Date:   Tue, 22 Sep 2020 17:30:07 +0100
From:   Alexandru Elisei <alexandru.elisei@....com>
To:     Will Deacon <will@...nel.org>
Cc:     mark.rutland@....com, sumit.garg@...aro.org, maz@...nel.org,
        linux-kernel@...r.kernel.org, swboyd@...omium.org,
        catalin.marinas@....com, linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH v6 0/7] arm_pmu: Use NMI for perf interrupt

Hi,

On 9/21/20 4:41 PM, Alexandru Elisei wrote:
> Hi Will,
>
> Thank you so much for reviewing the series!
>
> On 9/21/20 2:59 PM, Will Deacon wrote:
>> On Wed, Aug 19, 2020 at 02:34:12PM +0100, Alexandru Elisei wrote:
>>> The series makes the arm_pmu driver use NMIs for the perf interrupt when
>>> NMIs are available on the platform (currently, only arm64 + GICv3). To make
>>> it easier to play with the patches, I've pushed a branch at [1]:
>> This mostly looks good to me, but see some of the comments I left on the
>> code. One other thing I'm not sure about is whether or not we should tell
>> userspace that we're using an NMI for the sampling. Do any other
>> architectures have a conditional NMI?
> I'm not sure about other architectures being able to configure the perf interrupt
> as NMI or a regular interrupt, I'll try to find out. Regardless of what the other
> architecture do, I am of the opinion that we should spell out explicitly when the
> PMU is using pseudo-NMIs, because it makes a huge difference in the accuracy of
> the instrumentation and the overall usefulness of perf.

Coming back to this, looked at what other architectures are doing by grepping for
perf_pmu_register() and going from there, results below. I've found xtensa to
allow both regular IRQs and NMIs for PMU, based on a kernel config option (just
like arm64). However, the description for the config option states clearly the the
PMU IRQ will be an IRQ, while we don't have that for arm64 - the IRQ will be an
NMI automatically if the GIC is configured to use pseudo-NMIs. I think displaying
a message is the right thing to do, I'll do that for v7.

PMU IRQs for other architectures:

* alpha - PMU interrupt is always IRQ.
* arc - optional PMU interrupt; when present it's requested with
request_percpu_irq(); it prints to dmesg when overflow IRQ support has been detected.
* arm - no NMIs.
* c6x - seems like it doesn't have a PMU at all.
* csky - PMU interrupts is always IRQ.
* h8300 - seems like it doesn't have a PMU at all.
* hexagon - seems like it doesn't have a PMU at all.
* ia64 - perfmon interrupt is registered with register_percpu_irq(); it prints the
IRQ number.
* m64k - couldn't find anything resembling a PMU.
* microblaze - seems like it doesn't have a PMU.
* mips - regular IRQ; irq number and if it's shared with the timer interrupt is
printed.
* nds32 - regular IRQ; doesn't print anything regarding IRQ number.
* nios2 - seems like it doesn't have a PMU.
* openrisc - no PMU.
* parisc - no PMU IRQ, free-running counters?
* powerpc - no IRQ for IMC, hv_24x7 and hv_gpci PMUs; looks like for powerpc64,
the PMU interrupt is treated like an NMI if it is taken when interrupts are
"soft-masked", for powerpc32 it's always a regular interrupt; no information
displayed about the interrupt.
* riscv - they use regular IRQs only when multiplexing events; I haven't found any
information displayed about the PMU.
* s390 - no IRQ for cpum_cf_diag and cpm_cf; regular IRQ for cpum_sf; no dmesg output.
* sh - no IRQ.
* sparc - looks like it's always NMI; no information about IRQ is displayed.
* um - no PMU.
* x86 - the PMU interrupt is always a NMI, the lapic is configured to deliver the
PMI as an NMI (in arch/x86/events/core.c::perf_events_lapic_init()). Nothing about
interrupts printed in init_hw_perf_events();
* xtensa - the interrupt can be configurated as an NMI (EXTENSA_FAKE_NMI), but no
information about it is displayed.

Thanks,
Alex

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