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

[Gregory Price]

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/

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.


> 3. Move those algorithms in kernel. Will require generalization across
>    different hotpage trackers etc.
> 

In a longer discussion with Dan, we considered something a little more
abstract - like a system that monitors bandwidth and memory access stalls
and decide to promote X pages from Y device.  This carries a pretty tall
generalization cost, but it's pretty exciting to say the least.

Definitely worth a discussion for later.

>
> 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).
>

Without having looked at the patches yet, I would presume this interface
is at least gated to admin/root? (raw data is physical address info)

~Gregory