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

[Yosry Ahmed]

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?

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