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Date: Mon, 3 Jun 2024 09:52:15 -0700
From: Ian Rogers <irogers@...gle.com>
To: Yicong Yang <yangyicong@...wei.com>
Cc: will@...nel.org, mark.rutland@....com, acme@...nel.org,
namhyung@...nel.org, linux-arm-kernel@...ts.infradead.org,
linux-perf-users@...r.kernel.org, linux-kernel@...r.kernel.org,
peterz@...radead.org, mingo@...hat.com, alexander.shishkin@...ux.intel.com,
jolsa@...nel.org, james.clark@....com, dongli.zhang@...cle.com,
jonathan.cameron@...wei.com, prime.zeng@...ilicon.com, linuxarm@...wei.com,
yangyicong@...ilicon.com
Subject: Re: [PATCH 1/3] perf pmu: Limit PMU cpumask to online CPUs
On Mon, Jun 3, 2024 at 2:33 AM Yicong Yang <yangyicong@...wei.com> wrote:
>
> From: Yicong Yang <yangyicong@...ilicon.com>
>
> We'll initialize the PMU's cpumask from "cpumask" or "cpus" sysfs
> attributes if provided by the driver without checking the CPUs
> are online or not. In such case that CPUs provided by the driver
> contains the offline CPUs, we'll try to open event on the offline
> CPUs and then rejected by the kernel:
>
> [root@...alhost yang]# echo 0 > /sys/devices/system/cpu/cpu0/online
> [root@...alhost yang]# ./perf_static stat -e armv8_pmuv3_0/cycles/ --timeout 100
> 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.
>
> So it's better to do a double check in the userspace and only include
> the online CPUs from "cpumask" or "cpus" to avoid opening events on
> offline CPUs.
I see where you are coming from with this but I think it is wrong. The
cpus for an uncore PMU are a hint of the CPU to open on rather than
the set of valid CPUs. For example:
```
$ cat /sys/devices/uncore_imc_free_running_0/cpumask
0
$ perf stat -vv -e uncore_imc_free_running_0/data_read/ -C 1 -a sleep 0.1
Using CPUID GenuineIntel-6-8D-1
Attempt to add: uncore_imc_free_running_0/data_read/
..after resolving event: uncore_imc_free_running_0/event=0xff,umask=0x20/
Control descriptor is not initialized
------------------------------------------------------------
perf_event_attr:
type 24 (uncore_imc_free_running_0)
size 136
config 0x20ff (data_read)
sample_type IDENTIFIER
read_format TOTAL_TIME_ENABLED|TOTAL_TIME_RUNNING
disabled 1
inherit 1
exclude_guest 1
------------------------------------------------------------
sys_perf_event_open: pid -1 cpu 1 group_fd -1 flags 0x8
sys_perf_event_open failed, error -22
switching off cloexec flag
------------------------------------------------------------
perf_event_attr:
type 24 (uncore_imc_free_running_0)
size 136
config 0x20ff (data_read)
sample_type IDENTIFIER
read_format TOTAL_TIME_ENABLED|TOTAL_TIME_RUNNING
disabled 1
inherit 1
exclude_guest 1
------------------------------------------------------------
sys_perf_event_open: pid -1 cpu 1 group_fd -1 flags 0
sys_perf_event_open failed, error -22
switching off exclude_guest, exclude_host
------------------------------------------------------------
perf_event_attr:
type 24 (uncore_imc_free_running_0)
size 136
config 0x20ff (data_read)
sample_type IDENTIFIER
read_format TOTAL_TIME_ENABLED|TOTAL_TIME_RUNNING
disabled 1
inherit 1
------------------------------------------------------------
sys_perf_event_open: pid -1 cpu 1 group_fd -1 flags 0 = 3
uncore_imc_free_running_0/data_read/: 1: 4005984 102338957 102338957
uncore_imc_free_running_0/data_read/: 4005984 102338957 102338957
Performance counter stats for 'system wide':
244.51 MiB uncore_imc_free_running_0/data_read/
0.102320376 seconds time elapsed
```
So the CPU mask of the PMU says to open on CPU 0, but on the command
line when I passed "-C 1" it opened it on CPU 1. If the cpumask file
contained an offline CPU then this change would make it so the CPU map
in the tool were empty, however, a different CPU may be programmable
and online.
Fwiw, the tool will determine whether the mask is for all valid or a
hint by using the notion of a PMU being "core" or not. That notion
considers whether the mask was loading from a "cpumask" or "cpus"
file:
https://git.kernel.org/pub/scm/linux/kernel/git/perf/perf-tools-next.git/tree/tools/perf/util/pmu.c?h=perf-tools-next#n810
Thanks,
Ian
> Signed-off-by: Yicong Yang <yangyicong@...ilicon.com>
> ---
> tools/perf/util/pmu.c | 13 +++++++++++--
> 1 file changed, 11 insertions(+), 2 deletions(-)
>
> diff --git a/tools/perf/util/pmu.c b/tools/perf/util/pmu.c
> index 888ce9912275..51e8d10ee28b 100644
> --- a/tools/perf/util/pmu.c
> +++ b/tools/perf/util/pmu.c
> @@ -771,8 +771,17 @@ static struct perf_cpu_map *pmu_cpumask(int dirfd, const char *name, bool is_cor
> continue;
> cpus = perf_cpu_map__read(file);
> fclose(file);
> - if (cpus)
> - return cpus;
> + if (cpus) {
> + struct perf_cpu_map *intersect __maybe_unused;
> +
> + if (perf_cpu_map__is_subset(cpu_map__online(), cpus))
> + return cpus;
> +
> + intersect = perf_cpu_map__intersect(cpus, cpu_map__online());
> + perf_cpu_map__put(cpus);
> + if (intersect)
> + return intersect;
> + }
> }
>
> /* Nothing found, for core PMUs assume this means all CPUs. */
> --
> 2.24.0
>
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