Commit f8cbd99b authored by Ingo Molnar's avatar Ingo Molnar Committed by Linus Torvalds
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[PATCH] sched: voluntary kernel preemption 



This patch adds a new preemption model: 'Voluntary Kernel Preemption'.  The
3 models can be selected from a new menu:

            (X) No Forced Preemption (Server)
            ( ) Voluntary Kernel Preemption (Desktop)
            ( ) Preemptible Kernel (Low-Latency Desktop)

we still default to the stock (Server) preemption model.

Voluntary preemption works by adding a cond_resched()
(reschedule-if-needed) call to every might_sleep() check.  It is lighter
than CONFIG_PREEMPT - at the cost of not having as tight latencies.  It
represents a different latency/complexity/overhead tradeoff.

It has no runtime impact at all if disabled.  Here are size stats that show
how the various preemption models impact the kernel's size:

    text    data     bss     dec     hex filename
 3618774  547184  179896 4345854  424ffe vmlinux.stock
 3626406  547184  179896 4353486  426dce vmlinux.voluntary   +0.2%
 3748414  548640  179896 4476950  445016 vmlinux.preempt     +3.5%

voluntary-preempt is +0.2% of .text, preempt is +3.5%.

This feature has been tested for many months by lots of people (and it's
also included in the RHEL4 distribution and earlier variants were in Fedora
as well), and it's intended for users and distributions who dont want to
use full-blown CONFIG_PREEMPT for one reason or another.

Signed-off-by: default avatarIngo Molnar <mingo@elte.hu>
Signed-off-by: default avatarAndrew Morton <akpm@osdl.org>
Signed-off-by: default avatarLinus Torvalds <torvalds@osdl.org>
parent f704f56a

include/linux/kernel.h

+13 −5
Original line number Diff line number Diff line
@@ -58,15 +58,23 @@ struct completion; 
 * be biten later when the calling function happens to sleep when it is not

 * supposed to.

 */

#ifdef CONFIG_PREEMPT_VOLUNTARY

extern int cond_resched(void);

# define might_resched() cond_resched()

#else

# define might_resched() do { } while (0)

#endif



#ifdef CONFIG_DEBUG_SPINLOCK_SLEEP

#define might_sleep() __might_sleep(__FILE__, __LINE__)

#define might_sleep_if(cond) do { if (unlikely(cond)) might_sleep(); } while (0)

  void __might_sleep(char *file, int line);

# define might_sleep() \

	do { __might_sleep(__FILE__, __LINE__); might_resched(); } while (0)

#else

#define might_sleep() do {} while(0)

#define might_sleep_if(cond) do {} while (0)

# define might_sleep() do { might_resched(); } while (0)

#endif



#define might_sleep_if(cond) do { if (unlikely(cond)) might_sleep(); } while (0)



#define abs(x) ({				\

		int __x = (x);			\

		(__x < 0) ? -__x : __x;		\

kernel/Kconfig.preempt

+49 −8
Original line number Diff line number Diff line 


config PREEMPT

	bool "Preemptible Kernel"

choice

	prompt "Preemption Model"

	default PREEMPT_NONE



config PREEMPT_NONE

	bool "No Forced Preemption (Server)"

	help

	  This is the traditional Linux preemption model, geared towards

	  throughput. It will still provide good latencies most of the

	  time, but there are no guarantees and occasional longer delays

	  are possible.



	  Select this option if you are building a kernel for a server or

	  scientific/computation system, or if you want to maximize the

	  raw processing power of the kernel, irrespective of scheduling

	  latencies.



config PREEMPT_VOLUNTARY

	bool "Voluntary Kernel Preemption (Desktop)"

	help

	  This option reduces the latency of the kernel when reacting to

	  real-time or interactive events by allowing a low priority process to

	  be preempted even if it is in kernel mode executing a system call.

	  This allows applications to run more reliably even when the system is

	  This option reduces the latency of the kernel by adding more

	  "explicit preemption points" to the kernel code. These new

	  preemption points have been selected to reduce the maximum

	  latency of rescheduling, providing faster application reactions,

	  at the cost of slighly lower throughput.



	  This allows reaction to interactive events by allowing a

	  low priority process to voluntarily preempt itself even if it

	  is in kernel mode executing a system call. This allows

	  applications to run more 'smoothly' even when the system is

	  under load.



	  Say Y here if you are building a kernel for a desktop, embedded

	  or real-time system.  Say N if you are unsure.

	  Select this if you are building a kernel for a desktop system.



config PREEMPT

	bool "Preemptible Kernel (Low-Latency Desktop)"

	help

	  This option reduces the latency of the kernel by making

	  all kernel code (that is not executing in a critical section)

	  preemptible.  This allows reaction to interactive events by

	  permitting a low priority process to be preempted involuntarily

	  even if it is in kernel mode executing a system call and would

	  otherwise not be about to reach a natural preemption point.

	  This allows applications to run more 'smoothly' even when the

	  system is under load, at the cost of slighly lower throughput

	  and a slight runtime overhead to kernel code.



	  Select this if you are building a kernel for a desktop or

	  embedded system with latency requirements in the milliseconds

	  range.



endchoice



config PREEMPT_BKL

	bool "Preempt The Big Kernel Lock"

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