linux-kernel - Re: [PATCH 1/2] rwsem: check the lock before cpmxchg in down_write_trylock and rwsem_do

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Date:	Sat, 22 Jun 2013 08:10:11 +0800
From:	Alex Shi <alex.shi@...el.com>
To:	Tim Chen <tim.c.chen@...ux.intel.com>
CC:	Ingo Molnar <mingo@...e.hu>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Andi Kleen <andi@...stfloor.org>,
	Michel Lespinasse <walken@...gle.com>,
	Davidlohr Bueso <davidlohr.bueso@...com>,
	Matthew R Wilcox <matthew.r.wilcox@...el.com>,
	Dave Hansen <dave.hansen@...el.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Rik van Riel <riel@...hat.com>, linux-kernel@...r.kernel.org,
	linux-mm <linux-mm@...ck.org>
Subject: Re: [PATCH 1/2] rwsem: check the lock before cpmxchg in down_write_trylock
 and rwsem_do_wake

On 06/22/2013 07:51 AM, Tim Chen wrote:
> Doing cmpxchg will cause cache bouncing when checking
> sem->count. This could cause scalability issue
> in a large machine (e.g. a 80 cores box).
> 
> A pre-read of sem->count can mitigate this.
> 
> Signed-off-by: Alex Shi <alex.shi@...el.com>
> Signed-off-by: Tim Chen <tim.c.chen@...ux.intel.com>

Hi Tim,
there is a technical error in this patch.
the "From: " line should be 'Alex Shi', since he made the most input of
this patch.

And I still think split this patch to 4 smaller will make it more simple
to review, that I had sent you and Davidlohr.

could you like to re-send with my 4 patch version? :)

> ---
>  include/asm-generic/rwsem.h |    8 ++++----
>  lib/rwsem.c                 |   21 +++++++++++++--------
>  2 files changed, 17 insertions(+), 12 deletions(-)
> 
> diff --git a/include/asm-generic/rwsem.h b/include/asm-generic/rwsem.h
> index bb1e2cd..052d973 100644
> --- a/include/asm-generic/rwsem.h
> +++ b/include/asm-generic/rwsem.h
> @@ -70,11 +70,11 @@ static inline void __down_write(struct rw_semaphore *sem)
>  
>  static inline int __down_write_trylock(struct rw_semaphore *sem)
>  {
> -	long tmp;
> +	if (unlikely(&sem->count != RWSEM_UNLOCKED_VALUE))
> +		return 0;
>  
> -	tmp = cmpxchg(&sem->count, RWSEM_UNLOCKED_VALUE,
> -		      RWSEM_ACTIVE_WRITE_BIAS);
> -	return tmp == RWSEM_UNLOCKED_VALUE;
> +	return cmpxchg(&sem->count, RWSEM_UNLOCKED_VALUE,
> +		RWSEM_ACTIVE_WRITE_BIAS) == RWSEM_UNLOCKED_VALUE;
>  }
>  
>  /*
> diff --git a/lib/rwsem.c b/lib/rwsem.c
> index 19c5fa9..2072af5 100644
> --- a/lib/rwsem.c
> +++ b/lib/rwsem.c
> @@ -75,7 +75,7 @@ __rwsem_do_wake(struct rw_semaphore *sem, enum rwsem_wake_type wake_type)
>  			 * will block as they will notice the queued writer.
>  			 */
>  			wake_up_process(waiter->task);
> -		goto out;
> +		return sem;
>  	}
>  
>  	/* Writers might steal the lock before we grant it to the next reader.
> @@ -85,15 +85,21 @@ __rwsem_do_wake(struct rw_semaphore *sem, enum rwsem_wake_type wake_type)
>  	adjustment = 0;
>  	if (wake_type != RWSEM_WAKE_READ_OWNED) {
>  		adjustment = RWSEM_ACTIVE_READ_BIAS;
> - try_reader_grant:
> -		oldcount = rwsem_atomic_update(adjustment, sem) - adjustment;
> -		if (unlikely(oldcount < RWSEM_WAITING_BIAS)) {
> -			/* A writer stole the lock. Undo our reader grant. */
> +		while (1) {
> +			/* A writer stole the lock. */
> +			if (sem->count < RWSEM_WAITING_BIAS)
> +				return sem;
> +
> +			oldcount = rwsem_atomic_update(adjustment, sem)
> +								- adjustment;
> +			if (likely(oldcount >= RWSEM_WAITING_BIAS))
> +				break;
> +
> +			 /* A writer stole the lock.  Undo our reader grant. */
>  			if (rwsem_atomic_update(-adjustment, sem) &
>  						RWSEM_ACTIVE_MASK)
> -				goto out;
> +				return sem;
>  			/* Last active locker left. Retry waking readers. */
> -			goto try_reader_grant;
>  		}
>  	}
>  
> @@ -136,7 +142,6 @@ __rwsem_do_wake(struct rw_semaphore *sem, enum rwsem_wake_type wake_type)
>  	sem->wait_list.next = next;
>  	next->prev = &sem->wait_list;
>  
> - out:
>  	return sem;
>  }
>  
> 


-- 
Thanks
    Alex
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