linux-kernel - Re: [PATCH 1/5] memcg: disable kmem charging in nmi for unsupported arch

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Message-ID: <050484a9-c08c-40d2-b431-76903a639222@suse.cz>
Date: Fri, 16 May 2025 11:30:17 +0200
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>, Alexei Starovoitov <ast@...nel.org>,
 Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
 Harry Yoo <harry.yoo@...cle.com>, Yosry Ahmed <yosry.ahmed@...ux.dev>,
 Peter Zijlstra <peterz@...radead.org>,
 Mathieu Desnoyers <mathieu.desnoyers@...icios.com>, bpf@...r.kernel.org,
 linux-mm@...ck.org, cgroups@...r.kernel.org, linux-kernel@...r.kernel.org,
 Meta kernel team <kernel-team@...a.com>
Subject: Re: [PATCH 1/5] memcg: disable kmem charging in nmi for unsupported
 arch

On 5/16/25 08:49, Shakeel Butt wrote:
> The memcg accounting and stats uses this_cpu* and atomic* ops. There are
> archs which define CONFIG_HAVE_NMI but does not define
> CONFIG_ARCH_HAS_NMI_SAFE_THIS_CPU_OPS and ARCH_HAVE_NMI_SAFE_CMPXCHG, so
> memcg accounting for such archs in nmi context is not possible to
> support. Let's just disable memcg accounting in nmi context for such
> archs.
> 
> Signed-off-by: Shakeel Butt <shakeel.butt@...ux.dev>
> ---
>  include/linux/memcontrol.h |  5 +++++
>  mm/memcontrol.c            | 15 +++++++++++++++
>  2 files changed, 20 insertions(+)
> 
> diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
> index f7848f73f41c..53920528821f 100644
> --- a/include/linux/memcontrol.h
> +++ b/include/linux/memcontrol.h
> @@ -62,6 +62,11 @@ struct mem_cgroup_reclaim_cookie {
>  
>  #ifdef CONFIG_MEMCG
>  
> +#if defined(CONFIG_ARCH_HAS_NMI_SAFE_THIS_CPU_OPS) || \
> +	!defined(CONFIG_HAVE_NMI) || defined(ARCH_HAVE_NMI_SAFE_CMPXCHG)
> +#define MEMCG_SUPPORTS_NMI_CHARGING
> +#endif
> +
>  #define MEM_CGROUP_ID_SHIFT	16
>  
>  struct mem_cgroup_id {
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index e17b698f6243..dface07f69bb 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -2647,11 +2647,26 @@ static struct obj_cgroup *current_objcg_update(void)
>  	return objcg;
>  }
>  
> +#ifdef MEMCG_SUPPORTS_NMI_CHARGING
> +static inline bool nmi_charging_allowed(void)
> +{
> +	return true;
> +}
> +#else
> +static inline bool nmi_charging_allowed(void)
> +{
> +	return false;
> +}
> +#endif
> +
>  __always_inline struct obj_cgroup *current_obj_cgroup(void)
>  {
>  	struct mem_cgroup *memcg;
>  	struct obj_cgroup *objcg;
>  
> +	if (in_nmi() && !nmi_charging_allowed())

Exchange the two as the latter is compile-time constant, so it can shortcut
the in_nmi() check away in all the good cases?

> +		return NULL;
> +
>  	if (in_task()) {
>  		memcg = current->active_memcg;
>  		if (unlikely(memcg))