linux-kernel - Re: [PATCH 2/2 v3] lib: debugobjects: touch watchdog to avoid softlockup when !CONFIG

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Date:   Sat, 20 Jan 2018 08:06:55 +0800
From:   "Yang Shi" <yang.s@...baba-inc.com>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     longman@...hat.com, LKML <linux-kernel@...r.kernel.org>,
        Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH 2/2 v3] lib: debugobjects: touch watchdog to avoid
 softlockup when !CONFIG_PREEMPT



On 1/18/18 1:02 AM, Thomas Gleixner wrote:
> On Thu, 18 Jan 2018, Yang Shi wrote:
>> On 1/17/18 4:21 AM, Thomas Gleixner wrote:
>>> There are two things which can be done here:
>>>
>>>    1) The collected objects can be put on a global free list and work
>>>       scheduled to free them piecewise.
>>
>> I don't get your point here. objects free has already been done in a work.
>> free_object() -> schedule_work()
> 
> But it's doing it in a loop for each freed object.
> 
>> Do you mean free all of them out of the for loop in a batch? Then don't call
>> free_object() in the for loop?
> 
> Somehing like this:
> 
> diff --git a/lib/debugobjects.c b/lib/debugobjects.c
> index 2f5349c6e81a..d36940cdc658 100644
> --- a/lib/debugobjects.c
> +++ b/lib/debugobjects.c
> @@ -42,6 +42,7 @@ static struct debug_obj		obj_static_pool[ODEBUG_POOL_SIZE] __initdata;
>   static DEFINE_RAW_SPINLOCK(pool_lock);
>   
>   static HLIST_HEAD(obj_pool);
> +static HLIST_HEAD(obj_to_free);
>   
>   static int			obj_pool_min_free = ODEBUG_POOL_SIZE;
>   static int			obj_pool_free = ODEBUG_POOL_SIZE;
> @@ -714,13 +715,12 @@ EXPORT_SYMBOL_GPL(debug_object_active_state);
>   static void __debug_check_no_obj_freed(const void *address, unsigned long size)
>   {
>   	unsigned long flags, oaddr, saddr, eaddr, paddr, chunks;
> -	struct hlist_node *tmp;
> -	HLIST_HEAD(freelist);
>   	struct debug_obj_descr *descr;
>   	enum debug_obj_state state;
>   	struct debug_bucket *db;
> +	struct hlist_node *tmp;
>   	struct debug_obj *obj;
> -	int cnt;
> +	int cnt, freed = 0;
>   
>   	saddr = (unsigned long) address;
>   	eaddr = saddr + size;
> @@ -751,21 +751,22 @@ static void __debug_check_no_obj_freed(const void *address, unsigned long size)
>   				goto repeat;
>   			default:
>   				hlist_del(&obj->node);
> -				hlist_add_head(&obj->node, &freelist);
> +				/* Put them on the freelist */
> +				raw_spin_lock_irqsave(&pool_lock, flags);
> +				hlist_add_head(&obj->node, &obj_to_free);
> +				raw_spin_lock_irqrestore(&pool_lock, flags);
> +				freed++;
>   				break;
>   			}
>   		}
>   		raw_spin_unlock_irqrestore(&db->lock, flags);
>   
> -		/* Now free them */
> -		hlist_for_each_entry_safe(obj, tmp, &freelist, node) {
> -			hlist_del(&obj->node);
> -			free_object(obj);
> -		}
> -
>   		if (cnt > debug_objects_maxchain)
>   			debug_objects_maxchain = cnt;
>   	}
> +
> +	if (freed)
> +		schedule_work(.....);
> 
> The allocation side can look at the free list as well and grab objects from
> there if the pool level is low if that happens before the work can do that.

Thanks a lot for the example. I'm working on a preliminary patch, will 
send out for review once I finish my tests.

> 
> 
>>>
>>>    2) We can do a cond_resched() if not in atomic context and interrupts are
>>>       enabled.
>>
>> I did try this before I went with touching softlockup watchdog approach. The
>> problem is in_atomic() can't tell if it is in atomic context on non-preempt
>> kernel. For preempt kernel, it is easy.
> 
> Peter, can we do anything about that?

As Peter confirmed, in_atomic() is not functional on !PREEMPT kernel, 
shall we still use touching softlockup watchdog approach?

Thanks,
Yang

> 
> Thanks,
> 
> 	tglx
>