linux-kernel - [v2 1/9] sched/clock: broken stable to unstable transfer

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Message-Id: <1490368899-877997-2-git-send-email-pasha.tatashin@oracle.com>
Date:   Fri, 24 Mar 2017 11:21:31 -0400
From:   Pavel Tatashin <pasha.tatashin@...cle.com>
To:     x86@...nel.org, linux-kernel@...r.kernel.org, mingo@...hat.com,
        peterz@...radead.org, tglx@...utronix.de, hpa@...or.com
Subject: [v2 1/9] sched/clock: broken stable to unstable transfer

When it is determined that clock is actually unstable, and we switch from
stable to unstable the following function is eventually called:

__clear_sched_clock_stable()

In this function we set gtod_offset so the following holds true:

sched_clock() + raw_offset == ktime_get_ns() + gtod_offset

But instead of getting the latest timestamps, we use the last values
from scd, so instead of sched_clock() we use scd->tick_raw, and instead of
ktime_get_ns() we use scd->tick_gtod.

However, later, when we use gtod_offset sched_clock_local() we do not add
it to scd->tick_gtod to calculate the correct clock value when we determine
the boundaries for min/max clocks.

Signed-off-by: Pavel Tatashin <pasha.tatashin@...cle.com>
---
 kernel/sched/clock.c | 9 +++++----
 1 file changed, 5 insertions(+), 4 deletions(-)

diff --git a/kernel/sched/clock.c b/kernel/sched/clock.c
index a08795e..2bc1090 100644
--- a/kernel/sched/clock.c
+++ b/kernel/sched/clock.c
@@ -214,7 +214,7 @@ static inline u64 wrap_max(u64 x, u64 y)
  */
 static u64 sched_clock_local(struct sched_clock_data *scd)
 {
-	u64 now, clock, old_clock, min_clock, max_clock;
+	u64 now, clock, old_clock, min_clock, max_clock, gtod;
 	s64 delta;
 
 again:
@@ -231,9 +231,10 @@ static u64 sched_clock_local(struct sched_clock_data *scd)
 	 *		      scd->tick_gtod + TICK_NSEC);
 	 */
 
-	clock = scd->tick_gtod + gtod_offset + delta;
-	min_clock = wrap_max(scd->tick_gtod, old_clock);
-	max_clock = wrap_max(old_clock, scd->tick_gtod + TICK_NSEC);
+	gtod = scd->tick_gtod + gtod_offset;
+	clock = gtod + delta;
+	min_clock = wrap_max(gtod, old_clock);
+	max_clock = wrap_max(old_clock, gtod + TICK_NSEC);
 
 	clock = wrap_max(clock, min_clock);
 	clock = wrap_min(clock, max_clock);
-- 
1.8.3.1