4 files changed, 24 insertions, 1 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;
 }
 
diff --git a/arch/x86_64/kernel/traps.c b/arch/x86_64/kernel/traps.c
index 28e53342f294..ffc40cff1e07 100644
--- a/arch/x86_64/kernel/traps.c
+++ b/arch/x86_64/kernel/traps.c
@@ -1136,6 +1136,7 @@ asmlinkage void math_state_restore(void)
                 init_fpu(me);
         restore_fpu_checking(&me->thread.i387.fxsave);
         task_thread_info(me)->status |= TS_USEDFPU;
+        me->fpu_counter++;
 }
 
 void __init trap_init(void)
diff --git a/include/asm-x86_64/i387.h b/include/asm-x86_64/i387.h
index cba8a3b0cded..60c0f4853fdb 100644
--- a/include/asm-x86_64/i387.h
+++ b/include/asm-x86_64/i387.h
@@ -24,6 +24,7 @@ extern unsigned int mxcsr_feature_mask;
 extern void mxcsr_feature_mask_init(void);
 extern void init_fpu(struct task_struct *child);
 extern int save_i387(struct _fpstate __user *buf);
+extern asmlinkage void math_state_restore(void);
 
 /*
  * FPU lazy state save handling...
@@ -31,7 +32,9 @@ extern int save_i387(struct _fpstate __user *buf);
 
 #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)
 
 /* Ignore delayed exceptions from user space */
diff --git a/include/linux/sched.h b/include/linux/sched.h
index 34ed0d99b1bd..807556c5bcd2 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -865,6 +865,15 @@ struct task_struct {
         struct key *thread_keyring;     /* keyring private to this thread */
         unsigned char jit_keyring;      /* default keyring to attach requested keys to */
 #endif
+        /*
+         * fpu_counter contains the number of consecutive context switches
+         * that the FPU is used. If this is over a threshold, the lazy fpu
+         * saving becomes unlazy to save the trap. This is an unsigned char
+         * so that after 256 times the counter wraps and the behavior turns
+         * lazy again; this to deal with bursty apps that only use FPU for
+         * a short time
+         */
+        unsigned char fpu_counter;
         int oomkilladj; /* OOM kill score adjustment (bit shift). */
         char comm[TASK_COMM_LEN]; /* executable name excluding path
                                      - access with [gs]et_task_comm (which lock