Re: [PATCH] mm/memcontrol: Avoid stuck FLUSHING_CACHED_CHARGE on isolated CPU

[Waiman Long <longman@redhat.com>]

On 4/28/26 10:27 PM, Hui Zhu wrote:
> From: Hui Zhu <zhuhui@kylinos.cn>
>
> drain_all_stock() sets FLUSHING_CACHED_CHARGE before calling
> schedule_drain_work() to queue per-CPU drain work.  When the target
> CPU is isolated (cpu_is_isolated() == true), the work is silently
> not queued, but FLUSHING_CACHED_CHARGE stays set.  Every subsequent
> drain_all_stock() then sees the bit and skips this stock entirely,
> so the entry is effectively pinned until something else on that CPU
> runs drain_local_*_stock() and clears the bit -- which on a long-
> isolated CPU may never happen.
>
> The original idea was to actually perform the drain from the calling
> CPU on behalf of the isolated one, by adding a lock around the
> per-CPU stock so that a remote drainer could safely touch it.  In
> practice this turned out to be intrusive: the stock data structures
> and their fast paths (consume_stock(), refill_stock(), the obj_stock
> helpers) are deliberately designed around current-CPU-only access,
> and retrofitting cross-CPU serialisation onto them adds non-trivial
> locking and PREEMPT_RT concerns for very little gain.
>
> Looking at the actual amount of charge that can accumulate in a
> single per-CPU stock, it is bounded and small, so leaving an
> isolated CPU's stock undrained for a while is not a real problem.
> The only real bug is that the stuck FLUSHING_CACHED_CHARGE bit
> prevents future drain_all_stock() callers from re-attempting once
> the CPU is no longer isolated.
>
> Fix this minimally by clearing FLUSHING_CACHED_CHARGE when the work
> could not be queued because the target CPU is isolated.  The cached
> charge itself is left in place; it will be released the next time
> the CPU runs drain_local_*_stock() (e.g. after leaving isolation,
> or if the isolated CPU itself calls drain_all_stock() -- in that
> case cpu == curcpu causes drain_local_memcg_stock() to be invoked
> directly), and the next drain_all_stock() call is free to retry
> instead of skipping the stock forever.
>
> Fixes: 2d05068610a3 ("memcg: Prepare to protect against concurrent isolated cpuset change")

I don't think this is the right commit to blame as it didn't really 
change the logic other than adding RCU locking. I think commit 
6a792697a53a ("memcg: do not drain charge pcp caches on remote isolated 
cpus") is the right one as this is the commit that adds the 
cpu_is_isolated() check first.

Other than that, the patch looks good to me as the list of isolated CPUs 
is runtime changeable.

Cheers,
Longman

> Signed-off-by: Hui Zhu <zhuhui@kylinos.cn>
> ---
>   mm/memcontrol.c | 28 ++++++++++++++++++++++------
>   1 file changed, 22 insertions(+), 6 deletions(-)
>
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index c3d98ab41f1f..cee77b0a95f5 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -2219,7 +2219,8 @@ 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 void
> +schedule_drain_work(int cpu, struct work_struct *work, unsigned long *flags)
>   {
>   	/*
>   	 * Protect housekeeping cpumask read and work enqueue together
> @@ -2227,9 +2228,22 @@ static void schedule_drain_work(int cpu, struct work_struct *work)
>   	 * partition update only need to wait for an RCU GP and flush the
>   	 * pending work on newly isolated CPUs.
>   	 */
> -	guard(rcu)();
> -	if (!cpu_is_isolated(cpu))
> -		queue_work_on(cpu, memcg_wq, work);
> +	scoped_guard(rcu) {
> +		if (!cpu_is_isolated(cpu)) {
> +			queue_work_on(cpu, memcg_wq, work);
> +			return;
> +		}
> +	}
> +
> +	/*
> +	 * The target CPU is isolated: the drain work was not queued.
> +	 * Clear FLUSHING_CACHED_CHARGE so that future drain_all_stock()
> +	 * callers can re-attempt instead of skipping this stock forever.
> +	 * The cached charge is left in place; it will be released the
> +	 * next time the CPU itself runs drain_local_*_stock() (e.g.
> +	 * after leaving isolation), or by a follow-up mechanism.
> +	 */
> +	clear_bit(FLUSHING_CACHED_CHARGE, flags);
>   }
>   
>   /*
> @@ -2262,7 +2276,8 @@ void drain_all_stock(struct mem_cgroup *root_memcg)
>   			if (cpu == curcpu)
>   				drain_local_memcg_stock(&memcg_st->work);
>   			else
> -				schedule_drain_work(cpu, &memcg_st->work);
> +				schedule_drain_work(cpu, &memcg_st->work,
> +						    &memcg_st->flags);
>   		}
>   
>   		if (!test_bit(FLUSHING_CACHED_CHARGE, &obj_st->flags) &&
> @@ -2272,7 +2287,8 @@ void drain_all_stock(struct mem_cgroup *root_memcg)
>   			if (cpu == curcpu)
>   				drain_local_obj_stock(&obj_st->work);
>   			else
> -				schedule_drain_work(cpu, &obj_st->work);
> +				schedule_drain_work(cpu, &obj_st->work,
> +						    &obj_st->flags);
>   		}
>   	}
>   	migrate_enable();