Re: [PATCH] locking/rtmutex: Limit # of lock stealing for non-RT waiters

[Waiman Long <longman@redhat.com>]

On 6/8/22 11:22, Waiman Long wrote:
> Commit 48eb3f4fcfd3 ("locking/rtmutex: Implement equal priority lock
> stealing") allows unlimited number of lock stealing's for non-RT
> tasks. That can lead to lock starvation of non-RT top waiter if there
> is a constant incoming stream of non-RT lockers. This can cause task
> lockup in PREEMPT_RT kernel.
>
> Avoiding this problem and ensuring forward progress by limiting the
> number of times that a lock can be stolen from each waiter. This patch
> sets a threshold of 10. That number is arbitrary and can be changed
> if necessary.
>
> Fixes: 48eb3f4fcfd3 ("locking/rtmutex: Implement equal priority lock stealing")
> Signed-off-by: Waiman Long <longman@redhat.com>

Any comment on this patch?

Our QE team had verified that it fixed the lockup problem that they see 
in our PREEMPT_RT kernel.

Thanks,
Longman

> ---
>   kernel/locking/rtmutex.c        | 9 ++++++---
>   kernel/locking/rtmutex_common.h | 8 ++++++++
>   2 files changed, 14 insertions(+), 3 deletions(-)
>
> diff --git a/kernel/locking/rtmutex.c b/kernel/locking/rtmutex.c
> index 7779ee8abc2a..bdddb3dc36c2 100644
> --- a/kernel/locking/rtmutex.c
> +++ b/kernel/locking/rtmutex.c
> @@ -359,10 +359,13 @@ static inline bool rt_mutex_steal(struct rt_mutex_waiter *waiter,
>   	if (rt_prio(waiter->prio) || dl_prio(waiter->prio))
>   		return false;
>   
> -	return rt_mutex_waiter_equal(waiter, top_waiter);
> -#else
> -	return false;
> +	if (rt_mutex_waiter_equal(waiter, top_waiter) &&
> +	   (top_waiter->nr_steals < RT_MUTEX_LOCK_STEAL_MAX)) {
> +		top_waiter->nr_steals++;
> +		return true;
> +	}
>   #endif
> +	return false;
>   }
>   
>   #define __node_2_waiter(node) \
> diff --git a/kernel/locking/rtmutex_common.h b/kernel/locking/rtmutex_common.h
> index c47e8361bfb5..5858efe5cb0e 100644
> --- a/kernel/locking/rtmutex_common.h
> +++ b/kernel/locking/rtmutex_common.h
> @@ -26,6 +26,7 @@
>    * @task:		task reference to the blocked task
>    * @lock:		Pointer to the rt_mutex on which the waiter blocks
>    * @wake_state:		Wakeup state to use (TASK_NORMAL or TASK_RTLOCK_WAIT)
> + * @nr_steals:		Number of times the lock is stolen
>    * @prio:		Priority of the waiter
>    * @deadline:		Deadline of the waiter if applicable
>    * @ww_ctx:		WW context pointer
> @@ -36,11 +37,17 @@ struct rt_mutex_waiter {
>   	struct task_struct	*task;
>   	struct rt_mutex_base	*lock;
>   	unsigned int		wake_state;
> +	unsigned int		nr_steals;
>   	int			prio;
>   	u64			deadline;
>   	struct ww_acquire_ctx	*ww_ctx;
>   };
>   
> +/*
> + * The maximum number of times where lock can be stolen per waiter.
> + */
> +#define	RT_MUTEX_LOCK_STEAL_MAX	10
> +
>   /**
>    * rt_wake_q_head - Wrapper around regular wake_q_head to support
>    *		    "sleeping" spinlocks on RT
> @@ -194,6 +201,7 @@ static inline void rt_mutex_init_waiter(struct rt_mutex_waiter *waiter)
>   	RB_CLEAR_NODE(&waiter->tree_entry);
>   	waiter->wake_state = TASK_NORMAL;
>   	waiter->task = NULL;
> +	waiter->nr_steals = 0;
>   }
>   
>   static inline void rt_mutex_init_rtlock_waiter(struct rt_mutex_waiter *waiter)