Re: [PATCH] mm: vmscan: fix extreme overreclaim and swap floods

[Yosry Ahmed]

On Mon, Aug 8, 2022 at 6:54 AM Yosry Ahmed <yosryahmed@xxxxxxxxxx> wrote:
>
> On Tue, Aug 2, 2022 at 9:28 AM Johannes Weiner <hannes@xxxxxxxxxxx> wrote:
> >
> > During proactive reclaim, we sometimes observe severe overreclaim,
> > with several thousand times more pages reclaimed than requested.
> >
> > This trace was obtained from shrink_lruvec() during such an instance:
> >
> >     prio:0 anon_cost:1141521 file_cost:7767
> >     nr_reclaimed:4387406 nr_to_reclaim:1047 (or_factor:4190)
> >     nr=[7161123 345 578 1111]
> >
> > While he reclaimer requested 4M, vmscan reclaimed close to 16G, most
> > of it by swapping. These requests take over a minute, during which the
> > write() to memory.reclaim is unkillably stuck inside the kernel.
> >
> > Digging into the source, this is caused by the proportional reclaim
> > bailout logic. This code tries to resolve a fundamental conflict: to
> > reclaim roughly what was requested, while also aging all LRUs fairly
> > and in accordance to their size, swappiness, refault rates etc. The
> > way it attempts fairness is that once the reclaim goal has been
> > reached, it stops scanning the LRUs with the smaller remaining scan
> > targets, and adjusts the remainder of the bigger LRUs according to how
> > much of the smaller LRUs was scanned. It then finishes scanning that
> > remainder regardless of the reclaim goal.
> >
> > This works fine if priority levels are low and the LRU lists are
> > comparable in size. However, in this instance, the cgroup that is
> > targeted by proactive reclaim has almost no files left - they've
> > already been squeezed out by proactive reclaim earlier - and the
> > remaining anon pages are hot. Anon rotations cause the priority level
> > to drop to 0, which results in reclaim targeting all of anon (a lot)
> > and all of file (almost nothing). By the time reclaim decides to bail,
> > it has scanned most or all of the file target, and therefor must also
> > scan most or all of the enormous anon target. This target is thousands
> > of times larger than the reclaim goal, thus causing the overreclaim.
> >
> > The bailout code hasn't changed in years, why is this failing now?
> > The most likely explanations are two other recent changes in anon
> > reclaim:
> >
> > 1. Before the series starting with commit 5df741963d52 ("mm: fix LRU
> >    balancing effect of new transparent huge pages"), the VM was
> >    overall relatively reluctant to swap at all, even if swap was
> >    configured. This means the LRU balancing code didn't come into play
> >    as often as it does now, and mostly in high pressure situations
> >    where pronounced swap activity wouldn't be as surprising.
> >
> > 2. For historic reasons, shrink_lruvec() loops on the scan targets of
> >    all LRU lists except the active anon one, meaning it would bail if
> >    the only remaining pages to scan were active anon - even if there
> >    were a lot of them.
> >
> >    Before the series starting with commit ccc5dc67340c ("mm/vmscan:
> >    make active/inactive ratio as 1:1 for anon lru"), most anon pages
> >    would live on the active LRU; the inactive one would contain only a
> >    handful of preselected reclaim candidates. After the series, anon
> >    gets aged similarly to file, and the inactive list is the default
> >    for new anon pages as well, making it often the much bigger list.
> >
> >    As a result, the VM is now more likely to actually finish large
> >    anon targets than before.
> >
> > Change the code such that only one SWAP_CLUSTER_MAX-sized nudge toward
> > the larger LRU lists is made before bailing out on a met reclaim goal.
> >
> > This fixes the extreme overreclaim problem.
> >
> > Fairness is more subtle and harder to evaluate. No obvious misbehavior
> > was observed on the test workload, in any case. Conceptually, fairness
> > should primarily be a cumulative effect from regular, lower priority
> > scans. Once the VM is in trouble and needs to escalate scan targets to
> > make forward progress, fairness needs to take a backseat. This is also
> > acknowledged by the myriad exceptions in get_scan_count(). This patch
> > makes fairness decrease gradually, as it keeps fairness work static
> > over increasing priority levels with growing scan targets. This should
> > make more sense - although we may have to re-visit the exact values.
> >
> > Signed-off-by: Johannes Weiner <hannes@xxxxxxxxxxx>
> > ---
> >  mm/vmscan.c | 10 ++++------
> >  1 file changed, 4 insertions(+), 6 deletions(-)
> >
> > diff --git a/mm/vmscan.c b/mm/vmscan.c
> > index f7d9a683e3a7..1cc0c6666787 100644
> > --- a/mm/vmscan.c
> > +++ b/mm/vmscan.c
> > @@ -2897,8 +2897,8 @@ static void shrink_lruvec(struct lruvec *lruvec, struct scan_control *sc)
> >         enum lru_list lru;
> >         unsigned long nr_reclaimed = 0;
> >         unsigned long nr_to_reclaim = sc->nr_to_reclaim;
> > +       bool proportional_reclaim;
> >         struct blk_plug plug;
> > -       bool scan_adjusted;
> >
> >         get_scan_count(lruvec, sc, nr);
> >
> > @@ -2916,8 +2916,8 @@ static void shrink_lruvec(struct lruvec *lruvec, struct scan_control *sc)
> >          * abort proportional reclaim if either the file or anon lru has already
> >          * dropped to zero at the first pass.
> >          */
> > -       scan_adjusted = (!cgroup_reclaim(sc) && !current_is_kswapd() &&
> > -                        sc->priority == DEF_PRIORITY);
> > +       proportional_reclaim = (!cgroup_reclaim(sc) && !current_is_kswapd() &&
> > +                               sc->priority == DEF_PRIORITY);
> >
> >         blk_start_plug(&plug);
> >         while (nr[LRU_INACTIVE_ANON] || nr[LRU_ACTIVE_FILE] ||
> > @@ -2937,7 +2937,7 @@ static void shrink_lruvec(struct lruvec *lruvec, struct scan_control *sc)
> >
> >                 cond_resched();
> >
> > -               if (nr_reclaimed < nr_to_reclaim || scan_adjusted)
> > +               if (nr_reclaimed < nr_to_reclaim || proportional_reclaim)
> >                         continue;
> >
> >                 /*
> > @@ -2988,8 +2988,6 @@ static void shrink_lruvec(struct lruvec *lruvec, struct scan_control *sc)
> >                 nr_scanned = targets[lru] - nr[lru];
> >                 nr[lru] = targets[lru] * (100 - percentage) / 100;
> >                 nr[lru] -= min(nr[lru], nr_scanned);
> > -
> > -               scan_adjusted = true;
>
> Thanks for the great analysis of the problem!
>
> I have a question here. This fixes the overreclaim problem for
> proactive reclaim (and most other scenarios), but what about the case
> where proportional_reclaim (aka scan_adjusted) is set before we start
> shrinking lrus: global direct reclaim on DEF_PRIORITY? If we hit a
> memcg that has very few file pages and a ton of anon pages in this
> scenario (or vice versa), wouldn't we still overreclaim and possibly
> stall unnecessarily? or am I missing something here?

Never mind :) In this scenario we will keep iterating the LRUs anyway,
we don't attempt to make the scanning proportional. I guess the new
name (proportional_reclaim) confused me :)

>
> >         }
> >         blk_finish_plug(&plug);
> >         sc->nr_reclaimed += nr_reclaimed;
> > --
> > 2.37.1
> >
> >