Message-ID: <20230317131131.00005817@Huawei.com>
Date:   Fri, 17 Mar 2023 13:11:31 +0000
From:   Jonathan Cameron <Jonathan.Cameron@...wei.com>
To:     Jie Zhan <zhanjie9@...ilicon.com>
CC:     <will@...nel.org>, <mark.rutland@....com>,
        <mathieu.poirier@...aro.org>, <suzuki.poulose@....com>,
        <mike.leach@...aro.org>, <leo.yan@...aro.org>,
        <john.g.garry@...cle.com>, <james.clark@....com>,
        <peterz@...radead.org>, <mingo@...hat.com>, <acme@...nel.org>,
        <corbet@....net>, <zhangshaokun@...ilicon.com>,
        <shenyang39@...wei.com>, <hejunhao3@...wei.com>,
        <yangyicong@...ilicon.com>, <prime.zeng@...wei.com>,
        <suntao25@...wei.com>, <jiazhao4@...ilicon.com>,
        <linuxarm@...wei.com>, <linux-doc@...r.kernel.org>,
        <linux-kernel@...r.kernel.org>,
        <linux-arm-kernel@...ts.infradead.org>,
        <linux-perf-users@...r.kernel.org>, Rob Herring <robh@...nel.org>
Subject: Re: [RFC PATCH v1 0/4] HiSilicon Performance Monitor Control Unit

On Mon, 27 Feb 2023 16:49:46 +0800
Jie Zhan <zhanjie9@...ilicon.com> wrote:

> Please can anyone have a look at this PMCU patchset and provide some 
> comments?
> 
> It is much related to the ARM PMU.
> 
> We are looking forward to the feedback.
> 
> Any relevant comments/questions, with respect to software or hardware 
> design, use cases, coding, are welcome.
> 
> Kind regards,
> 
> Jie
> 
> 
> On 06/02/2023 14:51, Jie Zhan wrote:
> > HiSilicon Performance Monitor Control Unit (PMCU) is a device that offloads
> > PMU accesses from CPUs, handling the configuration, event switching, and
> > counter reading of core PMUs on Kunpeng SoC. It facilitates fine-grained
> > and multi-PMU-event CPU profiling, in which scenario the current 'perf'
> > scheme may lose events or drop sampling frequency. With PMCU, users can
> > reliably obtain the data of up to 240 PMU events with the sample interval
> > of events down to 1ms, while the software overhead of accessing PMUs, as
> > well as its impact on target workloads, is reduced.
> >
> > This patchset contains the documentation, driver, and user perf tool
> > support to enable using PMCU with the 'perf_event' framework.
> >
> > Here are two key questions requested for comments:
> >
> > - How do we make it compatible with arm_pmu drivers?
> >
> >    Hardware-wise, PMCU uses the existing core PMUs, so PMUs can be accessed
> >    from CPU and PMCU simultaneously. The current hardware can't guarantee
> >    mutual exclusive accesses. Hence, scheduling arm_pmu and PMCU events at
> >    the same time may mess up the operation of PMUs, delivering incorrect
> >    data for both events, e.g. unexpected events or sample periods.
> >    Software-wise, we probably need to prevent the two types of events from
> >    running at the same time, but currently there isn't a clear solution.

I've been thinking about this a bit and don't have a good answer yet.

So some thoughts that might get some discussion going (some are here
mostly to be shot down ;)

1. I suspect adding a hook into the specific pmu driver to reserve a counter is going
   to be controversial for this usecase.  But maybe there is a more generic
   way...  There are lock up detectors that use PMU counters and ensure the counters
   aren't also used for other purposes and that leads me to wonder if you can use
https://elixir.bootlin.com/linux/latest/source/kernel/events/core.c#L12700
perf_event_create_kernel_counter()
to do the same as opening a counter from userspace but then not use it.
I have no idea if this will work though or if enabling the event would be necessary
to prevent it being used elsewhere.

2. It might be possible to reuse any of the infrastructure that exists
   for userspace PMU counter access or maybe Rob Herring (+CC) has a suggestion based on
   his work on that feature.

3. It's not nice, but maybe could enforce this constraint just in userspace?
   We'd have to make sure that both drivers didn't do anything beyond not working
   correctly if the other driver is messing with the hardware.

4. We can't do the nasty trick of providing a second driver that binds to the
   PMU hardware to prevent it being used because I think the main arm PMU
   driver has suppress_bind_attrs = true.  Maybe we can make remove work?
   (original patch for this in 2018 added that line because of a crash on remove
    - not sure anyone looked at fixing the crash).

> >
> > - Currently we reply on a sysfs file for users to input event numbers. Is
> >    there a better way to pass many events?
> >
> >    The perf framework only allows three 64-bit config fields for custom PMU
> >    configs. Obviously, this can't satisfy our need for passing many events
> >    at a time. As an event number is 16-bit wide, the config fields can only
> >    take up to 12 events at a time, or up to 192 events even if we do a
> >    bitmap of events (and there are more than 192 available event numbers).
> >    Hence, the current design takes an array of event numbers from a sysfs
> >    file before starting profiling. However, this may go against the common
> >    way to schedule perf events through perf commands.
> >
> > Jie Zhan (4):
> >    docs: perf: Add documentation for HiSilicon PMCU
> >    drivers/perf: hisi: Add driver support for HiSilicon PMCU
> >    perf tool: Add HiSilicon PMCU data recording support
> >    perf tool: Add HiSilicon PMCU data decoding support
> >
> >   Documentation/admin-guide/perf/hisi-pmcu.rst |  183 +++
> >   Documentation/admin-guide/perf/index.rst     |    1 +
> >   drivers/perf/hisilicon/Kconfig               |   15 +
> >   drivers/perf/hisilicon/Makefile              |    1 +
> >   drivers/perf/hisilicon/hisi_pmcu.c           | 1096 ++++++++++++++++++
> >   tools/perf/arch/arm/util/auxtrace.c          |   61 +
> >   tools/perf/arch/arm64/util/Build             |    2 +-
> >   tools/perf/arch/arm64/util/hisi-pmcu.c       |  145 +++
> >   tools/perf/util/Build                        |    1 +
> >   tools/perf/util/auxtrace.c                   |    4 +
> >   tools/perf/util/auxtrace.h                   |    1 +
> >   tools/perf/util/hisi-pmcu.c                  |  305 +++++
> >   tools/perf/util/hisi-pmcu.h                  |   19 +
> >   13 files changed, 1833 insertions(+), 1 deletion(-)
> >   create mode 100644 Documentation/admin-guide/perf/hisi-pmcu.rst
> >   create mode 100644 drivers/perf/hisilicon/hisi_pmcu.c
> >   create mode 100644 tools/perf/arch/arm64/util/hisi-pmcu.c
> >   create mode 100644 tools/perf/util/hisi-pmcu.c
> >   create mode 100644 tools/perf/util/hisi-pmcu.h
> >
> >
> > base-commit: 830b3c68c1fb1e9176028d02ef86f3cf76aa2476  

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