On Tue, Nov 05, 2024 at 10:00:59AM +0800, Huang, Ying wrote: > Hi, Gregory, > >> > >> Several years ago, we have tried to use the access time tracking > >> mechanism of NUMA balancing to track the access time latency of unmapped > >> file cache folios. The original implementation is as follows, > >> > >> https://git.kernel.org/pub/scm/linux/kernel/git/vishal/tiering.git/commit/?h=tiering-0.8&id=5f2e64ce75c0322602c2ec8c70b64bb69b1f1329 > >> > >> What do you think about this? > >> > > > > Coming back around to explore this topic a bit more, dug into this old > > patch and the LRU patch by Keith - I'm struggling find a good option > > that doesn't over-complicate or propose something contentious. > > > > > > I did a browse through lore and did not see any discussion on this patch > > or on Keith's LRU patch, so i presume discussion on this happened largely > > off-list. So if you have any context as to why this wasn't RFC'd officially > > I would like more information. > > Thanks for doing this. There's no much discussion offline. We just > don't have enough time to work on the solution. > Exploring and testing this a little further, I brought this up to current folio work in 6.9 and found this solution to be unstable as-is. After some work to fix lock/reference issues, Johannes pointed out that __filemap_get_folio can be called from an atomic context - which means it may not be safe to do migrations in this context. We're back to looking at something like an LRU-esque system, but now we're thinking about isolating the folios in folio_mark_accessed into a task-local list, and then process the list on resume. Basically we're thinking 1) hook folio_mark_accessed and use PG_ACTIVE/PG_ACCESSED to determine whether the page is a promotion candidate. 2) if it is, isolate it from the LRU - which is safe because folio_mark_accessed already does this elsewhere, and place it onto current->promo_queue 3) set_notify_resume 4) add logic to resume_user_mode_work() to run through current->promo_queue and either promote the pages accordingly, or do folio_putback_lru on failure. Going to RFC this up ~Gregory
