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

[Michael Ellerman]

Madhavan Srinivasan <maddy at linux.ibm.com> writes:
> On 7/6/20 8:43 AM, Michael Ellerman wrote:
>> Kajol Jain <kjain at 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 at linux.ibm.com>
>>> Reviewed-by: Gautham R. Shenoy <ego at 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).

In theory yes.

But in reality no one will ever test that case, so the code will easily
bit rot.

Even if it doesn't bit rot, you've now created another state the system
can legally be in (hotplug init failed but PMU still probed), which you
have to test, document & support.

If the hotplug init fails then something is badly wrong, the best thing
we can do is bail on the PMU initialisation and hope the rest of the
system boots OK.

cheers