Re: [RFC PATCH 0/6] Support raw event and DT for perf on RISC-V

[Zong Li]

On Wed, Jul 1, 2020 at 8:52 AM Alan Kao <alankao@xxxxxxxxxxxxx> wrote:
>
> On Mon, Jun 29, 2020 at 11:19:09AM +0800, Zong Li wrote:
> > This patch set adds raw event support on RISC-V. In addition, we
> > introduce the DT mechanism to make our perf more generic and common.
> >
> > Currently, we set the hardware events by writing the mhpmeventN CSRs, it
> > would raise an illegal instruction exception and trap into m-mode to
> > emulate event selector CSRs access. It doesn't make sense because we
> > shouldn't write the m-mode CSRs in s-mode. Ideally, we should set event
> > selector through standard SBI call or the shadow CSRs of s-mode. We have
> > prepared a proposal of a new SBI extension, called "PMU SBI extension",
> > but we also discussing the feasibility of accessing these PMU CSRs on
> > s-mode at the same time, such as delegation mechanism, so I was
> > wondering if we could use SBI calls first and make the PMU SBI extension
> > as legacy when s-mode access mechanism is accepted by Foundation? or
> > keep the current situation to see what would happen in the future.
> >
> > This patch set also introduces the DT mechanism, we don't want to add too
> > much platform-dependency code in perf like other architectures, so we
> > put the mapping of generic hardware events to DT, then we can easy to
> > transfer generic hardware events to vendor's own hardware events without
> > any platfrom-dependency stuff in our perf.
> >
> > Zong Li (6):
> >   dt-bindings: riscv: Add YAML documentation for PMU
> >   riscv: dts: sifive: Add DT support for PMU
> >   riscv: add definition of hpmcounter CSRs
> >   riscv: perf: Add raw event support
> >   riscv: perf: introduce DT mechanism
> >   riscv: remove PMU menu of Kconfig
> >
>
> DT-based PMU registration looks good to me. Together with Anup's feedback,
> we can anticipate that the following items will be:
>
> - rewrite RISC-V PMU to a platform driver
> - propose SBI PMU extention
> - fixes: RV32 counter access, namings, etc.
>
> Yes, all are good directions towards better counting (`perf stat`) function.
> But as the original author of RISC-V perf port, please allow me to address
> the fundamental problems of RISC-V perf, again [0][1][2][3], that the sampling
> (`perf record`) function never earned enough respect.  Counting gives you a
> shallow view regarding an application, while sampling demystifies one for you.
>
> The problems are three-fold
> (1) Interrupt
> Sampling in perf requires that a HPM raises an interrupt when it overflows.
> Making RISC-V perf platform driver or not has nothing to do with this.  This
> requires more discussions in TGs.
> (2) S-mode access to PMU CSRs
> This is also addressed in this patch set but to me, it is kind of like a
> SBI-solves-them-all mindset to me.  Perf event is for performance monitoring
> thus we should eliminate any possible overhead if we can.  Setting event masks
> through SBI calls for counting maybe OK, but if we really take sampling and
> interrupt handling into consideration, it is questionable if it is still a
> viable way.
> (3) Registers, registers, registers
> There is just no enough CSR/function for perf sampling. The previous proposal
> explains why [2].
>
> Perf sampling is off-topic but somehow related, so I bring it up here just
> for your information.
>

Agree, sampling is an important measurement for perf, we should integrate it
to perf as soon as possible after overflow interrupt mechanism is standardized.

> As this patch set goes v2, the PMU porting guide in [0] should be removed since
> it contains no useful information anymore.
>

It seems that the document mentioned some hook functions, it is good for me to
reserve this document, maybe we could try to give some modification. I
would check that. Thanks

> [0] Documentation/riscv/pmu.rst
> [1] https://www.youtube.com/watch?v=Onvlcl4e2IU
> [2] https://github.com/riscv/riscv-isa-manual/issues/402
>     This proposal has been posted in Privileged Spec Task Group, in
> https://lists.riscv.org/g/tech-privileged-archive/message/488?p=,,,20,0,0,0::Created,,Proposal,20,2,40,32306071
> but never receive any feedback.
> [3] https://lists.riscv.org/g/tech-unixplatformspec/message/84
>     I intended to discuss [2] in the Unixplatform Spec Task Group at the
> online meeting, but obviously people were too busy knowing who the new
> RISC-V CTO is and what he has done to even follow the agenda.
>