Re: [PATCH net-next V4 1/3] sched/topology: Add NUMA-based CPUs spread API

[Tariq Toukan]

On 8/9/2022 1:02 PM, Valentin Schneider wrote:
> On 08/08/22 17:39, Tariq Toukan wrote:
> > On 8/4/2022 8:28 PM, Valentin Schneider wrote:
> > > On 28/07/22 22:12, Tariq Toukan wrote:
> > > > +static bool sched_cpus_spread_by_distance(int node, u16 *cpus, int ncpus)
> > >                                                          ^^^^^^^^^
> > > That should be a struct *cpumask.
> >
> > With cpumask, we'll lose the order.
>
> Right, I didn't get that from the changelog.

I'll make it clear when re-spinned.

> > > > +{
> > > > +	cpumask_var_t cpumask;
> > > > +	int first, i;
> > > > +
> > > > +	if (!zalloc_cpumask_var(&cpumask, GFP_KERNEL))
> > > > +		return false;
> > > > +
> > > > +	cpumask_copy(cpumask, cpu_online_mask);
> > > > +
> > > > +	first = cpumask_first(cpumask_of_node(node));
> > > > +
> > > > +	for (i = 0; i < ncpus; i++) {
> > > > +		int cpu;
> > > > +
> > > > +		cpu = sched_numa_find_closest(cpumask, first);
> > > > +		if (cpu >= nr_cpu_ids) {
> > > > +			free_cpumask_var(cpumask);
> > > > +			return false;
> > > > +		}
> > > > +		cpus[i] = cpu;
> > > > +		__cpumask_clear_cpu(cpu, cpumask);
> > > > +	}
> > > > +
> > > > +	free_cpumask_var(cpumask);
> > > > +	return true;
> > > > +}
> > >
> > > This will fail if ncpus > nr_cpu_ids, which shouldn't be a problem. It
> > > would make more sense to set *up to* ncpus, the calling code can then
> > > decide if getting fewer than requested is OK or not.
> > >
> > > I also don't get the fallback to cpumask_local_spread(), is that if the
> > > NUMA topology hasn't been initialized yet? It feels like most users of this
> > > would invoke it late enough (i.e. anything after early initcalls) to have
> > > the backing data available.
> >
> > I don't expect this to fail, as we invoke it late enough. Fallback is
> > there just in case, to preserve the old behavior instead of getting
> > totally broken.
>
> Then there shouldn't be a fallback method - the main method is expected to
> work.

I'll drop the fallback then.

> > > Finally, I think iterating only once per NUMA level would make more sense.
> >
> > Agree, although it's just a setup stage.
> > I'll check if it can work for me, based on the reference code below.
> >
> > > I've scribbled something together from those thoughts, see below. This has
> > > just the mlx5 bits touched to show what I mean, but that's just compile
> > > tested.
> >
> > My function returns a *sorted* list of the N closest cpus.
> > That is important. In many cases, drivers do not need all N irqs, but
> > only a portion of it, so it wants to use the closest subset of cpus.
>
> Are there cases where we can't figure this out in advance? From what I grok
> out of the two callsites you patched, all vectors will be used unless some
> error happens, so compressing the CPUs in a single cpumask seemed
> sufficient.

All vectors will be initialized to support the maximum number of traffic 
rings. However, the actual number of traffic rings can be controlled and 
set to a lower number N_actual < N. In this case, we'll be using only 
N_actual instances and we want them to be the first/closest.