Message-ID: <20250115134207.00002918@huawei.com>
Date: Wed, 15 Jan 2025 13:42:07 +0000
From: Jonathan Cameron <Jonathan.Cameron@...wei.com>
To: Neeraj Kumar <s.neeraj@...sung.com>
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Subject: Re: [RFC PATCH 0/4] CXL Hotness Monitoring Unit perf driver

On Fri, 3 Jan 2025 10:57:22 +0530
Neeraj Kumar <s.neeraj@...sung.com> wrote:

> On 27/11/24 04:34PM, Jonathan Cameron wrote:
> >On Thu, 21 Nov 2024 10:18:41 +0000
> >Jonathan Cameron <Jonathan.Cameron@...wei.com> wrote:
> >  
> >> The CXL specification release 3.2 is now available under a click through at
> >> https://computeexpresslink.org/cxl-specification/ and it brings new
> >> shiny toys.  
> >
> >If anyone wants to play, basic emulation on my CXL QEMU staging tree
> >https://gitlab.com/jic23/qemu/-/commit/e89b35d264c1bcc04807e7afab1254f35ffc8cb9
> >
> >Branch with a few other things on top is:
> >https://gitlab.com/jic23/qemu/-/commits/cxl-2024-11-27
> >
> >Note that this currently doesn't produce real data.  I have a plan
> >/ initial PoC / hack to hook that up via an addition to the QEMU cache
> >plugin and an external tool to emulate the hotness tracker counting
> >hardware. Will be a little while before I get that finished, so in
> >a meantime the above exercises the driver.
> >
> >Jonathan
> >  
> >>
> >> RFC reason
> >> - Whilst trace capture with a particular configuration is potentially useful
> >>   the intent is that CXL HMU units will be used to drive various forms of
> >>   hotpage migration for memory tiering setups. This driver doesn't do this
> >>   (yet), but rather provides data capture etc for experimentation and
> >>   for working out how to mostly put the allocations in the right place to
> >>   start with by tuning applications.
> >>
> >> CXL r3.2 introduces a CXL Hotness Monitoring Unit definition. The intent
> >> of this is to provide a way to establish which units of memory (typically
> >> pages or larger) in CXL attached memory are hot. The implementation details
> >> and algorithm are all implementation defined. The specification simply
> >> describes the 'interface' which takes the form of ring buffer of hotness
> >> records in a PCI BAR and defined capability, configuration and status
> >> registers.
> >>
> >> The hardware may have constraints on what it can track, granularity etc
> >> and on how accurately it tracks (e.g. counter exhaustion, inaccurate
> >> trackers). Some of these constraints are discoverable from the hardware
> >> registers, others such as loss of accuracy have no universally accepted
> >> measures as they are typically access pattern dependent. Sadly it is
> >> very unlikely any hardware will implement a truly precise tracker given
> >> the large resource requirements for tracking at a useful granularity.
> >>
> >> There are two fundamental operation modes:
> >>
> >> * Epoch based. Counters are checked after a period of time (Epoch) and
> >>   if over a threshold added to the hotlist.
> >> * Always on. Counters run until a threshold is reached, after that the
> >>   hot unit is added to the hotlist and the counter released.
> >>
> >> Counting can be filtered on:
> >>
> >> * Region of CXL DPA space (256MiB per bit in a bitmap).
> >> * Type of access - Trusted and non trusted or non trusted only, R/W/RW
> >>
> >> Sampling can be modified by:
> >>
> >> * Downsampling including potentially randomized downsampling.
> >>
> >> The driver presented here is intended to be useful in its own right but
> >> also to act as the first step of a possible path towards hotness monitoring
> >> based hot page migration. Those steps might look like.
> >>
> >> 1. Gather data - drivers provide telemetry like solutions to get that
> >>    data. May be enhanced, for example in this driver by providing the
> >>    HPA address rather than DPA Unit Address. Userspace can access enough
> >>    information to do this so maybe not.
> >> 2. Userspace algorithm development, possibly combined with userspace
