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. > > 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) FWIW, runs on x86. However, it triggers a lockdep warning in both start and stop due to the spin lock. Something to tidy up for RFCv2. J > > 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 >
