[PATCH] non lazy "sleazy" fpu implementation

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.

[akpm@osdl.org: place new task_struct field next to jit_keyring to save space]
Signed-off-by: Arjan van de Ven <arjan@linux.intel.com>
Signed-off-by: Andi Kleen <ak@suse.de>
Cc: Andi Kleen <ak@muc.de>
Signed-off-by: Andrew Morton <akpm@osdl.org>

1 files changed, 10 insertions, 0 deletions

diff --git a/arch/x86_64/kernel/process.c b/arch/x86_64/kernel/process.c
index 6fbd19564e4e..9e9a70e50c72 100644
--- a/arch/x86_64/kernel/process.c
+++ b/arch/x86_64/kernel/process.c
@@ -552,6 +552,10 @@ __switch_to(struct task_struct *prev_p, struct task_struct *next_p)
         int cpu = smp_processor_id();  
         struct tss_struct *tss = &per_cpu(init_tss, cpu);
 
+        /* we're going to use this soon, after a few expensive things */
+        if (next_p->fpu_counter>5)
+                prefetch(&next->i387.fxsave);
+
         /*
          * Reload esp0, LDT and the page table pointer:
          */
@@ -629,6 +633,12 @@ __switch_to(struct task_struct *prev_p, struct task_struct *next_p)
             || test_tsk_thread_flag(prev_p, TIF_IO_BITMAP))
                 __switch_to_xtra(prev_p, next_p, tss);
 
+        /* If the task has used fpu the last 5 timeslices, just do a full
+         * restore of the math state immediately to avoid the trap; the
+         * chances of needing FPU soon are obviously high now
+         */
+        if (next_p->fpu_counter>5)
+                math_state_restore();
         return prev_p;
 }