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Message-ID: <c0c69863-52f8-44ed-8d85-97531caf4dea@huaweicloud.com>
Date: Mon, 8 Dec 2025 11:10:36 +0800
From: Chen Ridong <chenridong@...weicloud.com>
To: Shakeel Butt <shakeel.butt@...ux.dev>
Cc: akpm@...ux-foundation.org, axelrasmussen@...gle.com, yuanchu@...gle.com,
weixugc@...gle.com, david@...nel.org, lorenzo.stoakes@...cle.com,
Liam.Howlett@...cle.com, vbabka@...e.cz, rppt@...nel.org, surenb@...gle.com,
mhocko@...e.com, corbet@....net, hannes@...xchg.org,
roman.gushchin@...ux.dev, muchun.song@...ux.dev, yuzhao@...gle.com,
zhengqi.arch@...edance.com, linux-mm@...ck.org, linux-doc@...r.kernel.org,
linux-kernel@...r.kernel.org, cgroups@...r.kernel.org, lujialin4@...wei.com,
chenridong@...wei.com
Subject: Re: [RFC PATCH -next 1/2] mm/mglru: use mem_cgroup_iter for global
reclaim
On 2025/12/5 6:29, Shakeel Butt wrote:
> Hi Chen,
>
> On Thu, Dec 04, 2025 at 12:31:23PM +0000, Chen Ridong wrote:
>> From: Chen Ridong <chenridong@...wei.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@...wei.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?
>
Thank you Shakeel for you reply.
IIUC, I think adding flush_reclaim_state here makes sense. Currently, shrink_one is only used for
root-level reclaim in gen-LRU, and flush_reclaim_state is only relevant during root reclaim.
Flushing after each lruvec is shrunk could help the reclaim loop terminate earlier, as
sc->nr_reclaimed += current->reclaim_state->reclaimed; may reach nr_to_reclaim sooner.
That said, I'm also wondering whether we should apply flush_reclaim_state for every iteration in
non-MGLLU reclaim as well. For non-root reclaim, it should be negligible since it effectively does
nothing. But for root-level reclaim under non-MGLRU, it might similarly help stop the iteration earlier.
>> -
>> - 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.
>
Will try to.
--
Best regards,
Ridong
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