Re: [RFC PATCH -next 1/2] mm/mglru: use mem_cgroup_iter for global reclaim

[Shakeel Butt <shakeel.butt@linux.dev>]

Hi Chen,

On Thu, Dec 04, 2025 at 12:31:23PM +0000, Chen Ridong wrote:
> From: Chen Ridong <chenridong@huawei.com>
> 
> The memcg LRU was originally introduced for global reclaim to enhance
> scalability. However, its implementation complexity has led to performance
> regressions when dealing with a large number of memory cgroups [1].
> 
> As suggested by Johannes [1], this patch adopts mem_cgroup_iter with
> cookie-based iteration for global reclaim, aligning with the approach
> already used in shrink_node_memcgs. This simplification removes the
> dedicated memcg LRU tracking while maintaining the core functionality.
> 
> It performed a stress test based on Zhao Yu's methodology [2] on a
> 1 TB, 4-node NUMA system. The results are summarized below:
> 
> 					memcg LRU    memcg iter
> stddev(pgsteal) / mean(pgsteal)            91.2%         75.7%
> sum(pgsteal) / sum(requested)             216.4%        230.5%
> 
> The new implementation demonstrates a significant improvement in
> fairness, reducing the standard deviation relative to the mean by
> 15.5 percentage points. While the reclaim accuracy shows a slight
> increase in overscan (from 85086871 to 90633890, 6.5%).
> 
> The primary benefits of this change are:
> 1. Simplified codebase by removing custom memcg LRU infrastructure
> 2. Improved fairness in memory reclaim across multiple cgroups
> 3. Better performance when creating many memory cgroups
> 
> [1] https://lore.kernel.org/r/20251126171513.GC135004@cmpxchg.org
> [2] https://lore.kernel.org/r/20221222041905.2431096-7-yuzhao@google.com
> Signed-off-by: Chen Ridong <chenridong@huawei.com>

Thanks a lot of this awesome work.

> ---
>  mm/vmscan.c | 117 ++++++++++++++++------------------------------------
>  1 file changed, 36 insertions(+), 81 deletions(-)
> 
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index fddd168a9737..70b0e7e5393c 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -4895,27 +4895,14 @@ static bool try_to_shrink_lruvec(struct lruvec *lruvec, struct scan_control *sc)
>  	return nr_to_scan < 0;
>  }
>  
> -static int shrink_one(struct lruvec *lruvec, struct scan_control *sc)
> +static void shrink_one(struct lruvec *lruvec, struct scan_control *sc)
>  {
> -	bool success;
>  	unsigned long scanned = sc->nr_scanned;
>  	unsigned long reclaimed = sc->nr_reclaimed;
> -	struct mem_cgroup *memcg = lruvec_memcg(lruvec);
>  	struct pglist_data *pgdat = lruvec_pgdat(lruvec);
> +	struct mem_cgroup *memcg = lruvec_memcg(lruvec);
>  
> -	/* lru_gen_age_node() called mem_cgroup_calculate_protection() */
> -	if (mem_cgroup_below_min(NULL, memcg))
> -		return MEMCG_LRU_YOUNG;
> -
> -	if (mem_cgroup_below_low(NULL, memcg)) {
> -		/* see the comment on MEMCG_NR_GENS */
> -		if (READ_ONCE(lruvec->lrugen.seg) != MEMCG_LRU_TAIL)
> -			return MEMCG_LRU_TAIL;
> -
> -		memcg_memory_event(memcg, MEMCG_LOW);
> -	}
> -
> -	success = try_to_shrink_lruvec(lruvec, sc);
> +	try_to_shrink_lruvec(lruvec, sc);
>  
>  	shrink_slab(sc->gfp_mask, pgdat->node_id, memcg, sc->priority);
>  
> @@ -4924,86 +4911,55 @@ static int shrink_one(struct lruvec *lruvec, struct scan_control *sc)
>  			   sc->nr_reclaimed - reclaimed);
>  
>  	flush_reclaim_state(sc);

Unrealted to your patch but why this flush_reclaim_state() is at
different place from the non-MGLRU code path?

> -
> -	if (success && mem_cgroup_online(memcg))
> -		return MEMCG_LRU_YOUNG;
> -
> -	if (!success && lruvec_is_sizable(lruvec, sc))
> -		return 0;
> -
> -	/* one retry if offlined or too small */
> -	return READ_ONCE(lruvec->lrugen.seg) != MEMCG_LRU_TAIL ?
> -	       MEMCG_LRU_TAIL : MEMCG_LRU_YOUNG;
>  }
>  
>  static void shrink_many(struct pglist_data *pgdat, struct scan_control *sc)

This function kind of become very similar to shrink_node_memcgs()
function other than shrink_one vs shrink_lruvec. Can you try to combine
them and see if it looks not-ugly? Otherwise the code looks good to me.