- make the preempt-enable test cheaper - only test for the (very rare) TIF_NEED_RESCHED

  condition, we test the preemption count in preempt_schedule(). This reduces the icache
  footprint and the overhead of preemption.

- plus optimize the irq-path preemption check a bit.

arch/i386/kernel/entry.S

@@ -235,7 +235,9 @@ ENTRY(resume_kernel)
 	jnz restore_all
 	incl TI_PRE_COUNT(%ebx)
 	sti
-	call SYMBOL_NAME(preempt_schedule)
+	movl TI_TASK(%ebx), %ecx	# ti->task
+	movl $0, (%ecx)			# current->state = TASK_RUNNING
+	call SYMBOL_NAME(schedule)
 	jmp ret_from_intr
 #endif
 

include/linux/spinlock.h

@@ -177,8 +177,7 @@ do { \
 do { \
 	--current_thread_info()->preempt_count; \
 	barrier(); \
-	if (unlikely(!(current_thread_info()->preempt_count) && \
-		test_thread_flag(TIF_NEED_RESCHED))) \
+	if (unlikely(test_thread_flag(TIF_NEED_RESCHED))) \
 		preempt_schedule(); \
 } while (0)
 

kernel/sched.c

@@ -841,6 +841,8 @@ asmlinkage void schedule(void)
  */
 asmlinkage void preempt_schedule(void)
 {
+	if (unlikely(preempt_get_count()))
+		return;
 	current->state = TASK_RUNNING;
 	schedule();
 }