Commit 9ea4c380 authored by Peter Zijlstra's avatar Peter Zijlstra Committed by Ingo Molnar
Browse files

locking: Optimize lock_bh functions 



Currently all _bh_ lock functions do two preempt_count operations:

  local_bh_disable();
  preempt_disable();

and for the unlock:

  preempt_enable_no_resched();
  local_bh_enable();

Since its a waste of perfectly good cycles to modify the same variable
twice when you can do it in one go; use the new
__local_bh_{dis,en}able_ip() functions that allow us to provide a
preempt_count value to add/sub.

So define SOFTIRQ_LOCK_OFFSET as the offset a _bh_ lock needs to
add/sub to be done in one go.

As a bonus it gets rid of the preempt_enable_no_resched() usage.

This reduces a 1000 loops of:

  spin_lock_bh(&bh_lock);
  spin_unlock_bh(&bh_lock);

from 53596 cycles to 51995 cycles. I didn't do enough measurements to
say for absolute sure that the result is significant but the the few
runs I did for each suggest it is so.

Reviewed-by: default avatarThomas Gleixner <tglx@linutronix.de>
Signed-off-by: default avatarPeter Zijlstra <peterz@infradead.org>
Cc: jacob.jun.pan@linux.intel.com
Cc: Mike Galbraith <bitbucket@online.de>
Cc: hpa@zytor.com
Cc: Arjan van de Ven <arjan@linux.intel.com>
Cc: lenb@kernel.org
Cc: rjw@rjwysocki.net
Cc: rui.zhang@intel.com
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Link: http://lkml.kernel.org/r/20131119151338.GF3694@twins.programming.kicks-ass.net


Signed-off-by: default avatarIngo Molnar <mingo@kernel.org>
parent c726099e

include/linux/preempt_mask.h

+15 −0
Original line number Diff line number Diff line
@@ -78,6 +78,21 @@ 
# define PREEMPT_CHECK_OFFSET 0

#endif



/*

 * The preempt_count offset needed for things like:

 *

 *  spin_lock_bh()

 *

 * Which need to disable both preemption (CONFIG_PREEMPT_COUNT) and

 * softirqs, such that unlock sequences of:

 *

 *  spin_unlock();

 *  local_bh_enable();

 *

 * Work as expected.

 */

#define SOFTIRQ_LOCK_OFFSET (SOFTIRQ_DISABLE_OFFSET + PREEMPT_CHECK_OFFSET)



/*

 * Are we running in atomic context?  WARNING: this macro cannot

 * always detect atomic context; in particular, it cannot know about

include/linux/rwlock_api_smp.h

+4 −8
Original line number Diff line number Diff line
@@ -172,8 +172,7 @@ static inline void __raw_read_lock_irq(rwlock_t *lock) 


static inline void __raw_read_lock_bh(rwlock_t *lock)

{

	local_bh_disable();

	preempt_disable();

	__local_bh_disable_ip(_RET_IP_, SOFTIRQ_LOCK_OFFSET);

	rwlock_acquire_read(&lock->dep_map, 0, 0, _RET_IP_);

	LOCK_CONTENDED(lock, do_raw_read_trylock, do_raw_read_lock);

}
@@ -200,8 +199,7 @@ static inline void __raw_write_lock_irq(rwlock_t *lock) 


static inline void __raw_write_lock_bh(rwlock_t *lock)

{

	local_bh_disable();

	preempt_disable();

	__local_bh_disable_ip(_RET_IP_, SOFTIRQ_LOCK_OFFSET);

	rwlock_acquire(&lock->dep_map, 0, 0, _RET_IP_);

	LOCK_CONTENDED(lock, do_raw_write_trylock, do_raw_write_lock);

}
@@ -250,8 +248,7 @@ static inline void __raw_read_unlock_bh(rwlock_t *lock) 
{

	rwlock_release(&lock->dep_map, 1, _RET_IP_);

	do_raw_read_unlock(lock);

	preempt_enable_no_resched();

	local_bh_enable_ip((unsigned long)__builtin_return_address(0));

	__local_bh_enable_ip(_RET_IP_, SOFTIRQ_LOCK_OFFSET);

}



static inline void __raw_write_unlock_irqrestore(rwlock_t *lock,
@@ -275,8 +272,7 @@ static inline void __raw_write_unlock_bh(rwlock_t *lock) 
{

	rwlock_release(&lock->dep_map, 1, _RET_IP_);

	do_raw_write_unlock(lock);

	preempt_enable_no_resched();

	local_bh_enable_ip((unsigned long)__builtin_return_address(0));

