[PATCH 3.16 41/86] x86/Kconfig: Select SCHED_SMT if SMP enabled

[Ben Hutchings]

3.16.68-rc1 review patch.  If anyone has any objections, please let me know.

------------------

From: Thomas Gleixner <tglx@xxxxxxxxxxxxx>

commit dbe733642e01dd108f71436aaea7b328cb28fd87 upstream.

CONFIG_SCHED_SMT is enabled by all distros, so there is not a real point to
have it configurable. The runtime overhead in the core scheduler code is
minimal because the actual SMT scheduling parts are conditional on a static
key.

This allows to expose the scheduler's SMT state static key to the
speculation control code. Alternatively the scheduler's static key could be
made always available when CONFIG_SMP is enabled, but that's just adding an
unused static key to every other architecture for nothing.

Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Reviewed-by: Ingo Molnar <mingo@xxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Andy Lutomirski <luto@xxxxxxxxxx>
Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Cc: Jiri Kosina <jkosina@xxxxxxx>
Cc: Tom Lendacky <thomas.lendacky@xxxxxxx>
Cc: Josh Poimboeuf <jpoimboe@xxxxxxxxxx>
Cc: Andrea Arcangeli <aarcange@xxxxxxxxxx>
Cc: David Woodhouse <dwmw@xxxxxxxxxxxx>
Cc: Tim Chen <tim.c.chen@xxxxxxxxxxxxxxx>
Cc: Andi Kleen <ak@xxxxxxxxxxxxxxx>
Cc: Dave Hansen <dave.hansen@xxxxxxxxx>
Cc: Casey Schaufler <casey.schaufler@xxxxxxxxx>
Cc: Asit Mallick <asit.k.mallick@xxxxxxxxx>
Cc: Arjan van de Ven <arjan@xxxxxxxxxxxxxxx>
Cc: Jon Masters <jcm@xxxxxxxxxx>
Cc: Waiman Long <longman9394@xxxxxxxxx>
Cc: Greg KH <gregkh@xxxxxxxxxxxxxxxxxxx>
Cc: Dave Stewart <david.c.stewart@xxxxxxxxx>
Cc: Kees Cook <keescook@xxxxxxxxxxxx>
Link: https://lkml.kernel.org/r/20181125185004.337452245@xxxxxxxxxxxxx
[bwh: Backported to 3.16: CONFIG_SCHED_SMT depended on CONFG_X86_HT, but that
 also follows CONFIG_SMP]
Signed-off-by: Ben Hutchings <ben@xxxxxxxxxxxxxxx>
---
 arch/x86/Kconfig | 8 +-------
 1 file changed, 1 insertion(+), 7 deletions(-)

--- a/arch/x86/Kconfig
+++ b/arch/x86/Kconfig
@@ -798,13 +798,7 @@ config NR_CPUS
 	  approximately eight kilobytes to the kernel image.
 
 config SCHED_SMT
-	bool "SMT (Hyperthreading) scheduler support"
-	depends on X86_HT
-	---help---
-	  SMT scheduler support improves the CPU scheduler's decision making
-	  when dealing with Intel Pentium 4 chips with HyperThreading at a
-	  cost of slightly increased overhead in some places. If unsure say
-	  N here.
+	def_bool y if SMP
 
 config SCHED_MC
 	def_bool y