Subject: Re: [PATCH v3 1/2] powerpc/perf/hv-24x7: Add cpu hotplug support

From: Michael Ellerman <[email-masked]>
Date: Mon, 06 Jul 2020 13:13:16 +1000

Kajol Jain <[email-masked]> writes:
> Patch here adds cpu hotplug functions to hv_24x7 pmu.
> A new cpuhp_state "CPUHP_AP_PERF_POWERPC_HV_24x7_ONLINE" enum
> is added.
>
> The online callback function updates the cpumask only if its
> empty. As the primary intention of adding hotplug support
> is to designate a CPU to make HCALL to collect the
> counter data.
>
> The offline function test and clear corresponding cpu in a cpumask
> and update cpumask to any other active cpu.
>
> Signed-off-by: Kajol Jain <[email-masked]>
> Reviewed-by: Gautham R. Shenoy <[email-masked]>
> ---
>  arch/powerpc/perf/hv-24x7.c | 45 +++++++++++++++++++++++++++++++++++++
>  include/linux/cpuhotplug.h  |  1 +
>  2 files changed, 46 insertions(+)
>
> diff --git a/arch/powerpc/perf/hv-24x7.c b/arch/powerpc/perf/hv-24x7.c
> index db213eb7cb02..ce4739e2b407 100644
> --- a/arch/powerpc/perf/hv-24x7.c
> +++ b/arch/powerpc/perf/hv-24x7.c
> @@ -31,6 +31,8 @@ static int interface_version;
>  /* Whether we have to aggregate result data for some domains. */
>  static bool aggregate_result_elements;
>  
> +static cpumask_t hv_24x7_cpumask;
> +
>  static bool domain_is_valid(unsigned domain)
>  {
>  	switch (domain) {
> @@ -1641,6 +1643,44 @@ static struct pmu h_24x7_pmu = {
>  	.capabilities = PERF_PMU_CAP_NO_EXCLUDE,
>  };
>  
> +static int ppc_hv_24x7_cpu_online(unsigned int cpu)
> +{
> +	/* Make this CPU the designated target for counter collection */

The comment implies every newly onlined CPU will become the target, but
actually it's only the first onlined CPU.

So I think the comment needs updating, or you could just drop the
comment, I think the code is fairly clear by itself.

> +	if (cpumask_empty(&hv_24x7_cpumask))
> +		cpumask_set_cpu(cpu, &hv_24x7_cpumask);
> +
> +	return 0;
> +}
> +
> +static int ppc_hv_24x7_cpu_offline(unsigned int cpu)
> +{
> +	int target = -1;

No need to initialise target, you assign to it unconditionally below.

> +	/* Check if exiting cpu is used for collecting 24x7 events */
> +	if (!cpumask_test_and_clear_cpu(cpu, &hv_24x7_cpumask))
> +		return 0;
> +
> +	/* Find a new cpu to collect 24x7 events */
> +	target = cpumask_last(cpu_active_mask);

Any reason to use cpumask_last() vs cpumask_first(), or a randomly
chosen CPU?

> +	if (target < 0 || target >= nr_cpu_ids)
> +		return -1;
> +
> +	/* Migrate 24x7 events to the new target */
> +	cpumask_set_cpu(target, &hv_24x7_cpumask);
> +	perf_pmu_migrate_context(&h_24x7_pmu, cpu, target);
> +
> +	return 0;
> +}
> +
> +static int hv_24x7_cpu_hotplug_init(void)
> +{
> +	return cpuhp_setup_state(CPUHP_AP_PERF_POWERPC_HV_24x7_ONLINE,
> +			  "perf/powerpc/hv_24x7:online",
> +			  ppc_hv_24x7_cpu_online,
> +			  ppc_hv_24x7_cpu_offline);
> +}
> +
>  static int hv_24x7_init(void)
>  {
>  	int r;
> @@ -1685,6 +1725,11 @@ static int hv_24x7_init(void)
>  	if (r)
>  		return r;
>  
> +	/* init cpuhotplug */
> +	r = hv_24x7_cpu_hotplug_init();
> +	if (r)
> +		pr_err("hv_24x7: CPU hotplug init failed\n");
> +

The hotplug initialisation shouldn't fail unless something is badly
wrong. I think you should just fail initialisation of the entire PMU if
that happens, which will make the error handling in the next patch much
simpler.

cheers

>  	r = perf_pmu_register(&h_24x7_pmu, h_24x7_pmu.name, -1);
>  	if (r)
>  		return r;