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

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Date:	Wed, 17 Aug 2016 09:41:59 +0800
From:	Wanpeng Li <kernellwp@...il.com>
To:	Jason Low <jason.low2@....com>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...hat.com>, imre.deak@...el.com,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Jason Low <jason.low2@...com>,
	Waiman Long <Waiman.Long@....com>,
	Davidlohr Bueso <dave@...olabs.net>,
	Tim Chen <tim.c.chen@...ux.intel.com>, terry.rudd@....com,
	"Paul E. McKenney" <paulmck@...ibm.com>
Subject: Re: [PATCH v2] locking/mutex: Prevent lock starvation when spinning
 is enabled

2016-08-11 2:44 GMT+08:00 Jason Low <jason.low2@....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