linux-kernel - Re: [RFC PATCH 02/10] memcg: decouple drain_obj

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Message-ID: <d9b2bc46-332a-44b1-bd43-41446b7f4228@suse.cz>
Date: Fri, 14 Mar 2025 10:57:30 +0100
From: Vlastimil Babka <vbabka@...e.cz>
To: Shakeel Butt <shakeel.butt@...ux.dev>,
 Andrew Morton <akpm@...ux-foundation.org>
Cc: Johannes Weiner <hannes@...xchg.org>, Michal Hocko <mhocko@...nel.org>,
 Roman Gushchin <roman.gushchin@...ux.dev>,
 Muchun Song <muchun.song@...ux.dev>,
 Sebastian Andrzej Siewior <bigeasy@...utronix.de>, linux-mm@...ck.org,
 cgroups@...r.kernel.org, linux-kernel@...r.kernel.org,
 Meta kernel team <kernel-team@...a.com>
Subject: Re: [RFC PATCH 02/10] memcg: decouple drain_obj_stock from local
 stock

On 3/14/25 07:15, Shakeel Butt wrote:
> Currently drain_obj_stock() can potentially call __refill_stock which
> accesses local cpu stock and thus requires memcg stock's local_lock.
> However if we look at the code paths leading to drain_obj_stock(), there
> is never a good reason to refill the memcg stock at all from it.
> 
> At the moment, drain_obj_stock can be called from reclaim, hotplug cpu
> teardown, mod_objcg_state() and refill_obj_stock(). For reclaim and
> hotplug there is no need to refill. For the other two paths, most
> probably the newly switched objcg would be used in near future and thus
> no need to refill stock with the older objcg.
> 
> In addition, __refill_stock() from drain_obj_stock() happens on rare
> cases, so performance is not really an issue. Let's just uncharge
> directly instead of refill which will also decouple drain_obj_stock from
> local cpu stock and local_lock requirements.
> 
> Signed-off-by: Shakeel Butt <shakeel.butt@...ux.dev>

Acked-by: Vlastimil Babka <vbabka@...e.cz>

> ---
>  mm/memcontrol.c | 7 ++++++-
>  1 file changed, 6 insertions(+), 1 deletion(-)
> 
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index c09a32e93d39..28cb75b5bc66 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -2855,7 +2855,12 @@ static struct obj_cgroup *drain_obj_stock(struct memcg_stock_pcp *stock)
>  
>  			mod_memcg_state(memcg, MEMCG_KMEM, -nr_pages);
>  			memcg1_account_kmem(memcg, -nr_pages);
> -			__refill_stock(memcg, nr_pages);
> +			if (!mem_cgroup_is_root(memcg)) {
> +				page_counter_uncharge(&memcg->memory, nr_pages);
> +				if (do_memsw_account())
> +					page_counter_uncharge(&memcg->memsw,
> +							      nr_pages);
> +			}
>  
>  			css_put(&memcg->css);
>  		}