[PATCH] locking/rtmutex: remove unused cmpxchg_relaxed

[PATCH] locking/rtmutex: remove unused cmpxchg_relaxed

[Alex Shi]

No one use this macro after it was introduced. Better to remove it?

Signed-off-by: Alex Shi <alex.shi@linux.alibaba.com>
Cc: Peter Zijlstra <peterz@infradead.org> 
Cc: Davidlohr Bueso <dave@stgolabs.net>
Cc: Ingo Molnar <mingo@redhat.com> 
Cc: Will Deacon <will@kernel.org> 
Cc: linux-kernel@vger.kernel.org 
---
 kernel/locking/rtmutex.c | 1 -
 1 file changed, 1 deletion(-)

diff --git a/kernel/locking/rtmutex.c b/kernel/locking/rtmutex.c
index 851bbb10819d..a849964a348d 100644
--- a/kernel/locking/rtmutex.c
+++ b/kernel/locking/rtmutex.c
@@ -141,7 +141,6 @@ static void fixup_rt_mutex_waiters(struct rt_mutex *lock)
  * set up.
  */
 #ifndef CONFIG_DEBUG_RT_MUTEXES
-# define rt_mutex_cmpxchg_relaxed(l,c,n) (cmpxchg_relaxed(&l->owner, c, n) == c)
 # define rt_mutex_cmpxchg_acquire(l,c,n) (cmpxchg_acquire(&l->owner, c, n) == c)
 # define rt_mutex_cmpxchg_release(l,c,n) (cmpxchg_release(&l->owner, c, n) == c)
 
-- 
1.8.3.1

Re: [PATCH] locking/rtmutex: remove unused cmpxchg_relaxed

[Davidlohr Bueso]

Cc'ing tglx.

On Tue, 21 Jan 2020, Alex Shi wrote:

>No one use this macro after it was introduced. Better to remove it?

You also need to remove it for the CONFIG_DEBUG_RT_MUTEXES=y case.

>
>Signed-off-by: Alex Shi <alex.shi@linux.alibaba.com>
>Cc: Peter Zijlstra <peterz@infradead.org>
>Cc: Davidlohr Bueso <dave@stgolabs.net>
>Cc: Ingo Molnar <mingo@redhat.com>
>Cc: Will Deacon <will@kernel.org>
>Cc: linux-kernel@vger.kernel.org
>---
> kernel/locking/rtmutex.c | 1 -
> 1 file changed, 1 deletion(-)
>
>diff --git a/kernel/locking/rtmutex.c b/kernel/locking/rtmutex.c
>index 851bbb10819d..a849964a348d 100644
>--- a/kernel/locking/rtmutex.c
>+++ b/kernel/locking/rtmutex.c
>@@ -141,7 +141,6 @@ static void fixup_rt_mutex_waiters(struct rt_mutex *lock)
>  * set up.
>  */
> #ifndef CONFIG_DEBUG_RT_MUTEXES
>-# define rt_mutex_cmpxchg_relaxed(l,c,n) (cmpxchg_relaxed(&l->owner, c, n) == c)
> # define rt_mutex_cmpxchg_acquire(l,c,n) (cmpxchg_acquire(&l->owner, c, n) == c)
> # define rt_mutex_cmpxchg_release(l,c,n) (cmpxchg_release(&l->owner, c, n) == c)

Hmm unrelated, but do we want CCAS for rtmutex fastpath? Ie:

     (l->owner == c && cmpxchg_acquire(&l->owner, c, n) == c)

That would optimize for the contended case and avoid the cmpxchg - it would
also help if we ever do the top-waiter spin thing.

Thanks,
Davidlohr

Re: [PATCH] locking/rtmutex: remove unused cmpxchg_relaxed

[Thomas Gleixner]

Davidlohr Bueso <dave@stgolabs.net> writes:
> On Tue, 21 Jan 2020, Alex Shi wrote:

  Subject: locking/rtmutex: remove unused cmpxchg_relaxed

should be

  Subject: locking/rtmutex: Remove unused rt_mutex_cmpxchg_relaxed()

You're not removing cmpxchg_relaxed, right?

>> No one use this macro after it was introduced. Better to remove it?

Please make that factual.

 The macro was never used at all. Remove it.

> You also need to remove it for the CONFIG_DEBUG_RT_MUTEXES=y case.

Yes.

> Hmm unrelated, but do we want CCAS for rtmutex fastpath? Ie:
>
>      (l->owner == c && cmpxchg_acquire(&l->owner, c, n) == c)
>
> That would optimize for the contended case and avoid the cmpxchg - it would
> also help if we ever do the top-waiter spin thing.

Not sure if it buys much, but it kinda makes sense.

