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SubjectRe: [PATCH] clocksource: Avoid selecting mult values that might overflow when adjusted
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On Thu, 2011-11-03 at 22:10 +0100, Ingo Molnar wrote:
> * John Stultz <john.stultz@linaro.org> wrote:
>
> > 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.
> >
> > CC: Yong Zhang <yong.zhang0@gmail.com>
> > CC: David Daney <ddaney.cavm@gmail.com>
> > CC: Thomas Gleixner <tglx@linutronix.de>
> > Reported-by: Chen Jie <chenj@lemote.com>
> > Reported-by: zhangfx <zhangfx@lemote.com>
> > Signed-off-by: John Stultz <john.stultz@linaro.org>
> > ---
> > include/linux/clocksource.h | 3 +-
> > kernel/time/clocksource.c | 53 ++++++++++++++++++++++++++++++++++--------
> > kernel/time/timekeeping.c | 3 ++
> > 3 files changed, 48 insertions(+), 11 deletions(-)
>
> This patch (included in tip:timers/urgent) causes the following boot
> warning x86:
>
> [ 0.000000] Fast TSC calibration using PIT
> [ 0.000000] ------------[ cut here ]------------
> [ 0.000000] WARNING: at kernel/time/timekeeping.c:855 do_timer+0x47f/0x4c0()
> [ 0.000000] Hardware name: System Product Name
> [ 0.000000] Adjusting more then 11%
> [ 0.000000] Modules linked in:
> [ 0.000000] Pid: 0, comm: swapper Not tainted 3.1.0-tip+ #161792
> [ 0.000000] Call Trace:
> [ 0.000000] <IRQ> [<ffffffff81042d0a>] warn_slowpath_common+0x7a/0xb0
> [ 0.000000] [<ffffffff81042de1>] warn_slowpath_fmt+0x41/0x50
> [ 0.000000] [<ffffffff8106e78f>] do_timer+0x47f/0x4c0
> [ 0.000000] [<ffffffff81073953>] tick_periodic+0x63/0x80
> [ 0.000000] [<ffffffff810739f1>] tick_handle_periodic+0x21/0x70
> [ 0.000000] [<ffffffff810046d8>] timer_interrupt+0x18/0x20
> [ 0.000000] [<ffffffff8109ff9e>] handle_irq_event_percpu+0x5e/0x220
> [ 0.000000] [<ffffffff810a019b>] handle_irq_event+0x3b/0x60
> [ 0.000000] [<ffffffff810a295c>] handle_level_irq+0x6c/0xd0
> [ 0.000000] [<ffffffff81003f34>] handle_irq+0x44/0xa0
>
> Full bootlog and config attached.
>
> i've excluded it from tip:master for now.

Thanks again for the bug report. I was able to reproduce it using the
jiffies clocksource. Looking at the code after a weekend of decent
sleep, I see the max_adjustment calculation is simply wrong (it was
proportional to the shift, not the mult - but adjustments are made on
mult).

This version simplifies the calculation and improves warn-on messages so
we also catch any overflow potential on clocksources that don't use the
clocksource_register_hz/khz interfaces.

Ingo: could you give it a whirl on your test box and verify it doesn't
have any trouble?

Yong: Can you also give this a test run to make sure you don't see any
problems?

thanks
-john

From 82c5b70fc5074b6bb6e05514afb6e9c73c740422 Mon Sep 17 00:00:00 2001
From: John Stultz <john.stultz@linaro.org>
Date: Mon, 31 Oct 2011 17:06:35 -0400
Subject: [PATCH] clocksource: Avoid selecting mult values that might overflow when adjusted

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.

v2: Fix max_adjustment calculation, and improve WARN_ONCE
messages.

CC: Yong Zhang <yong.zhang0@gmail.com>
CC: David Daney <ddaney.cavm@gmail.com>
CC: Thomas Gleixner <tglx@linutronix.de>
Reported-by: Chen Jie <chenj@lemote.com>
Reported-by: zhangfx <zhangfx@lemote.com>
Signed-off-by: John Stultz <john.stultz@linaro.org>
---
include/linux/clocksource.h | 3 +-
kernel/time/clocksource.c | 58 +++++++++++++++++++++++++++++++++++-------
kernel/time/timekeeping.c | 6 ++++
3 files changed, 56 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..cfc65e1 100644
--- a/kernel/time/clocksource.c
+++ b/kernel/time/clocksource.c
@@ -492,6 +492,22 @@ void clocksource_touch_watchdog(void)
}

/**
+ * clocksource_max_adjustment- Returns max adjustment amount
+ * @cs: Pointer to clocksource
+ *
+ */
+static u32 clocksource_max_adjustment(struct clocksource *cs)
+{
+ u64 ret;
+ /*
+ * We won't try to correct for more then 11% adjustments (110,000 ppm),
+ */
+ ret = (u64)cs->mult * 11;
+ do_div(ret,100);
+ return (u32)ret;
+}
+
+/**
* clocksource_max_deferment - Returns max time the clocksource can be deferred
* @cs: Pointer to clocksource
*
@@ -503,25 +519,28 @@ 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.
+ * Note: Here we subtract the maxadj to make sure we don't sleep for
+ * too long if there's a large negative adjustment.
*/
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 +659,6 @@ 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 +679,20 @@ 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
+ *
+ */
+ cs->maxadj = clocksource_max_adjustment(cs);
+ while ((cs->mult + cs->maxadj < cs->mult)
+ || (cs->mult - cs->maxadj > cs->mult)) {
+ cs->mult >>= 1;
+ cs->shift--;
+ cs->maxadj = clocksource_max_adjustment(cs);
+ }
+
cs->max_idle_ns = clocksource_max_deferment(cs);
}
EXPORT_SYMBOL_GPL(__clocksource_updatefreq_scale);
@@ -701,6 +733,12 @@ EXPORT_SYMBOL_GPL(__clocksource_register_scale);
*/
int clocksource_register(struct clocksource *cs)
{
+ /* calculate max adjustment for given mult/shift */
+ cs->maxadj = clocksource_max_adjustment(cs);
+ WARN_ONCE(cs->mult + cs->maxadj < cs->mult,
+ "Clocksource %s might overflow on 11%% adjustment\n",
+ cs->name);
+
/* calculate max idle time permitted for this clocksource */
cs->max_idle_ns = clocksource_max_deferment(cs);

diff --git a/kernel/time/timekeeping.c b/kernel/time/timekeeping.c
index 2b021b0e..2c04610 100644
--- a/kernel/time/timekeeping.c
+++ b/kernel/time/timekeeping.c
@@ -820,6 +820,12 @@ static void timekeeping_adjust(s64 offset)
} else
return;

+ WARN_ONCE(timekeeper.mult+adj >
+ timekeeper.clock->mult + timekeeper.clock->maxadj,
+ "Adjusting %s more then 11%% (%ld vs %ld)\n",
+ timekeeper.clock->name, (long)timekeeper.mult+adj,
+ (long)timekeeper.clock->mult +
+ timekeeper.clock->maxadj);
timekeeper.mult += adj;
timekeeper.xtime_interval += interval;
timekeeper.xtime_nsec -= offset;
--
1.7.3.2.146.gca209




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