Gregory Price <gourry@xxxxxxxxxx> writes: > 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. Sorry, I don't understand this, the above patch changes filemap_get_pages() and grab_cache_page_write_begin() instead of __filemap_get_folio(). > 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. If necessary, we can use a similar method for above solution too. And we can filter accessed once folios with folio_mark_accessed() firstly. That is, only promote a page if, - record the folio access time in folio_mark_accessed() only - when the folio are accessed again, and "access_time - record_time < threshold", promote the folio. > 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. Use a task_work? > Going to RFC this up -- Best Regards, Huang, Ying
