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

[Will Deacon]

Hi Geert,

On Tue, Dec 28, 2021 at 12:07:02PM +0100, Geert Uytterhoeven wrote:
> On Wed, Dec 8, 2021 at 9:19 PM Rob Herring <robh@xxxxxxxxxx> 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?

Looks like it, and perhaps that's also what is confusing systemd?
Rob -- how come you didn't see this during your testing?

Anywho, please can you try the untested diff below?

Thanks,

Will

--->8

diff --git a/arch/arm64/kernel/perf_event.c b/arch/arm64/kernel/perf_event.c
index 81cc9f0e718a..639f632aaa66 100644
--- a/arch/arm64/kernel/perf_event.c
+++ b/arch/arm64/kernel/perf_event.c
@@ -1214,6 +1214,14 @@ static struct ctl_table armv8_pmu_sysctl_table[] = {
        { }
 };
 
+static void armv8_pmu_register_sysctl_table(void)
+{
+       static u32 tbl_registered = 0;
+
+       if (!cmpxchg_relaxed(&tbl_registered, 0, 1))
+               register_sysctl("kernel", armv8_pmu_sysctl_table);
+}
+
 static int armv8_pmu_init(struct arm_pmu *cpu_pmu, char *name,
                          int (*map_event)(struct perf_event *event),
                          const struct attribute_group *events,
@@ -1248,8 +1256,7 @@ 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);
-
+       armv8_pmu_register_sysctl_table();
        return 0;
 }