Messages in this thread | | | From | Wanpeng Li <> | Date | Wed, 17 Aug 2016 09:41:59 +0800 | Subject | Re: [PATCH v2] locking/mutex: Prevent lock starvation when spinning is enabled |
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2016-08-11 2:44 GMT+08:00 Jason Low <jason.low2@hpe.com>: > Imre reported an issue where threads are getting starved when trying > to acquire a mutex. Threads acquiring a mutex can get arbitrarily delayed > sleeping on a mutex because other threads can continually steal the lock > in the fastpath and/or through optimistic spinning. > > Waiman has developed patches that allow waiters to return to optimistic > spinning, thus reducing the probability that starvation occurs. However, > Imre still sees this starvation problem in the workloads when optimistic > spinning is disabled. > > This patch adds an additional boolean to the mutex that gets used in > the CONFIG_SMP && !CONFIG_MUTEX_SPIN_ON_OWNER cases. The flag signifies > whether or not other threads need to yield to a waiter and gets set > when a waiter spends too much time waiting for the mutex. The threshold > is currently set to 16 wakeups, and once the wakeup threshold is exceeded, > other threads must yield to the top waiter. The flag gets cleared > immediately after the top waiter acquires the mutex.
There is a subtle difference between this patch and Waiman's. Waiman's patch will boost any waiter-spinner which is woken up, however, this patch will boost the top waiter once the number of any waiter-spinners woken up reaches the threshold. We can't get any benefit if the resource holder which top waiter is waiting for still not release the resource.
Regards, Wanpeng Li
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