Re: [PATCH v2] locking/mutex: Prevent lock starvation when spinning is enabled

Wanpeng Li
2016-08-17T01:42:02Z

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