Re: [PATCH v13 3/5] arm64: perf: Add userspace counter access disable switch

Geert Uytterhoeven
2021-12-28T11:08:41Z

Hi Rob,

On Wed, Dec 8, 2021 at 9:19 PM Rob Herring <robh@kernel.org> wrote:
> Like x86, some users may want to disable userspace PMU counter
> altogether. Add a sysctl 'perf_user_access' file to control userspace
> counter access. The default is '0' which is disabled. Writing '1'
> enables access.
>
> Note that x86 supports globally enabling user access by writing '2' to
> /sys/bus/event_source/devices/cpu/rdpmc. As there's not existing
> userspace support to worry about, this shouldn't be necessary for Arm.
> It could be added later if the need arises.

Thanks for your patch, which is now commit e2012600810c9ded ("arm64:
perf: Add userspace counter access disable switch") in arm64/for-next/core.

This is causing two issues on Renesas Salvator-XS with R-Car H3.
One during kernel boot:

hw perfevents: enabled with armv8_cortex_a53 PMU driver, 7
counters available
+sysctl duplicate entry: /kernel//perf_user_access
+CPU: 0 PID: 1 Comm: swapper/0 Not tainted
5.16.0-rc3-arm64-renesas-00003-ge2012600810c #1420
+Hardware name: Renesas Salvator-X 2nd version board based on r8a77951 (DT)
+Call trace:
+ dump_backtrace+0x0/0x190
+ show_stack+0x14/0x20
+ dump_stack_lvl+0x88/0xb0
+ dump_stack+0x14/0x2c
+ __register_sysctl_table+0x384/0x818
+ register_sysctl+0x20/0x28
+ armv8_pmu_init.constprop.0+0x118/0x150
+ armv8_a57_pmu_init+0x1c/0x28
+ arm_pmu_device_probe+0x1b4/0x558
+ armv8_pmu_device_probe+0x18/0x20
+ platform_probe+0x64/0xd0
+ really_probe+0xb4/0x2f8
+ __driver_probe_device+0x74/0xd8
+ driver_probe_device+0x3c/0xe0
+ __driver_attach+0x80/0x110
+ bus_for_each_dev+0x6c/0xc0
+ driver_attach+0x20/0x28
+ bus_add_driver+0x138/0x1e0
+ driver_register+0x60/0x110
+ __platform_driver_register+0x24/0x30
+ armv8_pmu_driver_init+0x18/0x20
+ do_one_initcall+0x15c/0x31c
+ kernel_init_freeable+0x2f0/0x354
+ kernel_init+0x20/0x120
+ ret_from_fork+0x10/0x20
hw perfevents: enabled with armv8_cortex_a57 PMU driver, 7
counters available

Presumably the same entry is added twice, once for the A53 PMU,
and a second time for the A57 PMU?

A second during systemd startup:

systemd-journald[326]: Failed to open runtime journal: No such
file or directory
systemd-journald[345]: File
/run/log/journal/09223238c0464b38ad4fc1d505d98e17/system.journal
corrupted or uncleanly shut down, renaming and replacing.

followed by lots of

systemd[<n>]: <foo>.service: Failed to connect stdout to the
journal socket, ignoring: Connection refused

failures.

> --- a/arch/arm64/kernel/perf_event.c
> +++ b/arch/arm64/kernel/perf_event.c
> @@ -286,6 +286,8 @@ static const struct attribute_group armv8_pmuv3_events_attr_group = {
> PMU_FORMAT_ATTR(event, "config:0-15");
> PMU_FORMAT_ATTR(long, "config1:0");
>
> +static int sysctl_perf_user_access __read_mostly;
> +
> static inline bool armv8pmu_event_is_64bit(struct perf_event *event)
> {
> return event->attr.config1 & 0x1;
> @@ -1104,6 +1106,19 @@ static int armv8pmu_probe_pmu(struct arm_pmu *cpu_pmu)
> return probe.present ? 0 : -ENODEV;
> }
>
> +static struct ctl_table armv8_pmu_sysctl_table[] = {
> + {
> + .procname = "perf_user_access",
> + .data = &sysctl_perf_user_access,
> + .maxlen = sizeof(unsigned int),
> + .mode = 0644,
> + .proc_handler = proc_dointvec_minmax,
> + .extra1 = SYSCTL_ZERO,
> + .extra2 = SYSCTL_ONE,
> + },
> + { }
> +};
> +
> static int armv8_pmu_init(struct arm_pmu *cpu_pmu, char *name,
> int (*map_event)(struct perf_event *event),
> const struct attribute_group *events,
> @@ -1136,6 +1151,8 @@ static int armv8_pmu_init(struct arm_pmu *cpu_pmu, char *name,
> cpu_pmu->attr_groups[ARMPMU_ATTR_GROUP_CAPS] = caps ?
> caps : &armv8_pmuv3_caps_attr_group;
>
> + register_sysctl("kernel", armv8_pmu_sysctl_table);
> +
> return 0;
> }

Gr{oetje,eeting}s,

Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@linux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
-- Linus Torvalds