On Wed, Jan 17, 2024 at 1:52 AM Zhongkun He <hezhongkun.hzk@xxxxxxxxxxxxx> wrote: > > > > > > > Please forgive me for adding additional information about this patch. > > > > > > I have finished the opt for introducing a folio_add_lru_tail(), but > > > there are many > > > questions: > > > 1) A new page can be move to LRU only by lru_add_fn, so > > > folio_add_lru_tail could not add pages to LRU for the following code > > > in folio_batch_move_lru(),which is added by Alex Shi for > > > serializing memcg changes in pagevec_lru_move_fn[1]. > > > > > > /* block memcg migration while the folio moves between lru */ > > > if (move_fn != lru_add_fn && !folio_test_clear_lru(folio)) > > > continue; > > > To achieve the goal, we need to add a new function like lru_add_fn > > > which does not have the lru flag and folio_add_lru_tail() > > > + if (move_fn != lru_add_fn && move_fn != lru_move_tail_fn_new && > > > + !folio_test_clear_lru(folio)) Hmm yeah, I guess it is a bit more plumbing to do. I prefer this though - not very fond of hacking current's flag just for a small optimization :) And I'd argue this is the "right" thing to do - draining the other LRU operation batches just so that we can successfully perform an add-to-tail seems hacky and wrong to me. > > > > > > 2) __read_swap_cache_async has six parameters, so there is no space to > > > add a new one, add_to_lru_head. Matthew's solution seems fine to me, no? i.e using a single flag parameter to encapsulate all boolean arguments. > > > > > > So it seems a bit hacky just for a special case for the reasons above. > > > > It's a lot of plumbing for sure. Adding a flag to current task_struct > > is a less-noisy yet-still-hacky solution. I am not saying we should do > > it, but it's an option. I am not sure how much task flags we have to > > spare. > > Got it. > > > > > > > > Back to the beginning, lru_add_drain() is the simplest option，which is common > > > below the __read_swap_cache_async(). Please see the function > > > swap_cluster_readahead() > > > and swap_vma_readahead(), of course it has been batched. > > > > > > Or we should leave this problem alone，before we can write back zswap > > > in batches. > > > > Calling lru_add_drain() for every written back page is an overkill > > imo. If we have writeback batching at some point, it may make more > > sense then. > > Agree. Agree. lru_add_drain() does quite a bit, and doing it for every written page makes batching less effective. And as argued above, I don't think we should do this. I'm fine with waiting til writeback batching too :) But that will be a bigger task. > > > > > Adding Michal Hocko was recently complaining [1] about lru_add_drain() > > being called unnecessarily elsewhere. > > Got it, thanks. > > > > [1]https://lore.kernel.org/linux-mm/ZaD9BNtXZfY2UtVI@tiehlicka/
