Re: [PATCH v4 04/14] mm/mglru: restructure the reclaim loop

[Chen Ridong <chenridong@huaweicloud.com>]

On 2026/4/7 19:57, Kairui Song via B4 Relay wrote:
> From: Kairui Song <kasong@tencent.com>
> 
> The current loop will calculate the scan number on each iteration. The
> number of folios to scan is based on the LRU length, with some unclear
> behaviors, eg, the scan number is only shifted by reclaim priority when
> aging is not needed or when at the default priority, and it couples
> the number calculation with aging and rotation.
> 
> Adjust, simplify it, and decouple aging and rotation. Just calculate the
> scan number for once at the beginning of the reclaim, always respect the
> reclaim priority, and make the aging and rotation more explicit.
> 
> This slightly changes how aging and offline memcg reclaim works:
> Previously, aging was always skipped at DEF_PRIORITY even when
> eviction was impossible. Now, aging is always triggered when it
> is necessary to make progress. The old behavior may waste a reclaim
> iteration only to escalate priority, potentially causing over-reclaim
> of slab and breaking reclaim balance in multi-cgroup setups.
> 
> Similar for offline memcg. Previously, offline memcg wouldn't be
> aged unless it didn't have any evictable folios. Now, we might age
> it if it has only 3 generations and the reclaim priority is less
> than DEF_PRIORITY, which should be fine. On one hand, offline memcg
> might still hold long-term folios, and in fact, a long-existing offline
> memcg must be pinned by some long-term folios like shmem. These folios
> might be used by other memcg, so aging them as ordinary memcg seems
> correct. Besides, aging enables further reclaim of an offlined memcg,
> which will certainly happen if we keep shrinking it. And offline
> memcg might soon be no longer an issue with reparenting.
> 
> And while at it, make it clear that unevictable memcg will get rotated
> so following reclaim will more likely to skip them, as a optimization.
> 
> Overall, the memcg LRU rotation, as described in mmzone.h,
> remains the same.
> 
> Reviewed-by: Axel Rasmussen <axelrasmussen@google.com>
> Signed-off-by: Kairui Song <kasong@tencent.com>
> ---
>  mm/vmscan.c | 69 +++++++++++++++++++++++++++++++------------------------------
>  1 file changed, 35 insertions(+), 34 deletions(-)
> 
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index 963362523782..462ca0fa2ba3 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -4913,49 +4913,36 @@ static int evict_folios(unsigned long nr_to_scan, struct lruvec *lruvec,
>  }
>  
>  static bool should_run_aging(struct lruvec *lruvec, unsigned long max_seq,
> -			     int swappiness, unsigned long *nr_to_scan)
> +			     struct scan_control *sc, int swappiness)
>  {
>  	DEFINE_MIN_SEQ(lruvec);
>  
> -	*nr_to_scan = 0;
>  	/* have to run aging, since eviction is not possible anymore */
>  	if (evictable_min_seq(min_seq, swappiness) + MIN_NR_GENS > max_seq)
>  		return true;
>  
> -	*nr_to_scan = lruvec_evictable_size(lruvec, swappiness);
> +	/* try to get away with not aging at the default priority */
> +	if (sc->priority == DEF_PRIORITY)
> +		return false;
> +
>  	/* better to run aging even though eviction is still possible */
>  	return evictable_min_seq(min_seq, swappiness) + MIN_NR_GENS == max_seq;
>  }
>  
> -/*
> - * For future optimizations:
> - * 1. Defer try_to_inc_max_seq() to workqueues to reduce latency for memcg
> - *    reclaim.
> - */
> -static long get_nr_to_scan(struct lruvec *lruvec, struct scan_control *sc, int swappiness)
> +static long get_nr_to_scan(struct lruvec *lruvec, struct scan_control *sc,
> +			   struct mem_cgroup *memcg, int swappiness)
>  {
> -	bool need_aging;
>  	unsigned long nr_to_scan;
> -	struct mem_cgroup *memcg = lruvec_memcg(lruvec);
> -	DEFINE_MAX_SEQ(lruvec);
> -
> -	if (mem_cgroup_below_min(sc->target_mem_cgroup, memcg))
> -		return -1;
> -
> -	need_aging = should_run_aging(lruvec, max_seq, swappiness, &nr_to_scan);
>  
> +	nr_to_scan = lruvec_evictable_size(lruvec, swappiness);
>  	/* try to scrape all its memory if this memcg was deleted */
> -	if (nr_to_scan && !mem_cgroup_online(memcg))
> +	if (!mem_cgroup_online(memcg))
>  		return nr_to_scan;
>  
>  	nr_to_scan = apply_proportional_protection(memcg, sc, nr_to_scan);
> +	nr_to_scan >>= sc->priority;
>  
> -	/* try to get away with not aging at the default priority */
> -	if (!need_aging || sc->priority == DEF_PRIORITY)
> -		return nr_to_scan >> sc->priority;
> -
> -	/* stop scanning this lruvec as it's low on cold folios */
> -	return try_to_inc_max_seq(lruvec, max_seq, swappiness, false) ? -1 : 0;
> +	return nr_to_scan;
>  }
>  
>  static bool should_abort_scan(struct lruvec *lruvec, struct scan_control *sc)
> @@ -4985,31 +4972,46 @@ static bool should_abort_scan(struct lruvec *lruvec, struct scan_control *sc)
>  	return true;
>  }
>  
> +/*
> + * For future optimizations:
> + * 1. Defer try_to_inc_max_seq() to workqueues to reduce latency for memcg
> + *    reclaim.
> + */
>  static bool try_to_shrink_lruvec(struct lruvec *lruvec, struct scan_control *sc)
>  {
> +	bool need_rotate = false;
>  	long nr_batch, nr_to_scan;
> -	unsigned long scanned = 0;
>  	int swappiness = get_swappiness(lruvec, sc);
> +	struct mem_cgroup *memcg = lruvec_memcg(lruvec);
> +
> +	nr_to_scan = get_nr_to_scan(lruvec, sc, memcg, swappiness);
> +	if (!nr_to_scan)
> +		need_rotate = true;
>  

