linux-kernel - Re: [PATCH v2 1/5] mutex: In mutex_can_spin_on_owner(), return false if task need

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Message-ID: <20140128202042.GN9012@linux.vnet.ibm.com>
Date:	Tue, 28 Jan 2014 12:20:42 -0800
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Jason Low <jason.low2@...com>
Cc:	mingo@...hat.com, peterz@...radead.org, Waiman.Long@...com,
	torvalds@...ux-foundation.org, tglx@...utronix.de,
	linux-kernel@...r.kernel.org, riel@...hat.com,
	akpm@...ux-foundation.org, davidlohr@...com, hpa@...or.com,
	andi@...stfloor.org, aswin@...com, scott.norton@...com,
	chegu_vinod@...com
Subject: Re: [PATCH v2 1/5] mutex: In mutex_can_spin_on_owner(), return false
 if task need_resched()

On Tue, Jan 28, 2014 at 11:13:12AM -0800, Jason Low wrote:
> The mutex_can_spin_on_owner() function should also return false if the
> task needs to be rescheduled to avoid entering the MCS queue when it
> needs to reschedule.
> 
> Signed-off-by: Jason Low <jason.low2@...com>

Reviewed-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>

But I cannot help asking how this affects performance.  (My guess is
"not much", but always good to know.)

> ---
>  kernel/locking/mutex.c |    3 +++
>  1 files changed, 3 insertions(+), 0 deletions(-)
> 
> diff --git a/kernel/locking/mutex.c b/kernel/locking/mutex.c
> index 4dd6e4c..85c6be1 100644
> --- a/kernel/locking/mutex.c
> +++ b/kernel/locking/mutex.c
> @@ -212,6 +212,9 @@ static inline int mutex_can_spin_on_owner(struct mutex *lock)
>  	struct task_struct *owner;
>  	int retval = 1;
> 
> +	if (need_resched())
> +		return 0;
> +
>  	rcu_read_lock();
>  	owner = ACCESS_ONCE(lock->owner);
>  	if (owner)
> -- 
> 1.7.1
> 

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