Re: [PATCH] mm/memcontrol: avoid stuck FLUSHING_CACHED_CHARGE bit on isolated cpus

[Shakeel Butt <shakeel.butt@linux.dev>]

On Fri, Aug 28, 2026 at 04:25:41PM +0200, Michal Hocko wrote:
> On Fri 28-08-26 09:46:21, Rik van Riel wrote:
> > drain_all_stock() can leave FLUSHING_CACHED_CHARGE set after the
> > work is dropped.  It sets the bit before checking isolation and
> > schedule_drain_work() checks isolation and queues in a separate RCU
> > critical section, so housekeeping_update()'s synchronize_rcu() can
> > race the second check.
> > 
> > drain_local_stock() only clears the bit for work that ran, so the bit
> > remains set and the stock is never drained again.
> > 
> > Reorganize the drain_all_stock() loop, reducing nesting, splitting
> > out local vs remote cpu handling, and skipping everything on isolated
> > cpus, which solves the stuck FLUSHING_CACHED_CHARGE flag.
> 
> Is there any reason why we cannot simply clear the flag if the work is
> not scheduled?
> ---
> 
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index 69b37f63a307..907ac63e5067 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -2261,8 +2261,10 @@ static bool is_memcg_drain_needed(struct memcg_stock_pcp *stock,
>  	return flush;
>  }
>  
> -static void schedule_drain_work(int cpu, struct work_struct *work)
> +static bool schedule_drain_work(int cpu, struct work_struct *work)
>  {
> +	int ret = false;
> +
>  	/*
>  	 * Protect housekeeping cpumask read and work enqueue together
>  	 * in the same RCU critical section so that later cpuset isolated
> @@ -2270,8 +2272,12 @@ static void schedule_drain_work(int cpu, struct work_struct *work)
>  	 * pending work on newly isolated CPUs.
>  	 */
>  	guard(rcu)();
> -	if (!cpu_is_isolated(cpu))
> -		queue_work_on(cpu, memcg_wq, work);
> +	if (!cpu_is_isolated(cpu)) {
> +		queue_work_on(cpu, memcg_wq, &memcg_st->work);
> +		ret = true;
> +	}
> +
> +	return ret;

Let's go with this patch. We can simplify above by inversing the check:

	if (cpu_is_isolated(cpu))
		return false;
	
	queue_work_on(cpu, memcg_wq, &memcg_st->work);
	return true;

>  }
>  
>  /*
> @@ -2303,8 +2309,8 @@ void drain_all_stock(struct mem_cgroup *root_memcg)
>  				      &memcg_st->flags)) {
>  			if (cpu == curcpu)
>  				drain_local_memcg_stock(&memcg_st->work);
> -			else
> -				schedule_drain_work(cpu, &memcg_st->work);
> +			else if (!schedule_drain_work(cpu, &memcg_st->work))
> +				clear_bit(FLUSHING_CACHED_CHARGE, &memcg_st->flags)
>  		}
>  
>  		if (!test_bit(FLUSHING_CACHED_CHARGE, &obj_st->flags) &&
> @@ -2313,8 +2319,8 @@ void drain_all_stock(struct mem_cgroup *root_memcg)
>  				      &obj_st->flags)) {
>  			if (cpu == curcpu)
>  				drain_local_obj_stock(&obj_st->work);
> -			else
> -				schedule_drain_work(cpu, &obj_st->work);
> +			else if (!schedule_drain_work(cpu, &obj_st->work))
> +				clear_bit(FLUSHING_CACHED_CHARGE, &obj_st->flags);
>  		}
>  	}
>  	migrate_enable();
> -- 
> Michal Hocko
> SUSE Labs