[tip: sched/core] cpuidle: Add comments about noinstr/__cpuidle usage

[tip-bot2 for Peter Zijlstra]

The following commit has been merged into the sched/core branch of tip:

Commit-ID:     0e985e9d22864e29d5d2b3d909ad15134d7f6d46
Gitweb:        https://git.kernel.org/tip/0e985e9d22864e29d5d2b3d909ad15134d7f6d46
Author:        Peter Zijlstra <peterz@xxxxxxxxxxxxx>
AuthorDate:    Thu, 12 Jan 2023 20:44:04 +01:00
Committer:     Ingo Molnar <mingo@xxxxxxxxxx>
CommitterDate: Fri, 13 Jan 2023 11:48:18 +01:00

cpuidle: Add comments about noinstr/__cpuidle usage

Add a few words on noinstr / __cpuidle usage.

Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx>
Link: https://lore.kernel.org/r/20230112195542.397238052@xxxxxxxxxxxxx
---
 drivers/cpuidle/cpuidle.c      | 12 ++++++++++++
 include/linux/compiler_types.h | 10 ++++++++++
 2 files changed, 22 insertions(+)

diff --git a/drivers/cpuidle/cpuidle.c b/drivers/cpuidle/cpuidle.c
index 73f7d8b..500d172 100644
--- a/drivers/cpuidle/cpuidle.c
+++ b/drivers/cpuidle/cpuidle.c
@@ -252,6 +252,18 @@ noinstr int cpuidle_enter_state(struct cpuidle_device *dev,
 		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))
diff --git a/include/linux/compiler_types.h b/include/linux/compiler_types.h
index d785890..dea5bf5 100644
--- a/include/linux/compiler_types.h
+++ b/include/linux/compiler_types.h
@@ -239,6 +239,16 @@ 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__ */