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Message-ID: <alpine.DEB.2.02.1405281143000.21720@ionos.tec.linutronix.de>
Date: Wed, 28 May 2014 11:43:16 +0200 (CEST)
From: Thomas Gleixner <tglx@...utronix.de>
To: Jason Low <jason.low2@...com>
cc: LKML <linux-kernel@...r.kernel.org>,
Ingo Molnar <mingo@...nel.org>,
Peter Zijlstra <peterz@...radead.org>,
Steven Rostedt <rostedt@...dmis.org>,
Lai Jiangshan <laijs@...fujitsu.com>
Subject: Re: [patch 6/6] rtmutex: Avoid pointless requeueing in the deadlock
detection chain walk
On Tue, 27 May 2014, Jason Low wrote:
> On Wed, May 21, 2014 at 8:25 PM, Thomas Gleixner <tglx@...utronix.de> wrote:
>
> > @@ -440,32 +452,41 @@ static int rt_mutex_adjust_prio_chain(st
> > get_task_struct(task);
> > raw_spin_lock_irqsave(&task->pi_lock, flags);
> >
> > - if (waiter == rt_mutex_top_waiter(lock)) {
> > - /*
> > - * The waiter became the top waiter on the
> > - * lock. Remove the previous top waiter from the tasks
> > - * pi waiters list and add waiter to it.
> > - */
> > - rt_mutex_dequeue_pi(task, prerequeue_top_waiter);
> > - rt_mutex_enqueue_pi(task, waiter);
> > - __rt_mutex_adjust_prio(task);
> > -
> > - } else if (prerequeue_top_waiter == waiter) {
> > - /*
> > - * The waiter was the top waiter on the lock. Remove
> > - * waiter from the tasks pi waiters list and add the
> > - * new top waiter to it.
> > - */
> > - rt_mutex_dequeue_pi(task, waiter);
> > - waiter = rt_mutex_top_waiter(lock);
> > - rt_mutex_enqueue_pi(task, waiter);
> > - __rt_mutex_adjust_prio(task);
> > -
> > - } else {
> > - /*
> > - * Nothing changed. No need to do any priority
> > - * adjustment.
> > - */
> > + /*
> > + * In case we are just following the lock chain for deadlock
> > + * detection we can avoid the whole requeue and priority
> > + * adjustment business.
> > + */
> > + if (requeue) {
> > + if (waiter == rt_mutex_top_waiter(lock)) {
> > + /*
> > + * The waiter became the top waiter on the
> > + * lock. Remove the previous top waiter from
> > + * the tasks pi waiters list and add waiter to
> > + * it.
> > + */
> > + rt_mutex_dequeue_pi(task, prerequeue_top_waiter);
> > + rt_mutex_enqueue_pi(task, waiter);
> > + __rt_mutex_adjust_prio(task);
> > +
> > + } else if (prerequeue_top_waiter == waiter) {
> > + /*
> > + * The waiter was the top waiter on the
> > + * lock. Remove waiter from the tasks pi
> > + * waiters list and add the new top waiter to
> > + * it.
> > + */
> > + rt_mutex_dequeue_pi(task, waiter);
> > + waiter = rt_mutex_top_waiter(lock);
> > + rt_mutex_enqueue_pi(task, waiter);
> > + __rt_mutex_adjust_prio(task);
> > +
> > + } else {
> > + /*
> > + * Nothing changed. No need to do any priority
> > + * adjustment.
> > + */
> > + }
> > }
> >
> > raw_spin_unlock_irqrestore(&task->pi_lock, flags);
>
> In the above case, could we go 1 step further and avoid taking the pi
> lock as well?
>
> if (requeue) {
> raw_spin_lock_irqsave(&task->pi_lock, flags);
>
> if (waiter == rt_mutex_top_waiter(lock)) {
> /*
> * The waiter became the top waiter on the
> * lock. Remove the previous top waiter from
> * the tasks pi waiters list and add waiter to
> * it.
> */
> rt_mutex_dequeue_pi(task, prerequeue_top_waiter);
> rt_mutex_enqueue_pi(task, waiter);
> __rt_mutex_adjust_prio(task);
>
> } else if (prerequeue_top_waiter == waiter) {
> /*
> * The waiter was the top waiter on the
> * lock. Remove waiter from the tasks pi
> * waiters list and add the new top waiter to
> * it.
> */
> rt_mutex_dequeue_pi(task, waiter);
> waiter = rt_mutex_top_waiter(lock);
> rt_mutex_enqueue_pi(task, waiter);
> __rt_mutex_adjust_prio(task);
>
> } else {
> /*
> * Nothing changed. No need to do any priority
> * adjustment.
> */
> }
>
> raw_spin_unlock_irqrestore(&task->pi_lock, flags);
> }
Indeed.
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