Re: [PATCH 1/3] powerpc/smp: Reintroduce cpu_core_mask

* Gautham R Shenoy [off-list ref] [2021-04-15 22:41:34]:

Hi Srikar,

Thanks for taking a look.

quoted

@@ -1485,12 +1486,36 @@ static void add_cpu_to_masks(int cpu)
 	add_cpu_to_smallcore_masks(cpu);

 	/* In CPU-hotplug path, hence use GFP_ATOMIC */
-	alloc_cpumask_var_node(&mask, GFP_ATOMIC, cpu_to_node(cpu));
+	ret = alloc_cpumask_var_node(&mask, GFP_ATOMIC, cpu_to_node(cpu));
 	update_mask_by_l2(cpu, &mask);

 	if (has_coregroup_support())
 		update_coregroup_mask(cpu, &mask);

+	if (chip_id == -1 || !ret) {
+		cpumask_copy(per_cpu(cpu_core_map, cpu), cpu_cpu_mask(cpu));
+		goto out;
+	}
+
+	if (shared_caches)
+		submask_fn = cpu_l2_cache_mask;
+
+	/* Update core_mask with all the CPUs that are part of submask */
+	or_cpumasks_related(cpu, cpu, submask_fn, cpu_core_mask);

If coregroups exist, we can add the cpus of the coregroup to the
cpu_core_mask thereby reducing the scope of the for_each_cpu() search
below. This will still cut down the time on Baremetal systems
supporting coregroups.

Yes, once we upstream coregroup support to Baremetal, we should look
at adding it. Also do note, number of CPUs we support for Baremetal is
comparatively lower than in PowerVM + QEMU. And more importantly the
number of cores per coregroup is also very low. So the optimization
may not yield too much of a benefit.

Its only in the QEMU case, where we end up having too many cores in
the same chip, where we see a drastic increase in the boot-up time.

-- 
Thanks and Regards
Srikar Dronamraju