var(&cpus, GFP_KERNEL))
> > > +		return;
> > > +
> > > +	curr_cpu = task_cpu(p);
> > > +	cache_cpu = mm->mm_sched_cpu;
> > > +#ifdef CONFIG_NUMA_BALANCING
> > > +	if (static_branch_likely(&sched_numa_balancing))
> > > +		pref_nid = p->numa_preferred_nid;
> > > +#endif
> > > +
> > > +	scoped_guard (cpus_read_lock) {
> > > +		get_scan_cpumasks(cpus, cache_cpu,
> > > +				  pref_nid, curr_cpu);
> > > +
> >
> > IIUC, `get_scan_cpumasks` ORs together the preferred NUMA node, cache CPU's node,
> > and current CPU's node. This could result in scanning multiple nodes, not preferring
> > the NUMA preferred node.
>
> So this used to be online_mask, and is now magically changed to this
> more limited mask.
>
> Could you split this change out and have it have a justification?

OK, we will do this and provide an explanation.

thanks,
Chenyu