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Groups > linux.kernel > #1720668

[tip:timers/urgent] time: Fix ktime_get_raw() incorrect base accumulation

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Commit-ID:  0bcdc0987cce9880436b70836c6a92bb8e744fd1
Gitweb:     http://git.kernel.org/tip/0bcdc0987cce9880436b70836c6a92bb8e744fd1
Author:     John Stultz <john.stultz@linaro.org>
AuthorDate: Fri, 25 Aug 2017 15:57:04 -0700
Committer:  Thomas Gleixner <tglx@linutronix.de>
CommitDate: Sat, 26 Aug 2017 16:06:12 +0200

time: Fix ktime_get_raw() incorrect base accumulation

In comqit fc6eead7c1e2 ("time: Clean up CLOCK_MONOTONIC_RAW time
handling"), the following code got mistakenly added to the update of the
raw timekeeper:

 /* Update the monotonic raw base */
 seconds = tk->raw_sec;
 nsec = (u32)(tk->tkr_raw.xtime_nsec >> tk->tkr_raw.shift);
 tk->tkr_raw.base = ns_to_ktime(seconds * NSEC_PER_SEC + nsec);

Which adds the raw_sec value and the shifted down raw xtime_nsec to the
base value.

But the read function adds the shifted down tk->tkr_raw.xtime_nsec value
another time, The result of this is that ktime_get_raw() users (which are
all internal users) see the raw time move faster then it should (the rate
at which can vary with the current size of tkr_raw.xtime_nsec), which has
resulted in at least problems with graphics rendering performance.

The change tried to match the monotonic base update logic:

 seconds = (u64)(tk->xtime_sec + tk->wall_to_monotonic.tv_sec);
 nsec = (u32) tk->wall_to_monotonic.tv_nsec;
 tk->tkr_mono.base = ns_to_ktime(seconds * NSEC_PER_SEC + nsec);

Which adds the wall_to_monotonic.tv_nsec value, but not the
tk->tkr_mono.xtime_nsec value to the base.

To fix this, simplify the tkr_raw.base accumulation to only accumulate the
raw_sec portion, and do not include the tkr_raw.xtime_nsec portion, which
will be added at read time.

Fixes: fc6eead7c1e2 ("time: Clean up CLOCK_MONOTONIC_RAW time handling")
Reported-and-tested-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: John Stultz <john.stultz@linaro.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Prarit Bhargava <prarit@redhat.com>
Cc: Kevin Brodsky <kevin.brodsky@arm.com>
Cc: Richard Cochran <richardcochran@gmail.com>
Cc: Stephen Boyd <stephen.boyd@linaro.org>
Cc: Will Deacon <will.deacon@arm.com>
Cc: Miroslav Lichvar <mlichvar@redhat.com>
Cc: Daniel Mentz <danielmentz@google.com>
Link: http://lkml.kernel.org/r/1503701824-1645-1-git-send-email-john.stultz@linaro.org

---
 kernel/time/timekeeping.c | 4 +---
 1 file changed, 1 insertion(+), 3 deletions(-)

diff --git a/kernel/time/timekeeping.c b/kernel/time/timekeeping.c
index cedafa0..7e7e61c 100644
--- a/kernel/time/timekeeping.c
+++ b/kernel/time/timekeeping.c
@@ -637,9 +637,7 @@ static inline void tk_update_ktime_data(struct timekeeper *tk)
 	tk->ktime_sec = seconds;
 
 	/* Update the monotonic raw base */
-	seconds = tk->raw_sec;
-	nsec = (u32)(tk->tkr_raw.xtime_nsec >> tk->tkr_raw.shift);
-	tk->tkr_raw.base = ns_to_ktime(seconds * NSEC_PER_SEC + nsec);
+	tk->tkr_raw.base = ns_to_ktime(tk->raw_sec * NSEC_PER_SEC);
 }
 
 /* must hold timekeeper_lock */

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Re: [RFC][PATCH 4/4] time: Clean up CLOCK_MONOTONIC_RAW time handling Chris Wilson <chris@chris-wilson.co.uk> - 2017-08-25 15:50 +0200
  Re: [RFC][PATCH 4/4] time: Clean up CLOCK_MONOTONIC_RAW time handling John Stultz <john.stultz@linaro.org> - 2017-08-25 21:00 +0200
    Re: [RFC][PATCH 4/4] time: Clean up CLOCK_MONOTONIC_RAW time handling John Stultz <john.stultz@linaro.org> - 2017-08-25 23:20 +0200
  [RFC][PATCH] time: Fix ktime_get_raw() issues caused by incorrect base accumulation John Stultz <john.stultz@linaro.org> - 2017-08-26 01:00 +0200
    Re: [RFC][PATCH] time: Fix ktime_get_raw() issues caused by incorrect base  accumulation Chris Wilson <chris@chris-wilson.co.uk> - 2017-08-26 12:30 +0200
    [tip:timers/urgent] time: Fix ktime_get_raw() incorrect base  accumulation tip-bot for John Stultz <tipbot@zytor.com> - 2017-08-26 16:20 +0200

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