[RFC] sched, x86: Prevent resched interrupts if task in kernel mode and !CONFIG_PREEMPT

[Kirill Tkhai <ktkhai@parallels.com>]

It's useless to send reschedule interrupts in such situations. The earliest
point, where schedule() call is possible, is sysret_careful(). But in that
function we directly test TIF_NEED_RESCHED.

So it's possible to get rid of that type of interrupts.

How about this idea? Is set_bit() cheap on x86 machines?
---
 arch/x86/kernel/entry_64.S |   10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/arch/x86/kernel/entry_64.S b/arch/x86/kernel/entry_64.S
index c653dc4..a046ba8 100644
--- a/arch/x86/kernel/entry_64.S
+++ b/arch/x86/kernel/entry_64.S
@@ -409,6 +409,13 @@ GLOBAL(system_call_after_swapgs)
 	movq_cfi rax,(ORIG_RAX-ARGOFFSET)
 	movq  %rcx,RIP-ARGOFFSET(%rsp)
 	CFI_REL_OFFSET rip,RIP-ARGOFFSET
+#if !defined(CONFIG_PREEMPT) || !defined(SMP)
+	/*
+	 * Tell resched_curr() do not send useless interrupts to us.
+	 * Kernel isn't preemptible till sysret_careful() anyway.
+	 */
+	LOCK ; bts $TIF_POLLING_NRFLAG,TI_flags+THREAD_INFO(%rsp,RIP-ARGOFFSET)
+#endif
 	testl $_TIF_WORK_SYSCALL_ENTRY,TI_flags+THREAD_INFO(%rsp,RIP-ARGOFFSET)
 	jnz tracesys
 system_call_fastpath:
@@ -427,6 +434,9 @@ GLOBAL(system_call_after_swapgs)
  * Has incomplete stack frame and undefined top of stack.
  */
 ret_from_sys_call:
+#if !defined(CONFIG_PREEMPT) || !defined(SMP)
+	LOCK ; btr $TIF_POLLING_NRFLAG,TI_flags+THREAD_INFO(%rsp,RIP-ARGOFFSET)
+#endif
 	movl $_TIF_ALLWORK_MASK,%edi
 	/* edi:	flagmask */
 sysret_check: