linux-kernel - Re: [PATCH 1/7] memcg: memcg_rstat_updated re-entrant safe against irqs

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Message-ID: <cb50a1c8-1f94-4a49-b5b3-8d2008c9f272@suse.cz>
Date: Wed, 14 May 2025 11:50:16 +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>,
 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 1/7] memcg: memcg_rstat_updated re-entrant safe against
 irqs

On 5/14/25 07:08, Shakeel Butt wrote:
> The function memcg_rstat_updated() is used to track the memcg stats
> updates for optimizing the flushes. At the moment, it is not re-entrant
> safe and the callers disabled irqs before calling. However to achieve
> the goal of updating memcg stats without irqs, memcg_rstat_updated()
> needs to be re-entrant safe against irqs.
> 
> This patch makes memcg_rstat_updated() re-entrant safe using this_cpu_*
> ops. On archs with CONFIG_ARCH_HAS_NMI_SAFE_THIS_CPU_OPS, this patch is
> also making memcg_rstat_updated() nmi safe.
> 
> Signed-off-by: Shakeel Butt <shakeel.butt@...ux.dev>

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

Some nits:

>  static inline void memcg_rstat_updated(struct mem_cgroup *memcg, int val)
>  {
> -	struct memcg_vmstats_percpu *statc;
> -	int cpu = smp_processor_id();
> +	struct memcg_vmstats_percpu __percpu *statc_pcpu;
> +	int cpu;
>  	unsigned int stats_updates;
>  
>  	if (!val)
>  		return;
>  
> +	/* Don't assume callers have preemption disabled. */
> +	cpu = get_cpu();
> +
>  	css_rstat_updated(&memcg->css, cpu);
> -	statc = this_cpu_ptr(memcg->vmstats_percpu);
> -	for (; statc; statc = statc->parent) {
> +	statc_pcpu = memcg->vmstats_percpu;

Wonder if extracting the this_cpu_ptr() statc pointer would still make the
code a bit simpler when accessing parent_pcpu and vmstats later on.

> +	for (; statc_pcpu; statc_pcpu = this_cpu_ptr(statc_pcpu)->parent_pcpu) {
>  		/*
>  		 * If @memcg is already flushable then all its ancestors are
>  		 * flushable as well and also there is no need to increase
>  		 * stats_updates.
>  		 */
> -		if (memcg_vmstats_needs_flush(statc->vmstats))
> +		if (memcg_vmstats_needs_flush(this_cpu_ptr(statc_pcpu)->vmstats))
>  			break;
>  
> -		stats_updates = READ_ONCE(statc->stats_updates) + abs(val);
> -		WRITE_ONCE(statc->stats_updates, stats_updates);
> +		stats_updates = this_cpu_add_return(statc_pcpu->stats_updates,
> +						    abs(val));
>  		if (stats_updates < MEMCG_CHARGE_BATCH)
>  			continue;
>  
> -		atomic64_add(stats_updates, &statc->vmstats->stats_updates);
> -		WRITE_ONCE(statc->stats_updates, 0);
> +		stats_updates = this_cpu_xchg(statc_pcpu->stats_updates, 0);
> +		if (stats_updates)

I think this is very likely to be true (given stats_updates >=
MEMCG_CHARGE_BATCH from above), only an irq can change it at this point? So
we could just do this unconditionally, and if we very rarely add a zero, it
doesn't matter?

> +			atomic64_add(stats_updates,
> +				&this_cpu_ptr(statc_pcpu)->vmstats->stats_updates);
>  	}
> +	put_cpu();
>  }
>  
>  static void __mem_cgroup_flush_stats(struct mem_cgroup *memcg, bool force)
> @@ -3716,7 +3722,7 @@ static void mem_cgroup_free(struct mem_cgroup *memcg)
>  
>  static struct mem_cgroup *mem_cgroup_alloc(struct mem_cgroup *parent)
>  {
> -	struct memcg_vmstats_percpu *statc, *pstatc;
> +	struct memcg_vmstats_percpu *statc, __percpu *pstatc_pcpu;
>  	struct mem_cgroup *memcg;
>  	int node, cpu;
>  	int __maybe_unused i;
> @@ -3747,9 +3753,9 @@ static struct mem_cgroup *mem_cgroup_alloc(struct mem_cgroup *parent)
>  
>  	for_each_possible_cpu(cpu) {
>  		if (parent)
> -			pstatc = per_cpu_ptr(parent->vmstats_percpu, cpu);
> +			pstatc_pcpu = parent->vmstats_percpu;
>  		statc = per_cpu_ptr(memcg->vmstats_percpu, cpu);
> -		statc->parent = parent ? pstatc : NULL;
> +		statc->parent_pcpu = parent ? pstatc_pcpu : NULL;
>  		statc->vmstats = memcg->vmstats;
>  	}
>