linux-kernel - Re: [PATCH 1/4] sched_ext: Introduce per-NUMA idle cpumasks

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Message-ID: <Z1ejJSBes62otQ0k@yury-ThinkPad>
Date: Mon, 9 Dec 2024 18:10:45 -0800
From: Yury Norov <yury.norov@...il.com>
To: Andrea Righi <arighi@...dia.com>
Cc: Tejun Heo <tj@...nel.org>, David Vernet <void@...ifault.com>,
	Changwoo Min <changwoo@...lia.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/4] sched_ext: Introduce per-NUMA idle cpumasks

On Tue, Dec 10, 2024 at 01:14:43AM +0100, Andrea Righi wrote:
> > And altogether this should look like:
> > 
> >  int scx_pick_idle_cpu_from_hop(struct cpumask *hop_cpus, struct cpumask *cpus_allowed)
> >  {
> >          int node, cpu, random_cpu;
> > 
> >          do {
> > 
> >                  /* Pick a 'random' CPU in the hop */
> >                  random_cpu = cpumask_any_and_distribute(hop_cpus, cpus_allowed);
> >                  if (random_cpu >= nr_cpu_ids)
> >                          continue;
> > 
> >                  node = cpu_to_node(random_cpu);
> > 
> >                  /* Find an idle CPU in the same node */
> >                  cpu = scx_pick_idle_cpu_from_node(node, cpus_allowed, flags);
> >                  if (cpu >= 0)
> >                          break;
> > 
> >                  /* No luck? Try other nodes */
> >          } while (cpumask_andnot(hop_cpus, hop_cpus, cpumask_of_node(node)));
> > 
> >          return cpu;
> >  }
> > 
> >  static s32 scx_pick_idle_cpu(const struct cpumask *cpus_allowed, s32 prev_cpu, u64 flags)
> >  {
> >         const struct cpumask *next, *prev = cpu_none_mask;
> >         int prev_node = cpu_to_node(prev_cpu);
> >  ...
> > 	for_each_numa_hop_mask(next, prev_node) {
> >                 cpumask_andnot(hop_cpus, next, prev);
> >                 cpu = scx_pick_idle_cpu_from_hop(hop_cpus, cpus_allowed);
> >                 prev = next;
> >         }
> >  ...
> >  }
> > 
> > Not tested, but should work.
> 
> Makes sense to me, I'll do some testing with this.

One thing you can do to optimize it is introducing a function that
pulls nodes from the hop_cpus:

 void sched_get_hop_nodes(nodemask_t *hop_nodes, const struct cpumask *hop_cpus)
 {
        int cpu;

        for_each_cpu(cpu, hop_cpus) {
               node_set(cpu_to_node(cpu);, hop_nodes);
               cpu = cpumask_next_zero(cpu, cpumask_of_node(node)
        }
 }

This should be O(N), but it will let you to avoid O(N*M) in the loop
condition inside scx_pick_idle_cpu_from_hop():

 int scx_pick_idle_cpu_from_hop(nodemask_t *hop_nodes, struct cpumask *cpus_allowed)
 {
        int node, idle_cpu, random_cpu;

        for_each_node_mask(node, &hop_nodes) {
                /* Pick a 'random' CPU in the node */
                random_cpu = cpumask_any_and_distribute(cpumask_of_node(node), cpus_allowed);
                if (random_cpu >= nr_cpu_ids)
                        continue;

                /* Find an idle CPU in the same node */
                idle_cpu = scx_pick_idle_cpu_from_node(node, cpus_allowed, flags);
                if (idle_cpu >= 0)
                        break;

        }

        return cpu;
 }

And at this point I'd also compare the above with non-randomized
version:

 static s32 scx_pick_idle_cpu(const struct cpumask *cpus_allowed, s32 prev_cpu, u64 flags)
 {
        const struct cpumask *next, *prev = cpu_none_mask;
        int prev_node = cpu_to_node(prev_cpu);
        nodemask_t hop_nodes;
 ...
	for_each_numa_hop_mask(next, prev_node) {
                if (!cpumask_and_andnot(hop_cpus, next, cpus_allow, prev))
                        goto cont;

                sched_get_hop_nodes(hop_nodes, hop_cpus);
                for_each_node_mask(node, hop_nodes) {
                        cpu = scx_pick_idle_cpu_from_node(node, cpus_allowed, flags);
                        if (cpu >= 0)
                                goto found;
                }

        cont:
                prev = next;
        }
 ...
 }

Don't know how it works, but it looks really good.

Thanks,
Yury