On Thu, Nov 21, 2024 at 02:58:52PM +0000, Jonathan Cameron wrote: > On Thu, 21 Nov 2024 09:24:43 -0500 > Gregory Price <gourry@xxxxxxxxxx> wrote: > > > On Thu, Nov 21, 2024 at 10:18:41AM +0000, Jonathan Cameron 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. > > > > FWIW this is what i was thinking about for this extension: > > > > https://lore.kernel.org/all/20240319172609.332900-1-gregory.price@xxxxxxxxxxxx/ > > Yup. I had that in mind. Forgot to actually add a link. > > > > > At least for testing CHMU stuff. So if anyone is poking at testing such > > things, they can feel free to use that for prototyping. However, I think > > there is general discomfort around userspace handling HPA/DPA. > > > > So it might look more like > > > > echo nr_pages > /sys/.../tiering/nodeN/promote_pages > > > > rather than handling the raw data from the CHMU to make decisions. > > Agreed, but I think we are far away from a point where we can implement that. > > Just working out how to tune the hardware to grab useful data is going > to take a while to figure out, let alone doing anything much with it. > > Without care you won't get a meaningful signal for what is actually > hot out of the box. Lots of reasons why including: > a) Exhaustion of tracking resources, due to looking at too large a window > or for too long. Will probably need some form of auto updating of > what is being scanning (coarse to fine might work though I'm doubtful, > scanning across small regions maybe). > b) Threshold too high, no detections. > c) Threshold too low, everything hot. > d) Wrong timescales. Hot is not a well defined thing. > e) Hardware that won't do tracking at fine enough granularity. > f) How does this even work with interleaving on larger pools :B It's pretend-addressing all the way down :D Lots of conceptually complex and fun questions here. ~Gregory
