[RFC patch 1/7] timekeeping: Do not unconditionally suspend NMI safe timekeepers

[Thomas Gleixner]

If the current clocksource can be accessed safely after timekeeping is
suspended then there is no reason to suspend the NMI safe timekeepers.

Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
---
 include/linux/clocksource.h |    3 ++-
 kernel/time/timekeeping.c   |    3 +++
 2 files changed, 5 insertions(+), 1 deletion(-)

--- a/include/linux/clocksource.h
+++ b/include/linux/clocksource.h
@@ -119,7 +119,8 @@ struct clocksource {
 #define CLOCK_SOURCE_VALID_FOR_HRES		0x20
 #define CLOCK_SOURCE_UNSTABLE			0x40
 #define CLOCK_SOURCE_SUSPEND_NONSTOP		0x80
-#define CLOCK_SOURCE_RESELECT			0x100
+#define CLOCK_SOURCE_SUSPEND_ACCESS_OK		0x100
+#define CLOCK_SOURCE_RESELECT			0x200
 
 /* simplify initialization of mask field */
 #define CLOCKSOURCE_MASK(bits) GENMASK_ULL((bits) - 1, 0)
--- a/kernel/time/timekeeping.c
+++ b/kernel/time/timekeeping.c
@@ -545,6 +545,9 @@ static void halt_fast_timekeeper(struct
 	static struct tk_read_base tkr_dummy;
 	struct tk_read_base *tkr = &tk->tkr_mono;
 
+	if (tkr->clock->flags & CLOCK_SOURCE_SUSPEND_ACCESS_OK)
+		return;
+
 	memcpy(&tkr_dummy, tkr, sizeof(tkr_dummy));
 	cycles_at_suspend = tk_clock_read(tkr);
 	tkr_dummy.clock = &dummy_clock;