Will it be simpler if we return directly here?

	if (!nr_to_scan)
		return ture;

I wonder if moving the aging check under `while (nr_to_scan > 0)` can change
behavior when the scan budget gets shifted down to 0.

In the old code, once `should_run_aging()` became true, reclaim could still go
through `try_to_inc_max_seq()` instead of being gated by the priority-shifted
scan budget. With this change, a small lruvec can skip the loop entirely, so a
lruvec that needs aging to make reclaim progress would neither scan nor age in
that reclaim round.

Does this have any observable impact on reclaim progress or reclaim balance,
e.g. by deferring aging until a later retry / higher priority and pushing more
pressure onto other memcgs?

> -	while (true) {
> +	while (nr_to_scan > 0) {
>  		int delta;
> +		DEFINE_MAX_SEQ(lruvec);
>  
> -		nr_to_scan = get_nr_to_scan(lruvec, sc, swappiness);
> -		if (nr_to_scan <= 0)
> +		if (mem_cgroup_below_min(sc->target_mem_cgroup, memcg)) {
> +			need_rotate = true;
>  			break;
> +		}
> +
> +		if (should_run_aging(lruvec, max_seq, sc, swappiness)) {
> +			if (try_to_inc_max_seq(lruvec, max_seq, swappiness, false))
> +				need_rotate = true;
> +			break;
> +		}
>  
>  		nr_batch = min(nr_to_scan, MAX_LRU_BATCH);
>  		delta = evict_folios(nr_batch, lruvec, sc, swappiness);
>  		if (!delta)
>  			break;
>  
> -		scanned += delta;
> -		if (scanned >= nr_to_scan)
> -			break;
> -
>  		if (should_abort_scan(lruvec, sc))
>  			break;
>  
> +		nr_to_scan -= delta;
>  		cond_resched();
>  	}
>  
> @@ -5035,8 +5037,7 @@ static bool try_to_shrink_lruvec(struct lruvec *lruvec, struct scan_control *sc)
>  			reclaim_throttle(pgdat, VMSCAN_THROTTLE_WRITEBACK);
>  	}
>  
> -	/* whether this lruvec should be rotated */
> -	return nr_to_scan < 0;
> +	return need_rotate;
>  }
>  
>  static int shrink_one(struct lruvec *lruvec, struct scan_control *sc)
> 

-- 
Best regards,
Ridong