Re: [PATCH v1 2/2] mm/slab/kvfree_rcu: Switch to WQ_MEM_RECLAIM wq

[Joel Fernandes <joelagnelf@nvidia.com>]

On Fri, Feb 28, 2025 at 01:13:56PM +0100, Uladzislau Rezki (Sony) wrote:
> Currently kvfree_rcu() APIs use a system workqueue which is
> "system_unbound_wq" to driver RCU machinery to reclaim a memory.
> 
> Recently, it has been noted that the following kernel warning can
> be observed:
> 
> <snip>
> workqueue: WQ_MEM_RECLAIM nvme-wq:nvme_scan_work is flushing !WQ_MEM_RECLAIM events_unbound:kfree_rcu_work
>   WARNING: CPU: 21 PID: 330 at kernel/workqueue.c:3719 check_flush_dependency+0x112/0x120
>   Modules linked in: intel_uncore_frequency(E) intel_uncore_frequency_common(E) skx_edac(E) ...
>   CPU: 21 UID: 0 PID: 330 Comm: kworker/u144:6 Tainted: G            E      6.13.2-0_g925d379822da #1
>   Hardware name: Wiwynn Twin Lakes MP/Twin Lakes Passive MP, BIOS YMM20 02/01/2023
>   Workqueue: nvme-wq nvme_scan_work
>   RIP: 0010:check_flush_dependency+0x112/0x120
>   Code: 05 9a 40 14 02 01 48 81 c6 c0 00 00 00 48 8b 50 18 48 81 c7 c0 00 00 00 48 89 f9 48 ...
>   RSP: 0018:ffffc90000df7bd8 EFLAGS: 00010082
>   RAX: 000000000000006a RBX: ffffffff81622390 RCX: 0000000000000027
>   RDX: 00000000fffeffff RSI: 000000000057ffa8 RDI: ffff88907f960c88
>   RBP: 0000000000000000 R08: ffffffff83068e50 R09: 000000000002fffd
>   R10: 0000000000000004 R11: 0000000000000000 R12: ffff8881001a4400
>   R13: 0000000000000000 R14: ffff88907f420fb8 R15: 0000000000000000
>   FS:  0000000000000000(0000) GS:ffff88907f940000(0000) knlGS:0000000000000000
>   CR2: 00007f60c3001000 CR3: 000000107d010005 CR4: 00000000007726f0
>   PKRU: 55555554
>   Call Trace:
>    <TASK>
>    ? __warn+0xa4/0x140
>    ? check_flush_dependency+0x112/0x120
>    ? report_bug+0xe1/0x140
>    ? check_flush_dependency+0x112/0x120
>    ? handle_bug+0x5e/0x90
>    ? exc_invalid_op+0x16/0x40
>    ? asm_exc_invalid_op+0x16/0x20
>    ? timer_recalc_next_expiry+0x190/0x190
>    ? check_flush_dependency+0x112/0x120
>    ? check_flush_dependency+0x112/0x120
>    __flush_work.llvm.1643880146586177030+0x174/0x2c0
>    flush_rcu_work+0x28/0x30
>    kvfree_rcu_barrier+0x12f/0x160
>    kmem_cache_destroy+0x18/0x120
>    bioset_exit+0x10c/0x150
>    disk_release.llvm.6740012984264378178+0x61/0xd0
>    device_release+0x4f/0x90
>    kobject_put+0x95/0x180
>    nvme_put_ns+0x23/0xc0
>    nvme_remove_invalid_namespaces+0xb3/0xd0
>    nvme_scan_work+0x342/0x490
>    process_scheduled_works+0x1a2/0x370
>    worker_thread+0x2ff/0x390
>    ? pwq_release_workfn+0x1e0/0x1e0
>    kthread+0xb1/0xe0
>    ? __kthread_parkme+0x70/0x70
>    ret_from_fork+0x30/0x40
>    ? __kthread_parkme+0x70/0x70
>    ret_from_fork_asm+0x11/0x20
>    </TASK>
>   ---[ end trace 0000000000000000 ]---
> <snip>
> 
> To address this switch to use of independent WQ_MEM_RECLAIM
> workqueue, so the rules are not violated from workqueue framework
> point of view.
> 
> Apart of that, since kvfree_rcu() does reclaim memory it is worth
> to go with WQ_MEM_RECLAIM type of wq because it is designed for
> this purpose.
> 
> Cc: <stable@vger.kernel.org>
> Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
> Cc: Keith Busch <kbusch@kernel.org>
> Closes: https://www.spinics.net/lists/kernel/msg5563270.html
> Fixes: 6c6c47b063b5 ("mm, slab: call kvfree_rcu_barrier() from kmem_cache_destroy()"),
> Reported-by: Keith Busch <kbusch@kernel.org>
> Signed-off-by: Uladzislau Rezki (Sony) <urezki@gmail.com>

