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[PATCH] sched: voluntary kernel preemption

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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>
This commit is contained in:
Ingo Molnar 2005-06-25 14:57:39 -07:00 committed by Linus Torvalds
parent f704f56af9
commit f8cbd99bd3
2 changed files with 63 additions and 14 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

20
include/linux/kernel.h
View File

@ -58,15 +58,23 @@ struct completion;
* be biten later when the calling function happens to sleep when it is not
* supposed to.
*/
#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);
#ifdef CONFIG_PREEMPT_VOLUNTARY
extern int cond_resched(void);
# define might_resched() cond_resched()
#else
#define might_sleep() do {} while(0)
#define might_sleep_if(cond) do {} while (0)
# define might_resched() do { } while (0)
#endif
#ifdef CONFIG_DEBUG_SPINLOCK_SLEEP
void __might_sleep(char *file, int line);
# define might_sleep() \
do { __might_sleep(__FILE__, __LINE__); might_resched(); } while (0)
#else
# 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; \

57
kernel/Kconfig.preempt
View File

@ -1,15 +1,56 @@
config PREEMPT
bool "Preemptible Kernel"
choice
prompt "Preemption Model"
default PREEMPT_NONE
config PREEMPT_NONE
bool "No Forced Preemption (Server)"
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 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 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"