	__local_bh_enable_ip(_RET_IP_, SOFTIRQ_LOCK_OFFSET);

}



#endif /* __LINUX_RWLOCK_API_SMP_H */

include/linux/spinlock_api_smp.h

+4 −8
Original line number Diff line number Diff line
@@ -131,8 +131,7 @@ static inline void __raw_spin_lock_irq(raw_spinlock_t *lock) 


static inline void __raw_spin_lock_bh(raw_spinlock_t *lock)

{

	local_bh_disable();

	preempt_disable();

	__local_bh_disable_ip(_RET_IP_, SOFTIRQ_LOCK_OFFSET);

	spin_acquire(&lock->dep_map, 0, 0, _RET_IP_);

	LOCK_CONTENDED(lock, do_raw_spin_trylock, do_raw_spin_lock);

}
@@ -174,20 +173,17 @@ static inline void __raw_spin_unlock_bh(raw_spinlock_t *lock) 
{

	spin_release(&lock->dep_map, 1, _RET_IP_);

	do_raw_spin_unlock(lock);

	preempt_enable_no_resched();

	local_bh_enable_ip((unsigned long)__builtin_return_address(0));

	__local_bh_enable_ip(_RET_IP_, SOFTIRQ_LOCK_OFFSET);

}



static inline int __raw_spin_trylock_bh(raw_spinlock_t *lock)

{

	local_bh_disable();

	preempt_disable();

	__local_bh_disable_ip(_RET_IP_, SOFTIRQ_LOCK_OFFSET);

	if (do_raw_spin_trylock(lock)) {

		spin_acquire(&lock->dep_map, 0, 1, _RET_IP_);

		return 1;

	}

	preempt_enable_no_resched();

	local_bh_enable_ip((unsigned long)__builtin_return_address(0));

	__local_bh_enable_ip(_RET_IP_, SOFTIRQ_LOCK_OFFSET);

	return 0;

}

include/linux/spinlock_api_up.h

+11 −5
Original line number Diff line number Diff line
@@ -24,11 +24,14 @@ 
 * flags straight, to suppress compiler warnings of unused lock

 * variables, and to add the proper checker annotations:

 */

#define ___LOCK(lock) \

  do { __acquire(lock); (void)(lock); } while (0)



#define __LOCK(lock) \

  do { preempt_disable(); __acquire(lock); (void)(lock); } while (0)

  do { preempt_disable(); ___LOCK(lock); } while (0)



#define __LOCK_BH(lock) \

  do { local_bh_disable(); __LOCK(lock); } while (0)

  do { __local_bh_disable_ip(_THIS_IP_, SOFTIRQ_LOCK_OFFSET); ___LOCK(lock); } while (0)



#define __LOCK_IRQ(lock) \

  do { local_irq_disable(); __LOCK(lock); } while (0)
@@ -36,12 +39,15 @@ 
#define __LOCK_IRQSAVE(lock, flags) \

  do { local_irq_save(flags); __LOCK(lock); } while (0)



#define ___UNLOCK(lock) \

  do { __release(lock); (void)(lock); } while (0)



#define __UNLOCK(lock) \

  do { preempt_enable(); __release(lock); (void)(lock); } while (0)

  do { preempt_enable(); ___UNLOCK(lock); } while (0)



#define __UNLOCK_BH(lock) \

  do { preempt_enable_no_resched(); local_bh_enable(); \

	  __release(lock); (void)(lock); } while (0)

  do { __local_bh_enable_ip(_THIS_IP_, SOFTIRQ_LOCK_OFFSET); \

       ___UNLOCK(lock); } while (0)



#define __UNLOCK_IRQ(lock) \

  do { local_irq_enable(); __UNLOCK(lock); } while (0)

kernel/softirq.c

+2 −2
Original line number Diff line number Diff line
@@ -107,7 +107,7 @@ static void __local_bh_disable(unsigned long ip, unsigned int cnt) 
	/*

	 * Were softirqs turned off above:

	 */

	if (softirq_count() == cnt)

	if (softirq_count() == (cnt & SOFTIRQ_MASK))

		trace_softirqs_off(ip);

	raw_local_irq_restore(flags);
@@ -133,7 +133,7 @@ static void __local_bh_enable(unsigned int cnt) 
{

	WARN_ON_ONCE(!irqs_disabled());



	if (softirq_count() == cnt)

	if (softirq_count() == (cnt & SOFTIRQ_MASK))

		trace_softirqs_on(_RET_IP_);

	preempt_count_sub(cnt);

}

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