RE: [EXT] Re: [RFC PATCH 0/4] CXL Hotness Monitoring Unit perf driver

[Ajay Joshi <ajayjoshi@micron.com>]

Micron Confidential


Micron Confidential
> From: Jonathan Cameron <Jonathan.Cameron@huawei.com>
> Sent: Wednesday, November 27, 2024 10:05 PM
>
>
> On Thu, 21 Nov 2024 10:18:41 +0000
> Jonathan Cameron <Jonathan.Cameron@huawei.com> wrote:
>
> > The CXL specification release 3.2 is now available under a click
> > through at
> >
> https://nam10.safelinks.protection.outlook.com/?url=https%3A%2F%2Fcom
> p
> > uteexpresslink.org%2Fcxl-
> specification%2F&data=05%7C02%7Cajayjoshi%40micron.com%7Ce59092c
> 80eed4878d9cc08dd0f016a78%7Cf38a5ecd28134862b11bac1d563c806f%
> 7C0%7C0%7C638683221020661525%7CUnknown%7CTWFpbGZsb3d8eyJF
> bXB0eU1hcGkiOnRydWUsIlYiOiIwLjAuMDAwMCIsIlAiOiJXaW4zMiIsIkFOIjoiT
> WFpbCIsIldUIjoyfQ%3D%3D%7C0%7C%7C%7C&sdata=A6OYPhky94PnkzYn
> 4bfB1usIFDQzR1GlY1QFK3hBVtY%3D&reserved=0 and it brings new shiny
> toys.
>
> If anyone wants to play, basic emulation on my CXL QEMU staging tree
> https://nam10.safelinks.protection.outlook.com/?url=https%3A%2F%2Fgitla
> b.com%2Fjic23%2Fqemu%2F-
> %2Fcommit%2Fe89b35d264c1bcc04807e7afab1254f35ffc8cb9&data=05%7
> C02%7Cajayjoshi%40micron.com%7Ce59092c80eed4878d9cc08dd0f016a7
> 8%7Cf38a5ecd28134862b11bac1d563c806f%7C0%7C0%7C638683221020
> 676260%7CUnknown%7CTWFpbGZsb3d8eyJFbXB0eU1hcGkiOnRydWUsIlYi
> OiIwLjAuMDAwMCIsIlAiOiJXaW4zMiIsIkFOIjoiTWFpbCIsIldUIjoyfQ%3D%3D
> %7C0%7C%7C%7C&sdata=Un0fB5v%2BBKTnQPldKKoRwOpw9GrGdDwBrXm
> JamKEIvA%3D&reserved=0

This is interesting. We are definitely trying this and let you know how it goes.

>
> Branch with a few other things on top is:
> https://nam10.safelinks.protection.outlook.com/?url=https%3A%2F%2Fgitla
> b.com%2Fjic23%2Fqemu%2F-%2Fcommits%2Fcxl-2024-11-
> 27&data=05%7C02%7Cajayjoshi%40micron.com%7Ce59092c80eed4878d9
> cc08dd0f016a78%7Cf38a5ecd28134862b11bac1d563c806f%7C0%7C0%7C
> 638683221020684284%7CUnknown%7CTWFpbGZsb3d8eyJFbXB0eU1hcGk
> iOnRydWUsIlYiOiIwLjAuMDAwMCIsIlAiOiJXaW4zMiIsIkFOIjoiTWFpbCIsIldUIj
> oyfQ%3D%3D%7C0%7C%7C%7C&sdata=V451%2BM9UKiC0RfBUviNTY3fZH
> UGHdjJEgGuR0DowJZM%3D&reserved=0
>
> 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
> >
> > $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
> >