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Message-Id: <1320097228-3612-1-git-send-email-john.stultz@linaro.org>
Date: Mon, 31 Oct 2011 14:40:28 -0700
From: John Stultz <john.stultz@...aro.org>
To: LKML <linux-kernel@...r.kernel.org>
Cc: John Stultz <john.stultz@...aro.org>,
Yong Zhang <yong.zhang0@...il.com>,
David Daney <ddaney.cavm@...il.com>,
Thomas Gleixner <tglx@...utronix.de>
Subject: [RFC][PATCH] clocksource: Avoid selecting mult values that might overflow when adjusted
Here's another rev of my earlier patch. Let me know if it helps.
-john
For some frequqencies, the clocks_calc_mult_shift() function will
unfortunately select mult values very close to 0xffffffff. This
has the potential to overflow when NTP adjusts the clock, adding
to the mult value.
This patch adds a clocksource.maxadj value, which provides
an approximation of an 11% adjustment(NTP limits adjustments to
500ppm and the tick adjustment is limited to 10%), which could
be made to the clocksource.mult value. This is then used to both
check that the current mult value won't overflow/underflow, as
well as warning us if the timekeeping_adjust() code pushes over
that 11% boundary.
UNTESTED! NOT FOR INCLUSION (YET)!
CC: Yong Zhang <yong.zhang0@...il.com>
CC: David Daney <ddaney.cavm@...il.com>
CC: Thomas Gleixner <tglx@...utronix.de>
Reported-by: Chen Jie <chenj@...ote.com>
Reported-by: zhangfx <zhangfx@...ote.com>
Signed-off-by: John Stultz <john.stultz@...aro.org>
---
include/linux/clocksource.h | 3 ++-
kernel/time/clocksource.c | 37 +++++++++++++++++++++++++++----------
kernel/time/timekeeping.c | 3 +++
3 files changed, 32 insertions(+), 11 deletions(-)
diff --git a/include/linux/clocksource.h b/include/linux/clocksource.h
index 139c4db..c86c940 100644
--- a/include/linux/clocksource.h
+++ b/include/linux/clocksource.h
@@ -156,6 +156,7 @@ extern u64 timecounter_cyc2time(struct timecounter *tc,
* @mult: cycle to nanosecond multiplier
* @shift: cycle to nanosecond divisor (power of two)
* @max_idle_ns: max idle time permitted by the clocksource (nsecs)
+ * @maxadj maximum adjustment value to mult (~11%)
* @flags: flags describing special properties
* @archdata: arch-specific data
* @suspend: suspend function for the clocksource, if necessary
@@ -172,7 +173,7 @@ struct clocksource {
u32 mult;
u32 shift;
u64 max_idle_ns;
-
+ u32 maxadj;
#ifdef CONFIG_ARCH_CLOCKSOURCE_DATA
struct arch_clocksource_data archdata;
#endif
diff --git a/kernel/time/clocksource.c b/kernel/time/clocksource.c
index cf52fda..2982996 100644
--- a/kernel/time/clocksource.c
+++ b/kernel/time/clocksource.c
@@ -503,25 +503,26 @@ static u64 clocksource_max_deferment(struct clocksource *cs)
/*
* Calculate the maximum number of cycles that we can pass to the
* cyc2ns function without overflowing a 64-bit signed result. The
- * maximum number of cycles is equal to ULLONG_MAX/cs->mult which
- * is equivalent to the below.
- * max_cycles < (2^63)/cs->mult
- * max_cycles < 2^(log2((2^63)/cs->mult))
- * max_cycles < 2^(log2(2^63) - log2(cs->mult))
- * max_cycles < 2^(63 - log2(cs->mult))
- * max_cycles < 1 << (63 - log2(cs->mult))
+ * maximum number of cycles is equal to ULLONG_MAX/(cs->mult+cs->maxadj)
+ * which is equivalent to the below.
+ * max_cycles < (2^63)/(cs->mult + cs->maxadj)
+ * max_cycles < 2^(log2((2^63)/(cs->mult + cs->maxadj)))
+ * max_cycles < 2^(log2(2^63) - log2(cs->mult + cs->maxadj))
+ * max_cycles < 2^(63 - log2(cs->mult + cs->maxadj))
+ * max_cycles < 1 << (63 - log2(cs->mult + cs->maxadj))
* Please note that we add 1 to the result of the log2 to account for
* any rounding errors, ensure the above inequality is satisfied and
* no overflow will occur.
*/
- max_cycles = 1ULL << (63 - (ilog2(cs->mult) + 1));
+ max_cycles = 1ULL << (63 - (ilog2(cs->mult + cs->maxadj) + 1));
/*
* The actual maximum number of cycles we can defer the clocksource is
* determined by the minimum of max_cycles and cs->mask.
*/
max_cycles = min_t(u64, max_cycles, (u64) cs->mask);
- max_nsecs = clocksource_cyc2ns(max_cycles, cs->mult, cs->shift);
+ max_nsecs = clocksource_cyc2ns(max_cycles, cs->mult+cs->maxadj,
+ cs->shift);
/*
* To ensure that the clocksource does not wrap whilst we are idle,
@@ -640,7 +641,6 @@ static void clocksource_enqueue(struct clocksource *cs)
void __clocksource_updatefreq_scale(struct clocksource *cs, u32 scale, u32 freq)
{
u64 sec;
-
/*
* Calc the maximum number of seconds which we can run before
* wrapping around. For clocksources which have a mask > 32bit
@@ -661,6 +661,23 @@ void __clocksource_updatefreq_scale(struct clocksource *cs, u32 scale, u32 freq)
clocks_calc_mult_shift(&cs->mult, &cs->shift, freq,
NSEC_PER_SEC / scale, sec * scale);
+
+ /*
+ * for clocksources that have large mults, to avoid overflow.
+ * Since mult may be adjusted by ntp, add an safety extra margin
+ *
+ * We won't try to correct for more then 11% adjustments (110,000 ppm),
+ * which approximates to 1/8 or 1/2^3. Thus 1 << (shift - 3) is the
+ * largest mult adjustment we'll support.
+ */
+ cs->maxadj = 1 << (cs->shift-3);
+ while ((cs->mult + cs->maxadj < cs->mult)
+ || (cs->mult - cs->maxadj > cs->mult)) {
+ cs->mult >>= 1;
+ cs->shift--;
+ cs->maxadj = 1 << (cs->shift-3);
+ }
+
cs->max_idle_ns = clocksource_max_deferment(cs);
}
EXPORT_SYMBOL_GPL(__clocksource_updatefreq_scale);
diff --git a/kernel/time/timekeeping.c b/kernel/time/timekeeping.c
index 2b021b0e..d37c9e3 100644
--- a/kernel/time/timekeeping.c
+++ b/kernel/time/timekeeping.c
@@ -820,6 +820,9 @@ static void timekeeping_adjust(s64 offset)
} else
return;
+ WARN_ONCE(timekeeper.mult+adj >
+ timekeeper.clock->mult + timekeeper.clock->maxadj,
+ "Adjusting more then 11%%");
timekeeper.mult += adj;
timekeeper.xtime_interval += interval;
timekeeper.xtime_nsec -= offset;
--
1.7.3.2.146.gca209
--
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