Messages in this thread | | | Date | Tue, 14 Jun 2022 11:06:17 -0400 | Subject | Re: [PATCH] locking/rtmutex: Limit # of lock stealing for non-RT waiters | From | Waiman Long <> |
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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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