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, 16 insertions, 3 deletions

diff --git a/arch/x86/kernel/traps_64.c b/arch/x86/kernel/traps_64.c
index dc0cb497eec3..adff76ea97c4 100644
--- a/arch/x86/kernel/traps_64.c
+++ b/arch/x86/kernel/traps_64.c
@@ -1124,10 +1124,23 @@ asmlinkage void __attribute__((weak)) mce_threshold_interrupt(void)
 asmlinkage void math_state_restore(void)
 {
         struct task_struct *me = current;
-        clts();                 /* Allow maths ops (or we recurse) */
 
-        if (!used_math())
-                init_fpu(me);
+        if (!used_math()) {
+                local_irq_enable();
+                /*
+                 * does a slab alloc which can sleep
+                 */
+                if (init_fpu(me)) {
+                        /*
+                         * ran out of memory!
+                         */
+                        do_group_exit(SIGKILL);
+                        return;
+                }
+                local_irq_disable();
+        }
+
+        clts();                 /* Allow maths ops (or we recurse) */
         restore_fpu_checking(&me->thread.xstate->fxsave);
         task_thread_info(me)->status |= TS_USEDFPU;
         me->fpu_counter++;