[PATCH] i386: add sleazy FPU optimization

i386 port of the sLeAZY-fpu feature.  Chuck reports that this gives him a +/-
0.4% improvement on his simple benchmark

x86_64 description follows:

Right now the kernel on x86-64 has a 100% lazy fpu behavior: after *every*
context switch a trap is taken for the first FPU use to restore the FPU
context lazily.  This is of course great for applications that have very
sporadic or no FPU use (since then you avoid doing the expensive save/restore
all the time).  However for very frequent FPU users...  you take an extra trap
every context switch.

The patch below adds a simple heuristic to this code: After 5 consecutive
context switches of FPU use, the lazy behavior is disabled and the context
gets restored every context switch.  If the app indeed uses the FPU, the trap
is avoided.  (the chance of the 6th time slice using FPU after the previous 5
having done so are quite high obviously).

After 256 switches, this is reset and lazy behavior is returned (until there
are 5 consecutive ones again).  The reason for this is to give apps that do
longer bursts of FPU use still the lazy behavior back after some time.

Signed-off-by: Chuck Ebbert <76306.1226@compuserve.com>
Signed-off-by: Arjan van de Ven <arjan@linux.intel.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Andi Kleen <ak@suse.de>

1 files changed, 4 insertions, 1 deletions

diff --git a/include/asm-i386/i387.h b/include/asm-i386/i387.h
index bc1d6edae1ed..434936c732d6 100644
--- a/include/asm-i386/i387.h
+++ b/include/asm-i386/i387.h
@@ -76,7 +76,9 @@ static inline void __save_init_fpu( struct task_struct *tsk )
 
 #define __unlazy_fpu( tsk ) do { \
         if (task_thread_info(tsk)->status & TS_USEDFPU) \
-                save_init_fpu( tsk ); \
+                save_init_fpu( tsk );                   \
+        else                                            \
+                tsk->fpu_counter = 0;                   \
 } while (0)
 
 #define __clear_fpu( tsk )                                      \
@@ -118,6 +120,7 @@ static inline void save_init_fpu( struct task_struct *tsk )
 extern unsigned short get_fpu_cwd( struct task_struct *tsk );
 extern unsigned short get_fpu_swd( struct task_struct *tsk );
 extern unsigned short get_fpu_mxcsr( struct task_struct *tsk );
+extern asmlinkage void math_state_restore(void);
 
 /*
  * Signal frame handlers...