linux-kernel - Re: [PATCH v4 0/6] sched: use runnable load based balance

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Date:	Tue, 07 May 2013 15:51:14 +0800
From:	Alex Shi <alex.shi@...el.com>
To:	Peter Zijlstra <peterz@...radead.org>
CC:	mingo@...hat.com, tglx@...utronix.de, akpm@...ux-foundation.org,
	arjan@...ux.intel.com, bp@...en8.de, pjt@...gle.com,
	namhyung@...nel.org, efault@....de, morten.rasmussen@....com,
	vincent.guittot@...aro.org, gregkh@...uxfoundation.org,
	preeti@...ux.vnet.ibm.com, viresh.kumar@...aro.org,
	linux-kernel@...r.kernel.org, len.brown@...el.com,
	rafael.j.wysocki@...el.com, jkosina@...e.cz,
	clark.williams@...il.com, tony.luck@...el.com,
	keescook@...omium.org, mgorman@...e.de, riel@...hat.com
Subject: Re: [PATCH v4 0/6] sched: use runnable load based balance

On 05/03/2013 03:55 PM, Alex Shi wrote:
> 
>> That should probably look like:
>>
>> 	preempt_disable();
>> 	raw_spin_unlock_irq();
>> 	preempt_enable_no_resched();
>> 	schedule();
>>
>> Otherwise you might find a performance regression on PREEMPT=y kernels.
> 
> Yes, right!
> Thanks a lot for reminder. The following patch will fix it.
>>

Peter, would you like to pick this patch?
> 
> ---
> 
> From 4c9b4b8a9b92bcbe6934637fd33c617e73dbda97 Mon Sep 17 00:00:00 2001
> From: Alex Shi <alex.shi@...el.com>
> Date: Fri, 3 May 2013 14:51:25 +0800
> Subject: [PATCH 8/8] rwsem: small optimizing rwsem_down_failed_common
> 
> Peter Zijlstra suggest adding a preempt_enable_no_resched() to prevent
> a unnecessary scheduler in raw_spin_unlock.
> And we also can pack 2 raw_spin_lock to save one. So has this patch.
> 
> Thanks Peter!
> 
> Signed-off-by: Alex Shi <alex.shi@...el.com>
> ---
>  lib/rwsem.c | 12 +++++++-----
>  1 file changed, 7 insertions(+), 5 deletions(-)
> 
> diff --git a/lib/rwsem.c b/lib/rwsem.c
> index ad5e0df..9aacf81 100644
> --- a/lib/rwsem.c
> +++ b/lib/rwsem.c
> @@ -212,23 +212,25 @@ rwsem_down_failed_common(struct rw_semaphore *sem,
>  		 adjustment == -RWSEM_ACTIVE_WRITE_BIAS)
>  		sem = __rwsem_do_wake(sem, RWSEM_WAKE_READ_OWNED);
>  
> -	raw_spin_unlock_irq(&sem->wait_lock);
> -
>  	/* wait to be given the lock */
>  	for (;;) {
> -		if (!waiter.task)
> +		if (!waiter.task) {
> +			raw_spin_unlock_irq(&sem->wait_lock);
>  			break;
> +		}
>  
> -		raw_spin_lock_irq(&sem->wait_lock);
> -		/* Try to get the writer sem, may steal from the head writer: */
> +		/* Try to get the writer sem, may steal from the head writer */
>  		if (flags == RWSEM_WAITING_FOR_WRITE)
>  			if (try_get_writer_sem(sem, &waiter)) {
>  				raw_spin_unlock_irq(&sem->wait_lock);
>  				return sem;
>  			}
> +		preempt_disable();
>  		raw_spin_unlock_irq(&sem->wait_lock);
> +		preempt_enable_no_resched();
>  		schedule();
>  		set_task_state(tsk, TASK_UNINTERRUPTIBLE);
> +		raw_spin_lock_irq(&sem->wait_lock);
>  	}
>  
>  	tsk->state = TASK_RUNNING;
> 


-- 
Thanks
    Alex
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