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

[Vlastimil Babka <vbabka@suse.cz>]

On 5/13/25 05:13, 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 against irqs.
> However it is using atomic_* ops which on x86, adds lock prefix to the
> instructions. Since this is per-cpu data, the this_cpu_* ops are
> preferred. However the percpu pointer is stored in struct mem_cgroup and
> doing the upward traversal through struct mem_cgroup may cause two cache
> misses as compared to traversing through struct memcg_vmstats_percpu
> pointer.
> 
> NOTE: explore if there is atomic_* ops alternative without lock prefix.

local_t might be what you want here
https://docs.kernel.org/core-api/local_ops.html

Or maybe just add __percpu to parent like this?

struct memcg_vmstats_percpu {
...
        struct memcg_vmstats_percpu __percpu *parent;
...
}

Yes, it means on each cpu's struct memcg_vmstats_percpu instance there will
be actually the same value stored (the percpu offset) instead of the
cpu-specific parent pointer, which might seem wasteful. But AFAIK this_cpu_*
is optimized enough thanks to the segment register usage, that it doesn't
matter? It shouldn't cause any extra cache miss you worry about, IIUC?

With that I think you could refactor that code to use e.g.
this_cpu_add_return() and this_cpu_xchg() on the stats_updates and obtain
the parent "pointer" in a way that's also compatible with these operations.

That is unless we want also nmi safety, then we're back to the issue of the
previous series...

> Signed-off-by: Shakeel Butt <shakeel.butt@linux.dev>
> ---
>  mm/memcontrol.c | 21 +++++++++++++--------
>  1 file changed, 13 insertions(+), 8 deletions(-)
> 
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index 6cfa3550f300..2c4c095bf26c 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -503,7 +503,7 @@ static inline int memcg_events_index(enum vm_event_item idx)
>  
>  struct memcg_vmstats_percpu {
>  	/* Stats updates since the last flush */
> -	unsigned int			stats_updates;
> +	atomic_t			stats_updates;
>  
>  	/* Cached pointers for fast iteration in memcg_rstat_updated() */
>  	struct memcg_vmstats_percpu	*parent;
> @@ -590,12 +590,15 @@ static bool memcg_vmstats_needs_flush(struct memcg_vmstats *vmstats)
>  static inline void memcg_rstat_updated(struct mem_cgroup *memcg, int val)
>  {
>  	struct memcg_vmstats_percpu *statc;
> -	int cpu = smp_processor_id();
> -	unsigned int stats_updates;
> +	int cpu;
> +	int stats_updates;
>  
>  	if (!val)
>  		return;
>  
> +	/* Don't assume callers have preemption disabled. */
> +	cpu = get_cpu();
> +
>  	cgroup_rstat_updated(memcg->css.cgroup, cpu);
>  	statc = this_cpu_ptr(memcg->vmstats_percpu);
>  	for (; statc; statc = statc->parent) {
> @@ -607,14 +610,16 @@ static inline void memcg_rstat_updated(struct mem_cgroup *memcg, int val)
>  		if (memcg_vmstats_needs_flush(statc->vmstats))
>  			break;
>  
> -		stats_updates = READ_ONCE(statc->stats_updates) + abs(val);
> -		WRITE_ONCE(statc->stats_updates, stats_updates);
> +		stats_updates = atomic_add_return(abs(val), &statc->stats_updates);
>  		if (stats_updates < MEMCG_CHARGE_BATCH)
>  			continue;
>  
> -		atomic64_add(stats_updates, &statc->vmstats->stats_updates);
> -		WRITE_ONCE(statc->stats_updates, 0);
> +		stats_updates = atomic_xchg(&statc->stats_updates, 0);
> +		if (stats_updates)
> +			atomic64_add(stats_updates,
> +				     &statc->vmstats->stats_updates);
>  	}
> +	put_cpu();
>  }
>  
>  static void __mem_cgroup_flush_stats(struct mem_cgroup *memcg, bool force)
> @@ -4155,7 +4160,7 @@ static void mem_cgroup_css_rstat_flush(struct cgroup_subsys_state *css, int cpu)
>  		mem_cgroup_stat_aggregate(&ac);
>  
>  	}
> -	WRITE_ONCE(statc->stats_updates, 0);
> +	atomic_set(&statc->stats_updates, 0);
>  	/* We are in a per-cpu loop here, only do the atomic write once */
>  	if (atomic64_read(&memcg->vmstats->stats_updates))
>  		atomic64_set(&memcg->vmstats->stats_updates, 0);