x86, fpu: lazy allocation of FPU area - v5

Only allocate the FPU area when the application actually uses FPU, i.e., in the
first lazy FPU trap. This could save memory for non-fpu using apps.

for example: on my system after boot, there are around 300 processes, with
only 17 using FPU.

Signed-off-by: Suresh Siddha <suresh.b.siddha@intel.com>
Cc: Arjan van de Ven <arjan@linux.intel.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

1 files changed, 15 insertions, 2 deletions

diff --git a/arch/x86/kernel/traps_32.c b/arch/x86/kernel/traps_32.c
index 8d136a73ce8e..471e694d6713 100644
--- a/arch/x86/kernel/traps_32.c
+++ b/arch/x86/kernel/traps_32.c
@@ -1148,9 +1148,22 @@ asmlinkage void math_state_restore(void)
         struct thread_info *thread = current_thread_info();
         struct task_struct *tsk = thread->task;
 
+        if (!tsk_used_math(tsk)) {
+                local_irq_enable();
+                /*
+                 * does a slab alloc which can sleep
+                 */
+                if (init_fpu(tsk)) {
+                        /*
+                         * ran out of memory!
+                         */
+                        do_group_exit(SIGKILL);
+                        return;
+                }
+                local_irq_disable();
+        }
+
         clts();                         /* Allow maths ops (or we recurse) */
-        if (!tsk_used_math(tsk))
-                init_fpu(tsk);
         restore_fpu(tsk);
         thread->status |= TS_USEDFPU;   /* So we fnsave on switch_to() */
         tsk->fpu_counter++;