linux-kernel - [PATCH V2 18/19] timekeeping: Let timekeeping_cycles_to

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Date: Mon, 25 Mar 2024 08:40:22 +0200
From: Adrian Hunter <adrian.hunter@...el.com>
To: Thomas Gleixner <tglx@...utronix.de>
Cc: Michael Ellerman <mpe@...erman.id.au>,
	Nicholas Piggin <npiggin@...il.com>,
	Christophe Leroy <christophe.leroy@...roup.eu>,
	"Aneesh Kumar K.V" <aneesh.kumar@...nel.org>,
	"Naveen N. Rao" <naveen.n.rao@...ux.ibm.com>,
	Heiko Carstens <hca@...ux.ibm.com>,
	Vasily Gorbik <gor@...ux.ibm.com>,
	Alexander Gordeev <agordeev@...ux.ibm.com>,
	Christian Borntraeger <borntraeger@...ux.ibm.com>,
	Sven Schnelle <svens@...ux.ibm.com>,
	Ingo Molnar <mingo@...hat.com>,
	Borislav Petkov <bp@...en8.de>,
	Dave Hansen <dave.hansen@...ux.intel.com>,
	x86@...nel.org,
	"H. Peter Anvin" <hpa@...or.com>,
	Andy Lutomirski <luto@...nel.org>,
	Vincenzo Frascino <vincenzo.frascino@....com>,
	John Stultz <jstultz@...gle.com>,
	Stephen Boyd <sboyd@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Randy Dunlap <rdunlap@...radead.org>,
	Bjorn Helgaas <bhelgaas@...gle.com>,
	Arnd Bergmann <arnd@...db.de>,
	Anna-Maria Behnsen <anna-maria@...utronix.de>,
	linuxppc-dev@...ts.ozlabs.org,
	linux-kernel@...r.kernel.org,
	linux-s390@...r.kernel.org
Subject: [PATCH V2 18/19] timekeeping: Let timekeeping_cycles_to_ns() handle both under and overflow

For the case !CONFIG_CLOCKSOURCE_VALIDATE_LAST_CYCLE, forego overflow
protection in the range (mask << 1) < delta <= mask, and interpret it
always as an inconsistency between CPU clock values. That allows
slightly neater code, and it is on a slow path so has no effect on
performance.

Suggested-by: Thomas Gleixner <tglx@...utronix.de>
Signed-off-by: Adrian Hunter <adrian.hunter@...el.com>
---
 kernel/time/timekeeping.c | 31 +++++++++++++------------------
 1 file changed, 13 insertions(+), 18 deletions(-)

diff --git a/kernel/time/timekeeping.c b/kernel/time/timekeeping.c
index 111dfdbd488f..4e18db1819f8 100644
--- a/kernel/time/timekeeping.c
+++ b/kernel/time/timekeeping.c
@@ -266,17 +266,14 @@ static inline u64 timekeeping_debug_get_ns(const struct tk_read_base *tkr)
 	 * Try to catch underflows by checking if we are seeing small
 	 * mask-relative negative values.
 	 */
-	if (unlikely((~delta & mask) < (mask >> 3))) {
+	if (unlikely((~delta & mask) < (mask >> 3)))
 		tk->underflow_seen = 1;
-		now = last;
-	}
 
-	/* Cap delta value to the max_cycles values to avoid mult overflows */
-	if (unlikely(delta > max)) {
+	/* Check for multiplication overflows */
+	if (unlikely(delta > max))
 		tk->overflow_seen = 1;
-		now = last + max;
-	}
 
+	/* timekeeping_cycles_to_ns() handles both under and overflow */
 	return timekeeping_cycles_to_ns(tkr, now);
 }
 #else
@@ -375,19 +372,17 @@ static inline u64 timekeeping_cycles_to_ns(const struct tk_read_base *tkr, u64 c
 	u64 mask = tkr->mask, delta = (cycles - tkr->cycle_last) & mask;
 
 	/*
-	 * This detects the case where the delta overflows the multiplication
-	 * with tkr->mult.
+	 * This detects both negative motion and the case where the delta
+	 * overflows the multiplication with tkr->mult.
 	 */
 	if (unlikely(delta > tkr->clock->max_cycles)) {
-		if (IS_ENABLED(CONFIG_CLOCKSOURCE_VALIDATE_LAST_CYCLE)) {
-			/*
-			 * Handle clocksource inconsistency between CPUs to prevent
-			 * time from going backwards by checking for the MSB of the
-			 * mask being set in the delta.
-			 */
-			if (unlikely(delta & ~(mask >> 1)))
-				return tkr->xtime_nsec >> tkr->shift;
-		}
+		/*
+		 * Handle clocksource inconsistency between CPUs to prevent
+		 * time from going backwards by checking for the MSB of the
+		 * mask being set in the delta.
+		 */
+		if (delta & ~(mask >> 1))
+			return tkr->xtime_nsec >> tkr->shift;
 
 		return delta_to_ns_safe(tkr, delta);
 	}
-- 
2.34.1