[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: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

1 files changed, 13 insertions, 5 deletions

diff --git a/include/linux/kernel.h b/include/linux/kernel.h
index e25b97062ce1..687ba8c9973d 100644
--- a/include/linux/kernel.h
+++ b/include/linux/kernel.h
@@ -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);
+  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;		\