linux-kernel - Re: [PATCH v6 2/2] perf/x86/amd/power: Add AMD accumulated power reporting mechanism

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Date:	Thu, 3 Mar 2016 09:50:11 +0100 (CET)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Huang Rui <ray.huang@....com>
cc:	Borislav Petkov <bp@...e.de>,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...nel.org>,
	Andy Lutomirski <luto@...capital.net>,
	Robert Richter <rric@...nel.org>,
	Jacob Shin <jacob.w.shin@...il.com>,
	Arnaldo Carvalho de Melo <acme@...nel.org>,
	Kan Liang <kan.liang@...el.com>, linux-kernel@...r.kernel.org,
	spg_linux_kernel@....com, x86@...nel.org,
	Suravee Suthikulpanit <suravee.suthikulpanit@....com>,
	Aravind Gopalakrishnan <Aravind.Gopalakrishnan@....com>,
	Borislav Petkov <bp@...en8.de>,
	Fengguang Wu <fengguang.wu@...el.com>,
	Guenter Roeck <linux@...ck-us.net>
Subject: Re: [PATCH v6 2/2] perf/x86/amd/power: Add AMD accumulated power
 reporting mechanism

On Thu, 3 Mar 2016, Huang Rui wrote:
> +/*
> + * The ratio of compute unit power accumulator sample period to the
> + * PTSC period.
> + */
> +static unsigned int cpu_pwr_sample_ratio;
> +static unsigned int cu_num;

Why do you need static storage for that information when the only purpose is
to printk it in init?

> +static void power_cpu_exit(int cpu)
> +{
> +	int target = nr_cpumask_bits;

What's that initialization for?

> +
> +	if (!cpumask_test_and_clear_cpu(cpu, &cpu_mask))
> +		return;
> +
> +	/*
> +	 * Find a new CPU on the same compute unit, if was set in cpumask
> +	 * and still some CPUs on compute unit. Then migrate event and
> +	 * context to new CPU.
> +	 */
> +	target = cpumask_any_but(topology_sibling_cpumask(cpu), cpu);
> +	if (target < nr_cpumask_bits) {
> +		cpumask_set_cpu(target, &cpu_mask);
> +		perf_pmu_migrate_context(&pmu_class, cpu, target);
> +	}
> +}
> +
> +static void power_cpu_init(int cpu)
> +{
> +	/*
> +	 * 1) If any CPU is set at cpu_mask in the same compute unit, do
> +	 * nothing.
> +	 * 2) If no CPU is set at cpu_mask in the same compute unit,
> +	 * set current STARTING CPU.
> +	 *
> +	 * If cpu_mask and topology_sibling_cpumask has intersected
> +	 * bits, that means any CPU is set in the same compute unit.
> +	 * But cpumask_weight(topology_sibling_cpumask(cpu)) == 1
> +	 * means no CPU is set on cpu_mask in the same compute unit
> +	 * before init current STARTING CPU.
> +	 */
> +	if (!cpumask_intersects(&cpu_mask, topology_sibling_cpumask(cpu)) &&
> +	    cpumask_weight(topology_sibling_cpumask(cpu)) == 1)
> +			cpumask_set_cpu(cpu, &cpu_mask);

I don't think you need that complexity.

	target = cpumask_any_but(topology_sibling_cpumask(cpu), cpu);
	if (target >= nr_cpumask_bits)
		cpumask_set_cpu(cpu, &cpu_mask);
	   
Simply because if there is a cpu aside of the new one already in the sibling
mask, then it is also in cpu_mask. Hmm?

> +static int
> +power_cpu_notifier(struct notifier_block *self, unsigned long action, void *hcpu)
> +{
> +	unsigned int cpu = (long)hcpu;
> +
> +	switch (action & ~CPU_TASKS_FROZEN) {
> +	case CPU_STARTING:
> +		power_cpu_init(cpu);
> +		break;
> +	case CPU_DOWN_PREPARE:
> +		power_cpu_exit(cpu);
> +		break;

And of course if CPU_DOWN_PREPARE fails and this is the last cpu in the
compute unit, nothing takes over the duty for this compute unit. So you need
to handle CPU_DOWN_FAILED ....

> +static int __init amd_power_pmu_init(void)
> +{
> +	int i, ret;
> +	u64 tmp;
> +
> +	if (!x86_match_cpu(cpu_match))
> +		return 0;
> +
> +	if (!boot_cpu_has(X86_FEATURE_ACC_POWER))
> +		return -ENODEV;
> +
> +	cu_num = boot_cpu_data.x86_max_cores / smp_num_siblings;
> +
> +	cpu_pwr_sample_ratio = cpuid_ecx(0x80000007);
> +
> +	if (rdmsrl_safe(MSR_F15H_CU_MAX_PWR_ACCUMULATOR, &tmp)) {
> +		pr_err("Failed to read max compute unit power accumulator MSR\n");
> +		return -ENODEV;
> +	}
> +	max_cu_acc_power = tmp;

Why do you need an intermediate 'tmp' for this?

> +	cpu_notifier_register_begin();
> +
> +	/* Choose one online core of each compute unit. */
> +	for (i = 0; i < boot_cpu_data.x86_max_cores; i += smp_num_siblings) {
> +		WARN_ON(cpumask_empty(topology_sibling_cpumask(i)));

Err. What guarantees that in each compute unit is one sibling online? And what
value has that WARN_ON? We don't care about the stack trace here, because it's
known already.

> +		cpumask_set_cpu(cpumask_any(topology_sibling_cpumask(i)), &cpu_mask);

Of course you just continue in that case and end up with:

       	      cpumask_set_cpu(nr_cpu_ids, &cpu_mask);

i.e. you try to do that on an invalid bit, which will trigger a justified
warning in cpumask_set_cpu() if CONFIG_DEBUG_PER_CPU_MAPS is enabled.

Aside of that this only handles a single socket. And why do you do the above
if you handle the same thing in the loop below?

> +	}
> +
> +	for_each_online_cpu(i)
> +		power_cpu_init(i);
> +
> +	__register_cpu_notifier(&power_cpu_notifier_nb);
> +
> +	ret = perf_pmu_register(&pmu_class, "power", -1);
> +	if (WARN_ON(ret)) {
> +		pr_warn("AMD Power PMU registration failed\n");

This still leaks the cpu notifier. .....

> +		goto out;
> +	}
> +
> +	pr_info("AMD Power PMU detected, %d compute units\n", cu_num);

Why is the number of compute units interesting at all?

Thanks,

	tglx