On Thu, 21 Nov 2024 10:18:41 +0000 Jonathan Cameron <Jonathan.Cameron@xxxxxxxxxx> 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. PoC of qemu plugin based approach to getting real data: https://lore.kernel.org/qemu-devel/20250124172905.84099-1-Jonathan.Cameron@xxxxxxxxxx/ Also available on gitlab.com/jic23/qemu cxl-2025-01-24 I have a minor revision to this driver to post after a spec clarification on units of one parameter. Will do that next week but for now that PoC ignores the parameter anyway :) Hopefully a cleaned up version of the above will provide us with a useful platform for algorithm and framework development. I've posted it early as there is a fundamental question for the QEMU maintainers on how they would prefer the plugin interaction to be done - once that's resolved it's just a question of wiring up more parameters and providing additional options for the actual counting implementation. For now it's an oracle with 32 bit counters for every page. Hardware folk tell me that might be a 'little too expensive'! It's not even that slow :) 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 >
