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

From: kajoljain <[email-masked]>
Date: Mon, 22 Jun 2020 11:18:01 +0530

On 6/19/20 10:28 AM, Gautham R Shenoy wrote:
> Hello Kajol,
> 
> On Thu, Jun 18, 2020 at 05:57:12PM +0530, Kajol Jain wrote:
>> 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 function update the cpumask only if its NULL.
>> As the primary intention for adding hotplug support
>> is to desiginate a CPU to make HCALL to collect the
>> count data.
>>
>> The offline function test and clear corresponding cpu in a cpumask
>> and update cpumask to any other active cpu.
>>
>> With this patchset, perf tool side does not need "-C <cpu>"
>> to be added.
>>
>> Signed-off-by: Kajol Jain <[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..fdc4ae155d60 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 */
>> +	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;
>> +
>> +	/* 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_any_but(cpu_active_mask, cpu);
> 
> cpumask_any_but() typically picks the first CPU in cpu_active_mask
> that is not @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);
> 
> 
> On a system with N CPUs numbered [O..N-1], can you please verify if
> the time required to sequentially offline CPUs [0..N-2] ,in that
> order, increase with this patch ?
> 
> I am asking this because we have encountered this problem once before
> at a customer site and the commit 9c9f8fb71fee ("powerpc/perf: Use
> cpumask_last() to determine the designated cpu for nest/core units.")
> was introduced to fix that problem.
> 

Hi Gautham,
     Thanks for reviewing the patch. So, cpu_active_mask has bit 'cpu' set
only if that cpu is available. Even if we offline cpu non-sequentially
it will update "cpu_acive_mask" accordingly.

This is some of test I tried:

command:# cat /sys/devices/hv_24x7/cpumask
0
command:# echo 0 > /sys/devices/system/cpu/cpu0/online
command:# echo 0 > /sys/devices/system/cpu/cpu2/online
command:# cat /sys/devices/hv_24x7/cpumask
1
command:# echo 0 > /sys/devices/system/cpu/cpu1/online
command:# cat /sys/devices/hv_24x7/cpumask
3

Please let me know if my understanding is fine.

Thanks,
Kajol Jain

>> +
>> +	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");
>> +
>>  	r = perf_pmu_register(&h_24x7_pmu, h_24x7_pmu.name, -1);
>>  	if (r)
>>  		return r;
>> diff --git a/include/linux/cpuhotplug.h b/include/linux/cpuhotplug.h
>> index 8377afef8806..16ed8f6f8774 100644
>> --- a/include/linux/cpuhotplug.h
>> +++ b/include/linux/cpuhotplug.h
>> @@ -180,6 +180,7 @@ enum cpuhp_state {
>>  	CPUHP_AP_PERF_POWERPC_CORE_IMC_ONLINE,
>>  	CPUHP_AP_PERF_POWERPC_THREAD_IMC_ONLINE,
>>  	CPUHP_AP_PERF_POWERPC_TRACE_IMC_ONLINE,
>> +	CPUHP_AP_PERF_POWERPC_HV_24x7_ONLINE,
>>  	CPUHP_AP_WATCHDOG_ONLINE,
>>  	CPUHP_AP_WORKQUEUE_ONLINE,
>>  	CPUHP_AP_RCUTREE_ONLINE,
>> -- 
>> 2.18.2
>>