Re: [PATCH 2/2] perf: arm_pmuv3: Don't use PMCCNTR_EL0 on SMT cores

[James Clark <james.clark@linaro.org>]

On 12/08/2025 9:08 am, Yicong Yang wrote:
> From: Yicong Yang <yangyicong@hisilicon.com>
> 
> CPU_CYCLES is expected to count the logical CPU (PE) clock. Currently it's
> preferred to use PMCCNTR_EL0 for counting CPU_CYCLES, but it'll count
> processor clock rather than the PE clock (ARM DDI0487 L.b D13.1.3) if
> one of the SMT siblings is not idle on a multi-threaded implementation.
> So don't use it on SMT cores.
> 
> When counting cycles on SMT CPU 2-3 and CPU 3 is idle, without this
> patch we'll get:
> [root@client1 tmp]# perf stat -e cycles -A -C 2-3 -- stress-ng -c 1
> --taskset 2 --timeout 1
> [...]
>   Performance counter stats for 'CPU(s) 2-3':
> 
> CPU2           2880457316      cycles
> CPU3           2880459810      cycles
>         1.254688470 seconds time elapsed
> 
> With this patch the idle state of CPU3 is observed as expected:
> [root@client1 ~]#  perf stat -e cycles -A -C 2-3 -- stress-ng -c 1
> --taskset 2 --timeout 1
> [...]
>   Performance counter stats for 'CPU(s) 2-3':
> 
> CPU2           2558580492      cycles
> CPU3               305749      cycles
>         1.113626410 seconds time elapsed
> 
> Signed-off-by: Yicong Yang <yangyicong@hisilicon.com>
> ---
>   drivers/perf/arm_pmuv3.c | 9 +++++++++
>   1 file changed, 9 insertions(+)
> 
> diff --git a/drivers/perf/arm_pmuv3.c b/drivers/perf/arm_pmuv3.c
> index 95c899d07df5..ed3149632b71 100644
> --- a/drivers/perf/arm_pmuv3.c
> +++ b/drivers/perf/arm_pmuv3.c
> @@ -1002,6 +1002,15 @@ static bool armv8pmu_can_use_pmccntr(struct pmu_hw_events *cpuc,
>   	if (has_branch_stack(event))
>   		return false;
>   
> +	/*
> +	 * The PMCCNTR_EL0 increments from the processor clock rather than
> +	 * the PE clock (ARM DDI0487 L.b D13.1.3) which means it'll continue
> +	 * counting on a WFI PE if one of its SMT silbing is not idle on a
> +	 * multi-threaded implementation. So don't use it on SMT cores.
> +	 */
> +	if (cpumask_weight(topology_sibling_cpumask(smp_processor_id())) > 1)
> +		return false;
> +

Isn't this something that's static to the PMU? If all CPUs in each PMU 
are always the same then this doesn't need to be probed every time and 
can be set once.

Also you can't call smp_processor_id() from here because this is also 
called in armpmu_event_init() -> __hw_perf_event_init() -> 
validate_group() before the event is actually scheduled on a CPU. With 
CONFIG_DEBUG_PREEMPT you'd see the error.

>   	return true;
>   }
>