Re: [PATCH 1/2] perf: arm_pmu: Only show online CPUs in device's "cpus" attribute

[Dongli Zhang]

Ping? Is there any plan to move forward with the patch from Yicong?

Thank you very much!

Dongli Zhang

On 4/18/24 9:32 AM, Dongli Zhang wrote:
> 
> 
> On 4/11/24 01:55, Yicong Yang wrote:
>> On 2024/4/10 23:34, Will Deacon wrote:
>>> On Wed, Apr 10, 2024 at 05:58:32PM +0800, Yicong Yang wrote:
>>>> From: Yicong Yang <yangyicong@xxxxxxxxxxxxx>
>>>>
>>>> When there're CPUs offline after system booting, perf will failed:
>>>> [root@localhost ~]# /home/yang/perf stat -a -e armv8_pmuv3_0/cycles/
>>>> Error:
>>>> The sys_perf_event_open() syscall returned with 19 (No such device) for event (cpu-clock).
>>>> /bin/dmesg | grep -i perf may provide additional information.
>>>>
>>>> This is due to PMU's "cpus" is not updated and still contains offline
>>>> CPUs and perf will try to open perf event on the offlined CPUs.
>>>>
>>>> Make "cpus" attribute only shows online CPUs and introduced a new
>>>> "supported_cpus" where users can get the range of the CPUs this
>>>> PMU supported monitoring.
>>>>
>>>> Signed-off-by: Yicong Yang <yangyicong@xxxxxxxxxxxxx>
>>>> ---
>>>>  drivers/perf/arm_pmu.c | 24 +++++++++++++++++++++++-
>>>>  1 file changed, 23 insertions(+), 1 deletion(-)
>>>
>>> Hmm. Is the complexity in the driver really worth it here? CPUs can be
>>> onlined and offlined after the perf_event_open() syscall has been
>>> executed, 
>>
>> Yes. So we have cpuhp callbacks to handle the cpu online/offline
>> and migrate the perf context.
>>
>>> so this feels like something userspace should be aware of and
>>> handle on a best-effort basis anyway.
>>>
>>
>> Looks like it's a convention for a PMU device to provide a "cpus" attribute (for core
>> PMUs) or "cpumask" attribute (for uncore PMUs) to indicates the CPUs on which the
>> events can be opened. If no such attributes provided, all online CPUs indicated. Perf
>> will check this and if user doesn't specify a certian range of CPUs the events will
>> be opened on all the CPUs PMU indicated.
>>
>>> Does x86 get away with this because CPU0 is never offlined?
>>>
>>
>> Checked on my x86 server there's no "cpus" or "cpumask" provided so perf will try
>> to open the events on all the online CPUs if no CPU range specified. But for their
>> hybrid platform there do have a "cpus" attribute[1] and it'll be updated when CPU
>> offline[2].
>>
>> The arm-cspmu also provides a "cpumask" to indicate supported online CPUs and an
>> "associated_cpus" to indicated the CPUs related to the PMU.
>>
>> [1] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/arch/x86/events/intel/core.c?h=v6.9-rc1#n5931
>> [2] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/arch/x86/events/intel/core.c?h=v6.9-rc1#n4949
>>
>> Thanks.
>>
>>
> 
> 
> The arm_dsu has the concepts of 'cpumask' as well. It also has 'associated_cpus'.
> 
> When the current cpumask offline, the cpuhp handler will migrate the cpumask to
> other associated_cpus.
> 
> # cat /sys/devices/arm_dsu_26/associated_cpus
> 4-5
> [root@lse-aarch64-bm-ol8 opc]# cat /sys/devices/arm_dsu_26/cpumask
> 4
> 
> 812 static int dsu_pmu_cpu_online(unsigned int cpu, struct hlist_node *node)
> 813 {
> 814         struct dsu_pmu *dsu_pmu = hlist_entry_safe(node, struct dsu_pmu,
> 815                                                    cpuhp_node);
> 816
> 817         if (!cpumask_test_cpu(cpu, &dsu_pmu->associated_cpus))
> 818                 return 0;
> 819
> 820         /* If the PMU is already managed, there is nothing to do */
> 821         if (!cpumask_empty(&dsu_pmu->active_cpu))
> 822                 return 0;
> 823
> 824         dsu_pmu_init_pmu(dsu_pmu);
> 825         dsu_pmu_set_active_cpu(cpu, dsu_pmu);
> 826
> 827         return 0;
> 828 }
> 829
> 830 static int dsu_pmu_cpu_teardown(unsigned int cpu, struct hlist_node *node)
> 831 {
> 832         int dst;
> 833         struct dsu_pmu *dsu_pmu = hlist_entry_safe(node, struct dsu_pmu,
> 834                                                    cpuhp_node);
> 835
> 836         if (!cpumask_test_and_clear_cpu(cpu, &dsu_pmu->active_cpu))
> 837                 return 0;
> 838
> 839         dst = dsu_pmu_get_online_cpu_any_but(dsu_pmu, cpu);
> 840         /* If there are no active CPUs in the DSU, leave IRQ disabled */
> 841         if (dst >= nr_cpu_ids)
> 842                 return 0;
> 843
> 844         perf_pmu_migrate_context(&dsu_pmu->pmu, cpu, dst);
> 845         dsu_pmu_set_active_cpu(dst, dsu_pmu);
> 846
> 847         return 0;
> 848 }
> 
> 
> However, I think the userspace perf tool looks more friendly (just return <not
> supported>) in this case when I offline all CPUs from cpumask of a DSU. Perhaps
> because it is NULL now.
> 
> # perf stat -e arm_dsu_26/l3d_cache_wb/
> ^C
>  Performance counter stats for 'system wide':
> 
>    <not supported>      arm_dsu_26/l3d_cache_wb/
> 
>        0.553294766 seconds time elapsed
> 
> 
> # cat /sys/devices/arm_dsu_26/associated_cpus
> 4-5
> # cat /sys/devices/arm_dsu_26/cpumask
> 4
> # echo 0 > /sys/devices/system/cpu/cpu4/online
> # cat /sys/devices/arm_dsu_26/cpumask
> 5
> # echo 0 > /sys/devices/system/cpu/cpu5/online
> # cat /sys/devices/arm_dsu_26/cpumask
> 
> #
> 
> Dongli Zhang