Subject: Re: [PATCH v2 7/7]powerpc/powernv: nest pmu cpumask and cpu hotplug support

From: Preeti U Murthy <[email-masked]>
Date: Tue, 16 Jun 2015 11:58:09 +0530

On 06/11/2015 10:47 AM, Madhavan Srinivasan wrote:
> Adds cpumask attribute to be used by each nest pmu since nest
> units are per-chip. Only one cpu (first online cpu) from each node/chip
> is designated to read counters.
> 
> On cpu hotplug, dying cpu is checked to see whether it is one of the
> designated cpus, if yes, next online cpu from the same node/chip is designated
> as new cpu to read counters.
> 
> Cc: Michael Ellerman <[email-masked]>
> Cc: Benjamin Herrenschmidt <[email-masked]>
> Cc: Paul Mackerras <[email-masked]>
> Cc: Anton Blanchard <[email-masked]>
> Cc: Sukadev Bhattiprolu <[email-masked]>
> Cc: Anshuman Khandual <[email-masked]>
> Cc: Stephane Eranian <[email-masked]>
> Cc: Preeti U Murthy <[email-masked]>
> Cc: Ingo Molnar <[email-masked]>
> Cc: Peter Zijlstra <[email-masked]>
> Signed-off-by: Madhavan Srinivasan <[email-masked]>
> ---
> +static void nest_change_cpu_context(int old_cpu, int new_cpu)
> +{
> +	int i;
> +
> +	if (new_cpu >= 0) {
> +		for (i = 0; per_nest_pmu_arr[i] != NULL; i++)
> +			perf_pmu_migrate_context(&per_nest_pmu_arr[i]->pmu,
> +							old_cpu, new_cpu);
> +	}
> +}
> +
> +static void nest_exit_cpu(int cpu)
> +{
> +	int i, nid, target = -1;
> +	const struct cpumask *l_cpumask;
> +	int src_chipid;
> +
> +	/*
> +	 * Check in the designated list for this cpu. Dont bother
> +	 * if not one of them.
> +	 */
> +	if (!cpumask_test_and_clear_cpu(cpu, &cpu_mask_nest_pmu))
> +		return;
> +
> +	/*
> +	 * Now that this cpu is one of the designated,
> +	 * find a new cpu a) which is not dying and

This comment is not right. nest_exit_cpu() is called in the hotplug
path, so another cpu cannot be dying in parallel. Hotplug operations are
done serially. The comment ought to be "a) which is online" instead.

> +	 * b) is in same node/chip.

node is not the same as a chip right ? And you are interested in cpus on
the same chip alone. So shouldn't the above comment be b) in the same chip ?

> +	 */
> +	nid = cpu_to_node(cpu);
> +	src_chipid = topology_physical_package_id(cpu);

What is the relation between a node and a chip ? Can't we have a
function which returns the cpumask of a chip straight away, since that
is what you seem to be more interested in ? You can then simply choose
the next cpu in this cpumask rather than going through the below loop.

> +	l_cpumask = cpumask_of_node(nid);
> +	for_each_cpu(i, l_cpumask) {
> +		if (i == cpu)
> +			continue;
> +		if (src_chipid == topology_physical_package_id(i)) {
> +			target = i;
> +			break;
> +		}
> +	}
> +
> +	/*
> +	 * Update the cpumask with the target cpu and
> +	 * migrate the context if needed
> +	 */
> +	if (target >= 0) {

You already check for target >= 0 here. So you don't need to check for
new_cpu >= 0 in nest_change_cpu_context() above ?

> +		cpumask_set_cpu(target, &cpu_mask_nest_pmu);
> +		nest_change_cpu_context (cpu, target);
> +	}
> +}
> +
> +static void nest_init_cpu(int cpu)
> +{
> +	int i, src_chipid;
> +
> +	/*
> +	 * Search for any existing designated thread from
> +	 * the incoming cpu's node/chip. If found, do nothing.
> +	 */
> +	src_chipid = topology_physical_package_id(cpu);
> +	for_each_cpu(i, &cpu_mask_nest_pmu)
> +		if (src_chipid == topology_physical_package_id(i))
> +			return;
> +
> +	/*
> +	 * Make incoming cpu as a designated thread for
> +	 * this node/chip
> +	 */
> +	cpumask_set_cpu(cpu, &cpu_mask_nest_pmu);

Why can't we simply check if cpu is the first one coming online in the
chip and designate it as the cpu_mask_nest_pmu for that chip ? If it is
not the first cpu, it means that another cpu in the same chip already
took over this duty and it needn't bother.

And shouldn't we also call nest_init() on this cpu, just like you do in
cpumask_chip() on all cpu_mask_nest_pmu cpus ?

> +}
> +
> +static int nest_cpu_notifier(struct notifier_block *self,
> +				unsigned long action, void *hcpu)
> +{
> +	long cpu = (long)hcpu;
> +
> +	switch (action & ~CPU_TASKS_FROZEN) {
> +	case CPU_DOWN_FAILED:

Why do we need to handle the DOWN_FAILED case ? In DOWN_PREPARE, you
have ensured that the function moves on to another cpu. So even if the
offline failed, its no issue. The duty is safely taken over.

> +	case CPU_STARTING:

I would suggest initializing nest in the CPU_ONLINE stage. This is
because CPU_STARTING phase can fail. In that case, we will be
unnecessarily initializing nest pre-maturely. CPU_ONLINE phase assures
that the cpu is successfully online and you can then initiate nest.

> +		nest_init_cpu(cpu);
> +		break;
> +	case CPU_DOWN_PREPARE:
> +		nest_exit_cpu(cpu);
> +		break;
> +	default:
> +		break;
> +	}
> +
> +	return NOTIFY_OK;
> +}
> +
> +static struct notifier_block nest_cpu_nb = {
> +	.notifier_call  = nest_cpu_notifier,
> +	.priority       = CPU_PRI_PERF + 1,
> +};
> +
> +void cpumask_chip(void)

This name ^^ is not apt IMO. You are initiating the cpumask necessary
for nest pmu. So why not call it nest_pmu_cpumask_init() ?

> +{
> +	const struct cpumask *l_cpumask;
> +	int cpu, nid;
> +
> +	if (!cpumask_empty(&cpu_mask_nest_pmu))

When can this condition become true ?

> +		return;
> +
> +	cpu_notifier_register_begin();
> +
> +	/*
> +	 * Nest PMUs are per-chip counters. So designate a cpu
> +	 * from each node/chip for counter collection.
> +	 */
> +	for_each_online_node(nid) {
> +		l_cpumask = cpumask_of_node(nid);
> +
> +		/* designate first online cpu in this node */
> +		cpu = cpumask_first(l_cpumask);
> +		cpumask_set_cpu(cpu, &cpu_mask_nest_pmu);
> +	}
> +
> +	/* Initialize Nest PMUs in each node using designated cpus */
> +	on_each_cpu_mask(&cpu_mask_nest_pmu, (smp_call_func_t)nest_init, NULL, 1);
> +
> +	__register_cpu_notifier(&nest_cpu_nb);
> +
> +	cpu_notifier_register_done();
> +}

Regards
Preeti U Murthy