Re: [PATCH V6] arm64: perf: Make exporting of pmu events configurable

[Srinivasarao Pathipati]

On 7/13/2022 9:35 AM, Srinivasarao Pathipati wrote:
> On 6/24/2022 5:48 PM, Will Deacon wrote:
> > On Thu, Jun 09, 2022 at 06:29:49PM +0530, Srinivasarao Pathipati wrote:
> > > The PMU export bit (PMCR_EL0.X) is getting reset during pmu reset,
> > > Make it configurable using sysctls to enable/disable at runtime.
> > >
> > > Signed-off-by: Srinivasarao Pathipati <quic_c_spathi@xxxxxxxxxxx>
> > > ---
> > > Changes since V5:
> > >     - removed configuring with kernel parameters.
> > > Changes since V4:
> > >     - Registering sysctls dynamically for only arm64 as suggested by 
> > > Will
> > >     - Not removed the code to configure with kernel parameters
> > >       as the sysctl's kernel parameter(sysctl.kernel.export_pmu_events)
> > >       is not working at early bootup. pmu_reset() getting called before
> > >       sysctl's kernel parameter is set.
> > > Changes since V3:
> > >     - export bit is now configurable with sysctl
> > >     - enabling export bit on reset instead of retaining
> > >
> > > Changes since V2:
> > >     Done below changes as per Will's comments
> > >     - enabling pmcr_x now configurable with kernel parameters and
> > >       by default it is disabled.
> > >
> > > Changes since V1:
> > >     - Preserving only PMCR_X bit as per Robin Murphy's comment.
> > >
> > > ---
> > >   Documentation/admin-guide/sysctl/kernel.rst | 11 +++++++++++
> > >   arch/arm64/kernel/perf_event.c              | 13 +++++++++++++
> > >   2 files changed, 24 insertions(+)
> > >
> > > diff --git a/Documentation/admin-guide/sysctl/kernel.rst 
> > > b/Documentation/admin-guide/sysctl/kernel.rst
> > > index ddccd10..c2ecd84 100644
> > > --- a/Documentation/admin-guide/sysctl/kernel.rst
> > > +++ b/Documentation/admin-guide/sysctl/kernel.rst
> > > @@ -267,6 +267,17 @@ domain names are in general different. For a 
> > > detailed discussion
> > >   see the ``hostname(1)`` man page.
> > >     +export_pmu_events (arm64 only)
> > > +==============================
> > > +
> > > +Controls the PMU export bit (PMCR_EL0.X), which enables the 
> > > exporting of
> > > +events over an IMPLEMENTATION DEFINED PMU event export bus to 
> > > another device.
> > > +
> > > +0: disables exporting of events (default).
> > > +
> > > +1: enables exporting of events.
> > > +
> > > +
> > >   firmware_config
> > >   ===============
> > >   diff --git a/arch/arm64/kernel/perf_event.c 
> > > b/arch/arm64/kernel/perf_event.c
> > > index cb69ff1..a8c32a0 100644
> > > --- a/arch/arm64/kernel/perf_event.c
> > > +++ b/arch/arm64/kernel/perf_event.c
> > > @@ -298,6 +298,7 @@ PMU_FORMAT_ATTR(long, "config1:0");
> > >   PMU_FORMAT_ATTR(rdpmc, "config1:1");
> > >     static int sysctl_perf_user_access __read_mostly;
> > > +static int sysctl_export_pmu_events __read_mostly;
> > >     static inline bool armv8pmu_event_is_64bit(struct perf_event 
> > > *event)
> > >   {
> > > @@ -1047,6 +1048,9 @@ static void armv8pmu_reset(void *info)
> > >       if (armv8pmu_has_long_event(cpu_pmu))
> > >           pmcr |= ARMV8_PMU_PMCR_LP;
> > >   +    if (sysctl_export_pmu_events)
> > > +        pmcr |= ARMV8_PMU_PMCR_X;
> > I think we need to do this in armv8pmu_start() rather than 
> > armv8pmu_reset(),
> > otherwise any changes to the sysctl at runtime won't take effect 
> > unless you
> > do something like re-online the CPU.
> >
> > Will
>
> Hi Will ,
>
> We think it may not work.   Say for example pmu->reset is called which 
> disables the exporting of events,
> and pmu->start not called. This would lead to missing of events.
>
> How about enabling it in both reset and start functions? do you see 
> any issue?
>
> --Srinivas

Hi Will ,

Could you please suggest ,better way to fix this.