[rfc] drm/ttm/memcg: simplest initial memcg/ttm integration (v2)

[Dave Airlie]

On Thu, 22 May 2025 at 00:43, Johannes Weiner <hannes at cmpxchg.org> wrote:
>
> On Wed, May 21, 2025 at 12:23:58PM +1000, Dave Airlie wrote:
> > >
> > > So in the GPU case, you'd charge on allocation, free objects into a
> > > cgroup-specific pool, and shrink using a cgroup-specific LRU
> > > list. Freed objects can be reused by this cgroup, but nobody else.
> > > They're reclaimed through memory pressure inside the cgroup, not due
> > > to the action of others. And all allocated memory is accounted for.
> > >
> > > I have to admit I'm pretty clueless about the gpu driver internals and
> > > can't really judge how feasible this is. But from a cgroup POV, if you
> > > want proper memory isolation between groups, it seems to me that's the
> > > direction you'd have to take this in.
> >
> > I've been digging into this a bit today, to try and work out what
> > various paths forward might look like and run into a few impedance
> > mismatches.
> >
> > 1. TTM doesn't pool objects, it pools pages. TTM objects are varied in
> > size, we don't need to keep any sort of special allocator that we
> > would need if we cached sized objects (size buckets etc). list_lru
> > doesn't work on pages, if we were pooling the ttm objects I can see
> > being able to enable list_lru. But I'm seeing increased complexity for
> > no major return, but I might dig a bit more into whether caching
> > objects might help.
> >
> > 2. list_lru isn't suitable for pages, AFAICS we have to stick the page
> > into another object to store it in the list_lru, which would mean we'd
> > be allocating yet another wrapper object. Currently TTM uses the page
> > LRU pointer to add it to the shrinker_list, which is simple and low
> > overhead.
>
> Why wouldn't you be able to use the page LRU list_head with list_lru?
>
> list_lru_add(&ttm_pool_lru, &page->lru, page_to_nid(page), page_memcg(page));

I for some reason got it into my head that list_lru objects weren't
list_head, not sure why, guess I shall spend next week exploring this
possibility.

Dave.