[PATCH v3 50/51] cpuidle: Comments about noinstr/__cpuidle

Peter Zijlstra
2023-01-12T20:12:20Z

Add a few words on noinstr / __cpuidle usage.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
---
drivers/cpuidle/cpuidle.c | 12 ++++++++++++
include/linux/compiler_types.h | 10 ++++++++++
2 files changed, 22 insertions(+)

--- a/drivers/cpuidle/cpuidle.c
+++ b/drivers/cpuidle/cpuidle.c
@@ -252,6 +252,18 @@ noinstr int cpuidle_enter_state(struct c
instrumentation_begin();
}

+ /*
+ * NOTE!!
+ *
+ * For cpuidle_state::enter() methods that do *NOT* set
+ * CPUIDLE_FLAG_RCU_IDLE RCU will be disabled here and these functions
+ * must be marked either noinstr or __cpuidle.
+ *
+ * For cpuidle_state::enter() methods that *DO* set
+ * CPUIDLE_FLAG_RCU_IDLE this isn't required, but they must mark the
+ * function calling ct_cpuidle_enter() as noinstr/__cpuidle and all
+ * functions called within the RCU-idle region.
+ */
entered_state = target_state->enter(dev, drv, index);

if (WARN_ONCE(!irqs_disabled(), "%ps leaked IRQ state", target_state->enter))
--- a/include/linux/compiler_types.h
+++ b/include/linux/compiler_types.h
@@ -233,6 +233,16 @@ struct ftrace_likely_data {

#define noinstr __noinstr_section(".noinstr.text")

+/*
+ * The __cpuidle section is used twofold:
+ *
+ * 1) the original use -- identifying if a CPU is 'stuck' in idle state based
+ * on it's instruction pointer. See cpu_in_idle().
+ *
+ * 2) supressing instrumentation around where cpuidle disables RCU; where the
+ * function isn't strictly required for #1, this is interchangeable with
+ * noinstr.
+ */
#define __cpuidle __noinstr_section(".cpuidle.text")

#endif /* __KERNEL__ */