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Date: Mon, 9 Jun 2014 20:48:49 -0700
From: Jason Low <jason.low2@...com>
To: Steven Rostedt <rostedt@...dmis.org>
Cc: Thomas Gleixner <tglx@...utronix.de>,
LKML <linux-kernel@...r.kernel.org>,
Peter Zijlstra <peterz@...radead.org>,
Ingo Molnar <mingo@...nel.org>,
Lai Jiangshan <laijs@...fujitsu.com>,
Jason Low <jason.low2@...com>, Brad Mouring <bmouring@...com>
Subject: Re: [patch V3 7/7] rtmutex: Avoid pointless requeueing in the
deadlock detection chain walk
On Mon, Jun 9, 2014 at 6:20 PM, Steven Rostedt <rostedt@...dmis.org> wrote:
> On Mon, 09 Jun 2014 20:28:10 -0000
> Thomas Gleixner <tglx@...utronix.de> wrote:
>
>> In case the dead lock detector is enabled we follow the lock chain to
>> the end in rt_mutex_adjust_prio_chain, even if we could stop earlier
>> due to the priority/waiter constellation.
>>
>> But once we are not longer the top priority waiter in a certain step
>> or the task holding the lock has already the same priority then there
>> is no point in dequeing and enqueing along the lock chain as there is
>> no change at all.
>>
>> So stop the requeueing at this point.
>>
>> Signed-off-by: Thomas Gleixner <tglx@...utronix.de>
>> ---
>> kernel/locking/rtmutex.c | 61 +++++++++++++++++++++++++++++++++++++++++------
>> 1 file changed, 54 insertions(+), 7 deletions(-)
>>
>> Index: tip/kernel/locking/rtmutex.c
>> ===================================================================
>> --- tip.orig/kernel/locking/rtmutex.c
>> +++ tip/kernel/locking/rtmutex.c
>> @@ -359,6 +359,7 @@ static int rt_mutex_adjust_prio_chain(st
>> struct rt_mutex *lock;
>> bool detect_deadlock;
>> unsigned long flags;
>> + bool requeue = true;
>>
>> detect_deadlock = rt_mutex_cond_detect_deadlock(orig_waiter, chwalk);
>>
>> @@ -436,18 +437,31 @@ static int rt_mutex_adjust_prio_chain(st
>> goto out_unlock_pi;
>> /*
>> * If deadlock detection is off, we stop here if we
>> - * are not the top pi waiter of the task.
>> + * are not the top pi waiter of the task. If deadlock
>> + * detection is enabled we continue, but stop the
>> + * requeueing in the chain walk.
>> */
>> - if (!detect_deadlock && top_waiter != task_top_pi_waiter(task))
>> - goto out_unlock_pi;
>> + if (top_waiter != task_top_pi_waiter(task)) {
>> + if (!detect_deadlock)
>> + goto out_unlock_pi;
>> + else
>> + requeue = false;
>> + }
>> }
>>
>> /*
>> - * When deadlock detection is off then we check, if further
>> - * priority adjustment is necessary.
>> + * If the waiter priority is the same as the task priority
>> + * then there is no further priority adjustment necessary. If
>> + * deadlock detection is off, we stop the chain walk. If its
>> + * enabled we continue, but stop the requeueing in the chain
>> + * walk.
>> */
>> - if (!detect_deadlock && waiter->prio == task->prio)
>> - goto out_unlock_pi;
>> + if (waiter->prio == task->prio) {
>> + if (!detect_deadlock)
>> + goto out_unlock_pi;
>> + else
>> + requeue = false;
>> + }
>>
>> /*
>> * We need to trylock here as we are holding task->pi_lock,
>> @@ -475,6 +489,39 @@ static int rt_mutex_adjust_prio_chain(st
>> }
>>
>> /*
>> + * If we just follow the lock chain for deadlock detection, no
>> + * need to do all the requeue operations. We avoid a truckload
>
> s/We/To/
>
>
>> + * of conditinals around the various places below and just do
>
> s/ and/, /
And s/conditinals/conditionals/
>> + * the minimum chain walk checks here.
>> + */
>> + if (!requeue) {
>> + /* Release the task */
>> + raw_spin_unlock_irqrestore(&task->pi_lock, flags);
>> + put_task_struct(task);
>> +
>> + /* If there is no owner of the lock, end of chain. */
>> + if (!rt_mutex_owner(lock)) {
>> + raw_spin_unlock(&lock->wait_lock);
>> + return 0;
>> + }
>> +
>> + /* Grab the next task, i.e. owner of @lock */
>> + task = rt_mutex_owner(lock);
>> + get_task_struct(task);
>> + raw_spin_lock_irqsave(&task->pi_lock, flags);
>> +
>> + /* Store whether owner is blocked itself and drop locks */
>> + next_lock = task_blocked_on(task);
>> + raw_spin_unlock_irqrestore(&task->pi_lock, flags);
>> + raw_spin_unlock(&lock->wait_lock);
>> +
>> + /* If owner is not blocked, end of chain. */
>> + if (!next_lock)
>> + goto out_put_task;
>
> On the loop back around, have something like:
>
> if (top_waiter) {
> if (!task_has_pi_waiters(task))
> goto out_unlock_pi;
>
> if (!requeue &&
> top_waiter != task_top_pi_waiter(task)) {
> if (!detect_deadlock)
> goto out_unlock_pi;
> else
> requeue = false;
> }
> }
>
> ??
>
>
>> + goto again;
>> + }
>> +
>> + /*
>> * Store the current top waiter before doing the requeue
>> * operation on @lock. We need it for the boost/deboost
>> * decision below.
>>
>
> --
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