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From: Zong Li <zong.li@sifive.com>
To: Alan Kao <alankao@andestech.com>
Cc: linux-riscv <linux-riscv@lists.infradead.org>,
	Palmer Dabbelt <palmer@dabbelt.com>,
	"linux-kernel@vger.kernel.org List"
	<linux-kernel@vger.kernel.org>,
	Paul Walmsley <paul.walmsley@sifive.com>
Subject: Re: [RFC PATCH 0/6] Support raw event and DT for perf on RISC-V
Date: Wed, 1 Jul 2020 11:15:27 +0800
Message-ID: <CANXhq0o322Pa1kUXjBb4b21Zxrz+zXB5D7Ljyj18j_=TMKfeVg@mail.gmail.com> (raw)
In-Reply-To: <20200701005129.GA27962@andestech.com>

On Wed, Jul 1, 2020 at 8:52 AM Alan Kao <alankao@andestech.com> 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.
>

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  parent reply index

Thread overview: 38+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-06-29  3:19 Zong Li
2020-06-29  3:19 ` [RFC PATCH 1/6] dt-bindings: riscv: Add YAML documentation for PMU Zong Li
2020-06-29  4:09   ` Anup Patel
2020-06-29  4:28     ` Zong Li
2020-06-29  4:37       ` Anup Patel
2020-06-29  6:35         ` Zong Li
2020-06-29  8:31           ` Anup Patel
2020-07-01  3:22             ` Zong Li
2020-06-29  3:19 ` [RFC PATCH 2/6] riscv: dts: sifive: Add DT support " Zong Li
2020-06-29  3:19 ` [RFC PATCH 3/6] riscv: add definition of hpmcounter CSRs Zong Li
2020-06-29  3:19 ` [RFC PATCH 4/6] riscv: perf: Add raw event support Zong Li
2020-06-29  4:17   ` Anup Patel
2020-06-29  4:35     ` Zong Li
2020-06-29  4:40       ` Anup Patel
2020-06-29  3:19 ` [RFC PATCH 5/6] riscv: perf: introduce DT mechanism Zong Li
2020-06-29  4:36   ` Anup Patel
2020-06-29  6:26     ` Zong Li
2020-06-29  3:19 ` [RFC PATCH 6/6] riscv: remove PMU menu of Kconfig Zong Li
2020-06-29  4:52 ` [RFC PATCH 0/6] Support raw event and DT for perf on RISC-V Anup Patel
2020-06-29  5:52   ` Zong Li
2020-06-29  8:27     ` Anup Patel
2020-06-29 12:53       ` Zong Li
2020-06-29 13:23         ` Anup Patel
2020-06-30  6:37           ` Zong Li
2020-06-30  7:39             ` Anup Patel
2020-06-30  8:04               ` Zong Li
2020-06-30 10:18                 ` Anup Patel
2020-06-30 11:38                   ` Anup Patel
2020-06-30 18:57                     ` Atish Patra
2020-07-01  2:14                       ` Zong Li
2020-07-01 11:43                         ` Anup Patel
2020-07-01  2:11                     ` Zong Li
2020-07-01  1:55                   ` Zong Li
2020-07-01  0:51 ` Alan Kao
2020-07-01  1:02   ` Atish Patra
2020-07-01  2:45     ` Alan Kao
2020-07-01  3:15   ` Zong Li [this message]
2020-07-01  4:13   ` Anup Patel

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