> >>    triggered migration by PA. Working out how to use different levels
> >>    of constrained hardware resources will be challenging.
> >> 3. Move those algorithms in kernel. Will require generalization across
> >>    different hotpage trackers etc.
> >>
> >> So far this driver just gives access to the raw data. I will probably kick
> >> of a longer discussion on how to do adaptive sampling needed to actually
> >> use these units for tiering etc, sometime soon (if no one one else beats
> >> me too it).  There is a follow up topic of how to virtualize this stuff
> >> for memory stranding cases (VM gets a fixed mixture of fast and slow
> >> memory and should do it's own tiering).
> >>
> >> More details in the Documentation patch but typical commands are:
> >>
> >> $perf record -a  -e cxl_hmu_mem0.0.0/epoch_type=0,access_type=6,\
> >>  hotness_threshold=1024,epoch_multiplier=4,epoch_scale=4,range_base=0,\
> >>  range_size=1024,randomized_downsampling=0,downsampling_factor=32,\
> >>  hotness_granual=12  
> 
> Facing issue while executing perf record on x86 emulation environment using following steps
> 
> 1. Tried applying CHMU Patch on branch cxl-for-6.13 using b4 utility. As
> base commit is not specified, with minor change able to apply patch.
> Compiled kernel with CONFIG_CXL_HMU
> 
> 2. Compiled jic23/cxl-2024-11-27 for x86_64-softmmu
> 
> 3. Launched Qemu with following CXL topology along with compiled kernel
> VM="-object memory-backend-ram,id=vmem1,share=on,size=512M \
>      -device pxb-cxl,bus_nr=12,bus=pcie.0,id=cxl.1 \
>      -device cxl-rp,port=0,bus=cxl.1,id=root_port13,chassis=0,slot=2 \
>      -device cxl-type3,bus=root_port13,volatile-memdev=vmem1,id=cxl-vmem1 \
>      -M cxl-fmw.0.targets.0=cxl.1,cxl-fmw.0.size=4G,cxl-fmw.0.interleave-granularity=8k"
> 
> 4. Created region and onlined this memory. Also run top utility on the newly created 
> numa node using numactl -m<node> top
> 
> 5. Compiled and installed perf utility in qemu environment, and able to
> see cxl_hmu_mem* entries in perf list
> 
> root@...UCXL2030mm:~# perf list
> <snip>
>   cxl_hmu_mem0.0.0/hotness_granual=0..0xffffffff,hotness_threshold=0..0xffffffff,downsampling_factor=0..255,.../modifier[Raw ev>
>   cxl_hmu_mem0.0.1/hotness_granual=0..0xffffffff,hotness_threshold=0..0xffffffff,downsampling_factor=0..255,.../modifier[Raw ev>
>   cxl_hmu_mem0.0.2/hotness_granual=0..0xffffffff,hotness_threshold=0..0xffffffff,downsampling_factor=0..255,.../modifier[Raw ev>
>   cxl_hmu_mem1.0.0/hotness_granual=0..0xffffffff,hotness_threshold=0..0xffffffff,downsampling_factor=0..255,.../modifier[Raw ev>
>   cxl_hmu_mem1.0.1/hotness_granual=0..0xffffffff,hotness_threshold=0..0xffffffff,downsampling_factor=0..255,.../modifier[Raw ev>
>   cxl_hmu_mem1.0.2/hotness_granual=0..0xffffffff,hotness_threshold=0..0xffffffff,downsampling_factor=0..255,.../modifier[Raw ev>
>   cxl_pmu_mem0.0/vid=0..0xffff,edge,mask=0..0xffffffff,.../modifier[Raw event descriptor]
>   cxl_pmu_mem0.1/vid=0..0xffff,edge,mask=0..0xffffffff,.../modifier[Raw event descriptor]
>   cxl_pmu_mem1.0/vid=0..0xffff,edge,mask=0..0xffffffff,.../modifier[Raw event descriptor]
>   cxl_pmu_mem1.1/vid=0..0xffff,edge,mask=0..0xffffffff,.../modifier[Raw event descriptor]
> <snip>
> 
> 6. Tried running perf command mentioned in Documentation/trace/cxl-hmu.rst
> 
> root@...UCXL2030mm:/home/cxl/cxl-linux-mainline/tools/perf# perf -v
> perf version 6.12.rc5.gc198a4f4a356
> root@...UCXL2030mm:/home/cxl/cxl-linux-mainline/tools/perf# perf record -a  -e cxl_hmu_mem0.0.0/epoch_type=0,access_type=6,hotness_threshold=1024,epoch_multiplier=4,epoch_scale=4,range_base=0,range_size=1024,randomized_downsampling=0,downsampling_factor=32,hotness_granual=12
> event syntax error: '..ess_granual=12'
>                                   \___ Unrecognized input

This is probably my mistake when cutting and pasting the example from a terminal.
Add a trailing / and something to run.

