On Thu, Nov 17, 2022 at 03:22:42PM -0700, Yu Zhao wrote: > On Thu, Nov 17, 2022 at 12:47 AM Minchan Kim <minchan@xxxxxxxxxx> wrote: > > > > On Tue, Nov 15, 2022 at 06:38:07PM -0700, Yu Zhao wrote: > > > The page reclaim isolates a batch of folios from the tail of one of > > > the LRU lists and works on those folios one by one. For a suitable > > > swap-backed folio, if the swap device is async, it queues that folio > > > for writeback. After the page reclaim finishes an entire batch, it > > > puts back the folios it queued for writeback to the head of the > > > original LRU list. > > > > > > In the meantime, the page writeback flushes the queued folios also by > > > batches. Its batching logic is independent from that of the page > > > reclaim. For each of the folios it writes back, the page writeback > > > calls folio_rotate_reclaimable() which tries to rotate a folio to the > > > tail. > > > > > > folio_rotate_reclaimable() only works for a folio after the page > > > reclaim has put it back. If an async swap device is fast enough, the > > > page writeback can finish with that folio while the page reclaim is > > > still working on the rest of the batch containing it. In this case, > > > that folio will remain at the head and the page reclaim will not retry > > > it before reaching there. > > > > > > This patch adds a retry to evict_folios(). After evict_folios() has > > > finished an entire batch and before it puts back folios it cannot free > > > immediately, it retries those that may have missed the rotation. > > > > Can we make something like this? > > This works for both the active/inactive LRU and MGLRU. I hope we fix both altogether. > > But it's not my prefered way because of these two subtle differences: > 1. Folios eligible for retry take an unnecessary round trip below -- > they are first added to the LRU list and then removed from there for > retry. For high speed swap devices, the LRU lock contention is already > quite high (>10% in CPU profile under heavy memory pressure). So I'm > hoping we can avoid this round trip. > 2. The number of retries of a folio on folio_wb_list is unlimited, > whereas this patch limits the retry to one. So in theory, we can spin > on a bunch of folios that keep failing. > > The most ideal solution would be to have the one-off retry logic in > shrink_folio_list(). But right now, that function is very cluttered. I > plan to refactor it (low priority at the moment), and probably after > that, we can add a generic retry for both the active/inactive LRU and > MGLRU. I'll raise its priority if you strongly prefer this. Please > feel free to let me know. Well, my preference for *ideal solution* is writeback completion drops page immediately without LRU rotating. IIRC, concern was softirq latency and locking relevant in the context at that time when I tried it.