Thanks,

        tglx

Re: [PATCH] locking/rtmutex: remove unused cmpxchg_relaxed

[Alex Shi]

在 2020/2/1 上午4:23, Thomas Gleixner 写道:
> Davidlohr Bueso <dave@stgolabs.net> writes:
>> On Tue, 21 Jan 2020, Alex Shi wrote:
> 
>   Subject: locking/rtmutex: remove unused cmpxchg_relaxed
> 
> should be
> 
>   Subject: locking/rtmutex: Remove unused rt_mutex_cmpxchg_relaxed()
> 
> You're not removing cmpxchg_relaxed, right?
> 
>>> No one use this macro after it was introduced. Better to remove it?
> 
> Please make that factual.
> 
>  The macro was never used at all. Remove it.
> 
>> You also need to remove it for the CONFIG_DEBUG_RT_MUTEXES=y case.
> 
> Yes.
> 
>> Hmm unrelated, but do we want CCAS for rtmutex fastpath? Ie:
>>
>>      (l->owner == c && cmpxchg_acquire(&l->owner, c, n) == c)
>>
>> That would optimize for the contended case and avoid the cmpxchg - it would
>> also help if we ever do the top-waiter spin thing.
> 
> Not sure if it buys much, but it kinda makes sense.
> 
> Thanks,
> 
>         tglx
> 
Thanks Thomas and David!
Is this following patch ok?

Thanks
Alex
---
From 4cf9e38a73c67c6894f3addb2ddca26bb51b1a28 Mon Sep 17 00:00:00 2001
From: Alex Shi <alex.shi@linux.alibaba.com>
Date: Tue, 21 Jan 2020 15:03:33 +0800
Subject: [PATCH v2] locking/rtmutex: optimize rt_mutex_cmpxchg_xxx series func

rt_mutex_cmpxchg_relexed isn't interested by anyone, so remove it.
And Davidlohr Bueso suggests check l->owner before cmpxchg to reduce
lock contention.

Signed-off-by: Alex Shi <alex.shi@linux.alibaba.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Davidlohr Bueso <dave@stgolabs.net>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Will Deacon <will@kernel.org>
Cc: linux-kernel@vger.kernel.org
---
 kernel/locking/rtmutex.c | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/kernel/locking/rtmutex.c b/kernel/locking/rtmutex.c
index 851bbb10819d..eb26f4e57ce4 100644
--- a/kernel/locking/rtmutex.c
+++ b/kernel/locking/rtmutex.c
@@ -141,9 +141,10 @@ static void fixup_rt_mutex_waiters(struct rt_mutex *lock)
  * set up.
  */
 #ifndef CONFIG_DEBUG_RT_MUTEXES
-# define rt_mutex_cmpxchg_relaxed(l,c,n) (cmpxchg_relaxed(&l->owner, c, n) == c)
-# define rt_mutex_cmpxchg_acquire(l,c,n) (cmpxchg_acquire(&l->owner, c, n) == c)
-# define rt_mutex_cmpxchg_release(l,c,n) (cmpxchg_release(&l->owner, c, n) == c)
+# define rt_mutex_cmpxchg_acquire(l,c,n)	\
+		(l->owner == c && cmpxchg_acquire(&l->owner, c, n) == c)
+# define rt_mutex_cmpxchg_release(l,c,n)	\
+		(l->owner == c && cmpxchg_release(&l->owner, c, n) == c)
 
 /*
  * Callers must hold the ->wait_lock -- which is the whole purpose as we force
@@ -202,7 +203,6 @@ static inline bool unlock_rt_mutex_safe(struct rt_mutex *lock,
 }
 
 #else
-# define rt_mutex_cmpxchg_relaxed(l,c,n)	(0)
 # define rt_mutex_cmpxchg_acquire(l,c,n)	(0)
 # define rt_mutex_cmpxchg_release(l,c,n)	(0)
 
-- 
1.8.3.1

Re: [PATCH] locking/rtmutex: remove unused cmpxchg_relaxed

[Davidlohr Bueso]

On Tue, 04 Feb 2020, Alex Shi wrote:

>Thanks Thomas and David!
>Is this following patch ok?

So if anything, this really wants to be two patches.

