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

[Madhavan Srinivasan <maddy@linux.ibm.com>]

On 7/6/20 8:43 AM, Michael Ellerman wrote:
> Kajol Jain <kjain@linux.ibm.com> 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 <kjain@linux.ibm.com>
>> Reviewed-by: Gautham R. Shenoy <ego@linux.vnet.ibm.com>
>> ---
>>   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.

We  did fail the PMU registration on failure of the hotplug
code (and yes error handling is much simpler), but on internal 
review/discussion,
what came up was that, hv_24x7 PMU will still be usable without
the hotplug code (with "-C" option to perf tool command line).

Maddy

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