linux-kernel - Re: [PATCH v5 1/6] rwsem: check the lock before cpmxchg in down_write

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Date:	Tue, 24 Sep 2013 16:22:53 -0700
From:	Jason Low <jason.low2@...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>,
	Alex Shi <alex.shi@...aro.org>,
	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>,
	Peter Hurley <peter@...leysoftware.com>,
	linux-kernel@...r.kernel.org, linux-mm <linux-mm@...ck.org>
Subject: Re: [PATCH v5 1/6] rwsem: check the lock before cpmxchg in down_write_trylock

Should we do something similar with __down_read_trylock, such as
the following?


Signed-off-by: Jason Low <jason.low2@...com>
---
 include/asm-generic/rwsem.h |    3 +++
 1 files changed, 3 insertions(+), 0 deletions(-)

diff --git a/include/asm-generic/rwsem.h b/include/asm-generic/rwsem.h
index bb1e2cd..47990dc 100644
--- a/include/asm-generic/rwsem.h
+++ b/include/asm-generic/rwsem.h
@@ -42,6 +42,9 @@ static inline int __down_read_trylock(struct
rw_semaphore *sem)
        long tmp;

        while ((tmp = sem->count) >= 0) {
+               if (sem->count != tmp)
+                       continue;
+
                if (tmp == cmpxchg(&sem->count, tmp,
                                   tmp + RWSEM_ACTIVE_READ_BIAS)) {
                        return 1;
-- 
1.7.1

On Tue, Sep 24, 2013 at 3:22 PM, Tim Chen <tim.c.chen@...ux.intel.com> wrote:
> Cmpxchg will cause the cacheline bouning when do the value checking,
> that cause scalability issue in a large machine (like a 80 core box).
>
> So a lock pre-read can relief this contention.
>
> Signed-off-by: Alex Shi <alex.shi@...el.com>
> ---
>  include/asm-generic/rwsem.h |    8 ++++----
>  1 files changed, 4 insertions(+), 4 deletions(-)
>
> diff --git a/include/asm-generic/rwsem.h b/include/asm-generic/rwsem.h
> index bb1e2cd..5ba80e7 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;
>  }
>
>  /*
> --
> 1.7.4.4
>
>
>
> --
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