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

[Alan Kao]

Tue, Jun 30, 2020 at 06:02:43PM -0700, Atish Patra wrote:
> On Tue, Jun 30, 2020 at 5:52 PM 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.
> >
> > As this patch set goes v2, the PMU porting guide in [0] should be removed since
> > it contains no useful information anymore.
> >
> > [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.
> >
> 
> Sorry. The last meeting's agenda was derailed for numerous reasons.
> Are you okay with discussing this during the next meeting ?
> I have not scheduled one yet but will probably schedule it on next
> Wednesday (8th July) if there is no objection.
> I can check with Anup if he can present the SBI PMU extension as well.

Thanks for the oppertunity. 
But I don't think that the time is enough for every important topic to be
covered.  What I provided in the previous citation [2] is a proposal,
which need expert to judge and critique after thorough reading.

The TG Chair should decide the priority of the items.  If there is any chance
for our proposal, I can give brief introductions.

> 
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> 
> 
> 
> -- 
> Regards,
> Atish