sched: Add CONFIG_SCHED_CORE help text

Hugh noted that the SCHED_CORE Kconfig option could do with a help
text.

Requested-by: Hugh Dickins <hughd@google.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Randy Dunlap <rdunlap@infradead.org>
Acked-by: Hugh Dickins <hughd@google.com>
Link: https://lkml.kernel.org/r/YKyhtwhEgvtUDOyl@hirez.programming.kicks-ass.net

diff --git a/kernel/Kconfig.preempt b/kernel/Kconfig.preempt
index ea1e3331c0ba3a178bed2f57192fc1a005f98757..bd7c4147b9a8105e15e081687c4e7217f37c3aec 100644 (file)
--- a/kernel/Kconfig.preempt
+++ b/kernel/Kconfig.preempt
@@ -104,4 +104,18 @@ config SCHED_CORE
        bool "Core Scheduling for SMT"
        default y
        depends on SCHED_SMT
+       help
+         This option permits Core Scheduling, a means of coordinated task
+         selection across SMT siblings. When enabled -- see
+         prctl(PR_SCHED_CORE) -- task selection ensures that all SMT siblings
+         will execute a task from the same 'core group', forcing idle when no
+         matching task is found.
+
+         Use of this feature includes:
+          - mitigation of some (not all) SMT side channels;
+          - limiting SMT interference to improve determinism and/or performance.
+
+         SCHED_CORE is default enabled when SCHED_SMT is enabled -- when
+         unused there should be no impact on performance.
+