 perf record -a  -e cxl_hmu_mem0.0.0/epoch_type=0,access_type=6,hotness_threshold=1024,epoch_multiplier=4,epoch_scale=4,range_base=0,range_size=1024,randomized_downsampling=0,downsampling_factor=32,hotness_granual=12/ -- sleep 10

Jonathan

> 
> 
> 
> Are there any steps i am missing?
> 
> Regards,
> Neeraj	 
> 
> >>
> >> $perf report --dump-raw-traces
> >>
> >> Example output.  With a counter_width of 16 (0x10) the least significant
> >> 4 bytes are the counter value and the unit index is bits 16-63.
> >> Here all units are over the threshold and the indexes are 0,1,2 etc.
> >>
> >> . ... CXL_HMU data: size 33512 bytes
> >> Header 0: units: 29c counter_width 10
> >> Header 1 : deadbeef
> >> 0000000000000283
> >> 0000000000010364
> >> 0000000000020366
> >> 000000000003033c
> >> 0000000000040343
> >> 00000000000502ff
> >> 000000000006030d
> >> 000000000007031a
> >>
> >> Which will produce a list of hotness entries.
> >> Bits[N-1:0] counter value
> >> Bits[63:N] Unit ID (combine with unit size and DPA base + HDM decoder
> >>   config to get to a Host Physical Address)
> >>
> >> Specific RFC questions.
> >> - What should be in the header added to the aux buffer.
> >>   Currently just the minimum is provided. Number of records
> >>   and the counter width needed to decode them.
> >> - Should we reset the counters when doing sampling "-F X"
> >>   If the frequency is higher than the epoch we never see any hot units.
> >>   If so, when should we reset them?
> >>
> >> Note testing has been light and on emulation only + as perf tool is
> >> a pain to build on a striped back VM,  build testing has all be on
> >> arm64 so far.  The driver loads though on both arm64 and x86 so
> >> any problems are likely in the perf tool arch specific code
> >> which is build tested (on wrong machine)
> >>
> >> The QEMU emulation needs some cleanup, but I should be able to post
> >> that shortly to let people actually play with this.  There are lots
> >> of open questions there on how 'right' we want the emulation to be
> >> and what counting uarch to emulate.
> >>
> >> Jonathan Cameron (4):
> >>   cxl: Register devices for CXL Hotness Monitoring Units (CHMU)
> >>   cxl: Hotness Monitoring Unit via a Perf AUX Buffer.
> >>   perf: Add support for CXL Hotness Monitoring Units (CHMU)
> >>   hwtrace: Document CXL Hotness Monitoring Unit driver
> >>
> >>  Documentation/trace/cxl-hmu.rst     | 197 +++++++
> >>  Documentation/trace/index.rst       |   1 +
> >>  drivers/cxl/Kconfig                 |   6 +
> >>  drivers/cxl/Makefile                |   3 +
> >>  drivers/cxl/core/Makefile           |   1 +
> >>  drivers/cxl/core/core.h             |   1 +
> >>  drivers/cxl/core/hmu.c              |  64 ++
> >>  drivers/cxl/core/port.c             |   2 +
> >>  drivers/cxl/core/regs.c             |  14 +
> >>  drivers/cxl/cxl.h                   |   5 +
> >>  drivers/cxl/cxlpci.h                |   1 +
> >>  drivers/cxl/hmu.c                   | 880 ++++++++++++++++++++++++++++
> >>  drivers/cxl/hmu.h                   |  23 +
> >>  drivers/cxl/pci.c                   |  26 +-
> >>  tools/perf/arch/arm/util/auxtrace.c |  58 ++
> >>  tools/perf/arch/x86/util/auxtrace.c |  76 +++
> >>  tools/perf/util/Build               |   1 +
> >>  tools/perf/util/auxtrace.c          |   4 +
> >>  tools/perf/util/auxtrace.h          |   1 +
> >>  tools/perf/util/cxl-hmu.c           | 367 ++++++++++++
> >>  tools/perf/util/cxl-hmu.h           |  18 +
> >>  21 files changed, 1748 insertions(+), 1 deletion(-)
> >>  create mode 100644 Documentation/trace/cxl-hmu.rst
> >>  create mode 100644 drivers/cxl/core/hmu.c
> >>  create mode 100644 drivers/cxl/hmu.c
> >>  create mode 100644 drivers/cxl/hmu.h
> >>  create mode 100644 tools/perf/util/cxl-hmu.c
> >>  create mode 100644 tools/perf/util/cxl-hmu.h
> >>  
> >  
> 

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