linux-kernel - Re: [PATCH 4/4] mm: Do periodic rescheduling when freeing objects in kmem_free_up

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Date:   Thu, 21 Mar 2019 23:00:35 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Waiman Long <longman@...hat.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Christoph Lameter <cl@...ux.com>,
        Pekka Enberg <penberg@...nel.org>,
        David Rientjes <rientjes@...gle.com>,
        Joonsoo Kim <iamjoonsoo.kim@....com>,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        selinux@...r.kernel.org, Paul Moore <paul@...l-moore.com>,
        Stephen Smalley <sds@...ho.nsa.gov>,
        Eric Paris <eparis@...isplace.org>,
        Oleg Nesterov <oleg@...hat.com>
Subject: Re: [PATCH 4/4] mm: Do periodic rescheduling when freeing objects in
 kmem_free_up_q()

On Thu, Mar 21, 2019 at 05:45:12PM -0400, Waiman Long wrote:
> If the freeing queue has many objects, freeing all of them consecutively
> may cause soft lockup especially on a debug kernel. So kmem_free_up_q()
> is modified to call cond_resched() if running in the process context.
> 
> Signed-off-by: Waiman Long <longman@...hat.com>
> ---
>  mm/slab_common.c | 11 ++++++++++-
>  1 file changed, 10 insertions(+), 1 deletion(-)
> 
> diff --git a/mm/slab_common.c b/mm/slab_common.c
> index dba20b4208f1..633a1d0f6d20 100644
> --- a/mm/slab_common.c
> +++ b/mm/slab_common.c
> @@ -1622,11 +1622,14 @@ EXPORT_SYMBOL_GPL(kmem_free_q_add);
>   * kmem_free_up_q - free all the objects in the freeing queue
>   * @head: freeing queue head
>   *
> - * Free all the objects in the freeing queue.
> + * Free all the objects in the freeing queue. The caller cannot hold any
> + * non-sleeping locks.
>   */
>  void kmem_free_up_q(struct kmem_free_q_head *head)
>  {
>  	struct kmem_free_q_node *node, *next;
> +	bool do_resched = !in_irq();
> +	int cnt = 0;
>  
>  	for (node = head->first; node; node = next) {
>  		next = node->next;
> @@ -1634,6 +1637,12 @@ void kmem_free_up_q(struct kmem_free_q_head *head)
>  			kmem_cache_free(node->cachep, node);
>  		else
>  			kfree(node);
> +		/*
> +		 * Call cond_resched() every 256 objects freed when in
> +		 * process context.
> +		 */
> +		if (do_resched && !(++cnt & 0xff))
> +			cond_resched();

Why not just: cond_resched() ?

>  	}
>  }
>  EXPORT_SYMBOL_GPL(kmem_free_up_q);
> -- 
> 2.18.1
>