Re: [RFC PATCH v3 0/8] mm: workingset reporting

[Gregory Price]

On Wed, Mar 27, 2024 at 03:53:39PM -0700, Yuanchu Xie wrote:
> On Wed, Mar 27, 2024 at 2:44 PM Gregory Price
> <gregory.price@xxxxxxxxxxxx> wrote:
> >
> > Please note that this proposed interface (move_phys_pages) is very
> > unlikely to be received upstream due to side channel concerns. Instead,
> > it's more likely that the tiering component will expose a "promote X
> > pages from tier A to tier B", and the kernel component would then
> > use/consume hotness information to determine which pages to promote.
> 
> I see that mm/memory-tiers.c only has support for demotion. What kind
> of hotness information do devices typically provide? The OCP proposal
> is not very specific about this.
> A list of hot pages with configurable threshold?
> Access frequency for all pages at configured granularity?
> Is there a way to tell which NUMA node is accessing them, for page promotion?

(caveat: i'm not a memory-tiers maintainer, you may want to poke at them
directly for more information, this is simply spitballing an idea)

I don't know of any public proposals of explicit hotness information
provided by hardware yet, just the general proposal.

For the sake of simplicity, I would make the assumption that you have
the least information possible - a simple list of "hot addresses" in
Host Physcal Address format.

I.e. there's some driver function that amounts to:

uint32_t device_get_hot_addresses(uint64_t *addresses, uint32_t buf_max);

Where the return value is number of addresses the device returned, and
the buf_max is the number of addresses that can be read.

Drives providing this functionality would then register this as a
callback when its memory becomes a member of some numa node.


Re: source node -
Devices have no real way of determining upstream source information.

> >
> > (Just as one example, there are many more realistic designs)
> >
> > So if there is a way to expose workingset data to the mm/memory_tiers.c
> > component instead of via sysfs/cgroup - that is preferable.
> 
> Appreciate the feedback. The data in its current form might be useful
> to inform demotion decisions, but for promotion, are you aware of any
> recent developments? I would like to encode hotness as workingset data
> as well.

There were some recent patches to DAMON about promotion/demotion.  You
might look there.

~Gregory