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

Wanpeng Li
2016-08-18T00:39:07Z

2016-08-18 2:30 GMT+08:00 Jason Low <jason.low2@hpe.com>:
> Hi Wanpeng,
>
> On Wed, 2016-08-17 at 09:41 +0800, Wanpeng Li wrote:
>> 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.
>
> Correct, since when spinning is disabled, we still want to generally
> allow other threads to steal the lock even if there are waiters in order
> to keep performance good, and only yield the lock when a waiter is
> getting 'starved'.
>
>> We can't get any benefit if the
>> resource holder which top waiter is waiting for still not release the
>> resource.
>
> If the resource holder does not release the resource, that sounds like
> an issue with the lock holder.
>
> Unless you're referring to how this doesn't provide immediate benefit to
> the top waiter,

Yes.

> in which case, I think that is okay since the goal of
> the patch is to prevent starvation. We tried disabling 'lock stealing'
> anytime there are waiters and that proved to reduce performance by quite
> a bit in some workloads.

Thanks for the clarification. :)

Regards,
Wanpeng Li