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

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