Re: [PATCH 1/2] mm: multi-gen LRU: retry folios written back while isolated

[Minchan Kim <minchan@kernel.org>]

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@kernel.org> 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.