sched: Add CONFIG_SCHED_CORE help text

mainline inclusion
from mainline-v5.14-rc1
commit 7b419f47
category: bugfix
bugzilla: https://gitee.com/openeuler/kernel/issues/I5OOWG
CVE: NA

Reference: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=7b419f47facd286c6723daca6ad69ec355473f78

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Hugh noted that the SCHED_CORE Kconfig option could do with a help
text.
Requested-by: NHugh Dickins <hughd@google.com>
Signed-off-by: NPeter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: NRandy Dunlap <rdunlap@infradead.org>
Acked-by: NHugh Dickins <hughd@google.com>
Link: https://lkml.kernel.org/r/YKyhtwhEgvtUDOyl@hirez.programming.kicks-ass.netSigned-off-by: NLin Shengwang <linshengwang1@huawei.com>
Reviewed-by: Nlihua <hucool.lihua@huawei.com>
Signed-off-by: NZheng Zengkai <zhengzengkai@huawei.com>

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.
+