BTW, there is a path in RCU-tasks that involves queuing work on system_wq
which is !WQ_RECLAIM. While I don't anticipate an issue such as the one fixed
by this patch, I am wondering if we should move these to their own WQ_RECLAIM
queues for added robustness since otherwise that will result in CB invocation
(And thus memory freeing delays). Paul?

kernel/rcu/tasks.h:       queue_work_on(cpuwq, system_wq, &rtpcp_next->rtp_work);
kernel/rcu/tasks.h:       queue_work_on(cpuwq, system_wq, &rtpcp_next->rtp_work);

For this patch:
Reviewed-by: Joel Fernandes <joelagnelf@nvidia.com>

thanks,

 - Joel


> ---
>  mm/slab_common.c | 14 ++++++++++----
>  1 file changed, 10 insertions(+), 4 deletions(-)
> 
> diff --git a/mm/slab_common.c b/mm/slab_common.c
> index 4030907b6b7d..4c9f0a87f733 100644
> --- a/mm/slab_common.c
> +++ b/mm/slab_common.c
> @@ -1304,6 +1304,8 @@ module_param(rcu_min_cached_objs, int, 0444);
>  static int rcu_delay_page_cache_fill_msec = 5000;
>  module_param(rcu_delay_page_cache_fill_msec, int, 0444);
>  
> +static struct workqueue_struct *rcu_reclaim_wq;
> +
>  /* Maximum number of jiffies to wait before draining a batch. */
>  #define KFREE_DRAIN_JIFFIES (5 * HZ)
>  #define KFREE_N_BATCHES 2
> @@ -1632,10 +1634,10 @@ __schedule_delayed_monitor_work(struct kfree_rcu_cpu *krcp)
>  	if (delayed_work_pending(&krcp->monitor_work)) {
>  		delay_left = krcp->monitor_work.timer.expires - jiffies;
>  		if (delay < delay_left)
> -			mod_delayed_work(system_unbound_wq, &krcp->monitor_work, delay);
> +			mod_delayed_work(rcu_reclaim_wq, &krcp->monitor_work, delay);
>  		return;
>  	}
> -	queue_delayed_work(system_unbound_wq, &krcp->monitor_work, delay);
> +	queue_delayed_work(rcu_reclaim_wq, &krcp->monitor_work, delay);
>  }
>  
>  static void
> @@ -1733,7 +1735,7 @@ kvfree_rcu_queue_batch(struct kfree_rcu_cpu *krcp)
>  			// "free channels", the batch can handle. Break
>  			// the loop since it is done with this CPU thus
>  			// queuing an RCU work is _always_ success here.
> -			queued = queue_rcu_work(system_unbound_wq, &krwp->rcu_work);
> +			queued = queue_rcu_work(rcu_reclaim_wq, &krwp->rcu_work);
>  			WARN_ON_ONCE(!queued);
>  			break;
>  		}
> @@ -1883,7 +1885,7 @@ run_page_cache_worker(struct kfree_rcu_cpu *krcp)
>  	if (rcu_scheduler_active == RCU_SCHEDULER_RUNNING &&
>  			!atomic_xchg(&krcp->work_in_progress, 1)) {
>  		if (atomic_read(&krcp->backoff_page_cache_fill)) {
> -			queue_delayed_work(system_unbound_wq,
> +			queue_delayed_work(rcu_reclaim_wq,
>  				&krcp->page_cache_work,
>  					msecs_to_jiffies(rcu_delay_page_cache_fill_msec));
>  		} else {
> @@ -2120,6 +2122,10 @@ void __init kvfree_rcu_init(void)
>  	int i, j;
>  	struct shrinker *kfree_rcu_shrinker;
>  
> +	rcu_reclaim_wq = alloc_workqueue("kvfree_rcu_reclaim",
> +			WQ_UNBOUND | WQ_MEM_RECLAIM, 0);
> +	WARN_ON(!rcu_reclaim_wq);
> +
>  	/* Clamp it to [0:100] seconds interval. */
>  	if (rcu_delay_page_cache_fill_msec < 0 ||
>  		rcu_delay_page_cache_fill_msec > 100 * MSEC_PER_SEC) {
> -- 
> 2.39.5
>