>---
>From 4cf9e38a73c67c6894f3addb2ddca26bb51b1a28 Mon Sep 17 00:00:00 2001
>From: Alex Shi <alex.shi@linux.alibaba.com>
>Date: Tue, 21 Jan 2020 15:03:33 +0800
>Subject: [PATCH v2] locking/rtmutex: optimize rt_mutex_cmpxchg_xxx series func
>
>rt_mutex_cmpxchg_relexed isn't interested by anyone, so remove it.
>And Davidlohr Bueso suggests check l->owner before cmpxchg to reduce
>lock contention.
>
>Signed-off-by: Alex Shi <alex.shi@linux.alibaba.com>
>Cc: Thomas Gleixner <tglx@linutronix.de>
>Cc: Davidlohr Bueso <dave@stgolabs.net>
>Cc: Ingo Molnar <mingo@redhat.com>
>Cc: Will Deacon <will@kernel.org>
>Cc: linux-kernel@vger.kernel.org
>---
> kernel/locking/rtmutex.c | 8 ++++----
> 1 file changed, 4 insertions(+), 4 deletions(-)
>
>diff --git a/kernel/locking/rtmutex.c b/kernel/locking/rtmutex.c
>index 851bbb10819d..eb26f4e57ce4 100644
>--- a/kernel/locking/rtmutex.c
>+++ b/kernel/locking/rtmutex.c
>@@ -141,9 +141,10 @@ static void fixup_rt_mutex_waiters(struct rt_mutex *lock)
>  * set up.
>  */
> #ifndef CONFIG_DEBUG_RT_MUTEXES
>-# define rt_mutex_cmpxchg_relaxed(l,c,n) (cmpxchg_relaxed(&l->owner, c, n) == c)
>-# define rt_mutex_cmpxchg_acquire(l,c,n) (cmpxchg_acquire(&l->owner, c, n) == c)
>-# define rt_mutex_cmpxchg_release(l,c,n) (cmpxchg_release(&l->owner, c, n) == c)
>+# define rt_mutex_cmpxchg_acquire(l,c,n)	\
>+		(l->owner == c && cmpxchg_acquire(&l->owner, c, n) == c)
>+# define rt_mutex_cmpxchg_release(l,c,n)	\
>+		(l->owner == c && cmpxchg_release(&l->owner, c, n) == c)

Thomas, should I resend the top-waiter spin series? Otherwise yeah,
I see little point to the CCAS fastpath thing.

Thanks,
Davidlohr

[PATCH 1/2] locking/rtmutex: Remove unused rt_mutex_cmpxchg_relaxed()

[Alex Shi]

This macro isn't interested by anyone, so remove it.

Signed-off-by: Alex Shi <alex.shi@linux.alibaba.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Davidlohr Bueso <dave@stgolabs.net>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Will Deacon <will@kernel.org>
Cc: linux-kernel@vger.kernel.org
---
 kernel/locking/rtmutex.c | 2 --
 1 file changed, 2 deletions(-)

diff --git a/kernel/locking/rtmutex.c b/kernel/locking/rtmutex.c
index 851bbb10819d..7ad22eade1cc 100644
--- a/kernel/locking/rtmutex.c
+++ b/kernel/locking/rtmutex.c
@@ -141,7 +141,6 @@ static void fixup_rt_mutex_waiters(struct rt_mutex *lock)
  * set up.
  */
 #ifndef CONFIG_DEBUG_RT_MUTEXES
-# define rt_mutex_cmpxchg_relaxed(l,c,n) (cmpxchg_relaxed(&l->owner, c, n) == c)
 # define rt_mutex_cmpxchg_acquire(l,c,n) (cmpxchg_acquire(&l->owner, c, n) == c)
 # define rt_mutex_cmpxchg_release(l,c,n) (cmpxchg_release(&l->owner, c, n) == c)
 
@@ -202,7 +201,6 @@ static inline bool unlock_rt_mutex_safe(struct rt_mutex *lock,
 }
 
 #else
-# define rt_mutex_cmpxchg_relaxed(l,c,n)	(0)
 # define rt_mutex_cmpxchg_acquire(l,c,n)	(0)
 # define rt_mutex_cmpxchg_release(l,c,n)	(0)
 
-- 
1.8.3.1

[PATCH 2/2] locking/rtmutex: optimize rt_mutex_cmpxchgs

[Alex Shi]

Checking l->owner first to skip time cost cmpxchgs.

Suggested-by: Davidlohr Bueso <dave@stgolabs.net>
Signed-off-by: Alex Shi <alex.shi@linux.alibaba.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Davidlohr Bueso <dave@stgolabs.net>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Will Deacon <will@kernel.org>
Cc: linux-kernel@vger.kernel.org
---
 kernel/locking/rtmutex.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/kernel/locking/rtmutex.c b/kernel/locking/rtmutex.c
index 7ad22eade1cc..e8df466af8ab 100644
--- a/kernel/locking/rtmutex.c
+++ b/kernel/locking/rtmutex.c
@@ -141,8 +141,10 @@ static void fixup_rt_mutex_waiters(struct rt_mutex *lock)
  * set up.
  */
 #ifndef CONFIG_DEBUG_RT_MUTEXES
-# define rt_mutex_cmpxchg_acquire(l,c,n) (cmpxchg_acquire(&l->owner, c, n) == c)
-# define rt_mutex_cmpxchg_release(l,c,n) (cmpxchg_release(&l->owner, c, n) == c)
+# define rt_mutex_cmpxchg_acquire(l, c, n)	\
+			(l->owner == c && cmpxchg_acquire(&l->owner, c, n) == c)
+# define rt_mutex_cmpxchg_release(l, c, n)	\
+			(l->owner == c && cmpxchg_release(&l->owner, c, n) == c)
 
 /*
  * Callers must hold the ->wait_lock -- which is the whole purpose as we force
-- 
1.8.3.1