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Message-Id: <20210429013037.3958717-6-paulmck@kernel.org>
Date: Wed, 28 Apr 2021 18:30:37 -0700
From: "Paul E. McKenney" <paulmck@...nel.org>
To: tglx@...utronix.de
Cc: linux-kernel@...r.kernel.org, john.stultz@...aro.org,
sboyd@...nel.org, corbet@....net, Mark.Rutland@....com,
maz@...nel.org, kernel-team@...com, neeraju@...eaurora.org,
ak@...ux.intel.com, feng.tang@...el.com, zhengjun.xing@...el.com,
"Paul E. McKenney" <paulmck@...nel.org>,
Xing Zhengjun <zhengjun.xing@...ux.intel.com>
Subject: [PATCH v11 clocksource 6/6] clocksource: Reduce clocksource-skew threshold for TSC
Currently, WATCHDOG_THRESHOLD is set to detect a 62.5-millisecond skew in
a 500-millisecond WATCHDOG_INTERVAL. This requires that clocks be skewed
by more than 12.5% in order to be marked unstable. Except that a clock
that is skewed by that much is probably destroying unsuspecting software
right and left. And given that there are now checks for false-positive
skews due to delays between reading the two clocks, it should be possible
to greatly decrease WATCHDOG_THRESHOLD, at least for fine-grained clocks
such as TSC.
Therefore, add a new uncertainty_margin field to the clocksource
structure that contains the maximum uncertainty in nanoseconds for
the corresponding clock. This field may be initialized manually,
as it is for clocksource_tsc_early and clocksource_jiffies, which
is copied to refined_jiffies. If the field is not initialized
manually, it will be computed at clock-registry time as the period
of the clock in question based on the scale and freq parameters to
__clocksource_update_freq_scale() function. If either of those two
parameters are zero, the tens-of-milliseconds WATCHDOG_THRESHOLD is
used as a cowardly alternative to dividing by zero. No matter how the
uncertainty_margin field is calculated, it is bounded below by twice
WATCHDOG_MAX_SKEW, that is, by 100 microseconds.
Note that manually initialized uncertainty_margin fields are not adjusted,
but there is a WARN_ON_ONCE() that triggers if any such field is less than
twice WATCHDOG_MAX_SKEW. This WARN_ON_ONCE() is intended to discourage
production use of the one-nanosecond uncertainty_margin values that are
used to test the clock-skew code itself.
The actual clock-skew check uses the sum of the uncertainty_margin fields
of the two clocksource structures being compared. Integer overflow is
avoided because the largest computed value of the uncertainty_margin
fields is one billion (10^9), and double that value fits into an
unsigned int. However, if someone manually specifies (say) UINT_MAX,
they will get what they deserve.
Note that the refined_jiffies uncertainty_margin field is initialized to
TICK_NSEC, which means that skew checks involving this clocksource will
be sufficently forgiving. In a similar vein, the clocksource_tsc_early
uncertainty_margin field is initialized to 32*NSEC_PER_MSEC, which
replicates the current behavior and allows custom setting if needed
in order to address the rare skews detected for this clocksource in
current mainline.
Link: https://lore.kernel.org/lkml/20210425224540.GA1312438@paulmck-ThinkPad-P17-Gen-1/
Link: https://lore.kernel.org/lkml/20210420064934.GE31773@xsang-OptiPlex-9020/
Link: https://lore.kernel.org/lkml/20210106004013.GA11179@paulmck-ThinkPad-P72/
Link: https://lore.kernel.org/lkml/20210414043435.GA2812539@paulmck-ThinkPad-P17-Gen-1/
Link: https://lore.kernel.org/lkml/20210419045155.GA596058@paulmck-ThinkPad-P17-Gen-1/
Suggested-by: Thomas Gleixner <tglx@...utronix.de>
Cc: John Stultz <john.stultz@...aro.org>
Cc: Stephen Boyd <sboyd@...nel.org>
Cc: Jonathan Corbet <corbet@....net>
Cc: Mark Rutland <Mark.Rutland@....com>
Cc: Marc Zyngier <maz@...nel.org>
Cc: Andi Kleen <ak@...ux.intel.com>
Cc: Xing Zhengjun <zhengjun.xing@...ux.intel.com>
Cc: Feng Tang <feng.tang@...el.com>
Signed-off-by: Paul E. McKenney <paulmck@...nel.org>
---
arch/x86/kernel/tsc.c | 1 +
include/linux/clocksource.h | 3 +++
kernel/time/clocksource.c | 30 ++++++++++++++++++++++++++----
kernel/time/jiffies.c | 15 ++++++++-------
4 files changed, 38 insertions(+), 11 deletions(-)
diff --git a/arch/x86/kernel/tsc.c b/arch/x86/kernel/tsc.c
index 56289170753c..6e11c9619437 100644
--- a/arch/x86/kernel/tsc.c
+++ b/arch/x86/kernel/tsc.c
@@ -1127,6 +1127,7 @@ static int tsc_cs_enable(struct clocksource *cs)
static struct clocksource clocksource_tsc_early = {
.name = "tsc-early",
.rating = 299,
+ .uncertainty_margin = 32 * NSEC_PER_MSEC,
.read = read_tsc,
.mask = CLOCKSOURCE_MASK(64),
.flags = CLOCK_SOURCE_IS_CONTINUOUS |
diff --git a/include/linux/clocksource.h b/include/linux/clocksource.h
index 83a3ebff7456..8f87c1a6f323 100644
--- a/include/linux/clocksource.h
+++ b/include/linux/clocksource.h
@@ -42,6 +42,8 @@ struct module;
* @shift: Cycle to nanosecond divisor (power of two)
* @max_idle_ns: Maximum idle time permitted by the clocksource (nsecs)
* @maxadj: Maximum adjustment value to mult (~11%)
+ * @uncertainty_margin: Maximum uncertainty in nanoseconds per half second.
+ * Zero says to use default WATCHDOG_THRESHOLD.
* @archdata: Optional arch-specific data
* @max_cycles: Maximum safe cycle value which won't overflow on
* multiplication
@@ -93,6 +95,7 @@ struct clocksource {
u32 shift;
u64 max_idle_ns;
u32 maxadj;
+ u32 uncertainty_margin;
#ifdef CONFIG_ARCH_CLOCKSOURCE_DATA
struct arch_clocksource_data archdata;
#endif
diff --git a/kernel/time/clocksource.c b/kernel/time/clocksource.c
index f71f375df544..c228f3727191 100644
--- a/kernel/time/clocksource.c
+++ b/kernel/time/clocksource.c
@@ -122,17 +122,17 @@ static int clocksource_watchdog_kthread(void *data);
static void __clocksource_change_rating(struct clocksource *cs, int rating);
/*
- * Interval: 0.5sec Threshold: 0.0625s
+ * Interval: 0.5sec Threshold: 0.0312s, when doubled: 0.0625s
*/
#define WATCHDOG_INTERVAL (HZ >> 1)
-#define WATCHDOG_THRESHOLD (NSEC_PER_SEC >> 4)
+#define WATCHDOG_THRESHOLD (NSEC_PER_SEC >> 5)
/*
* Maximum permissible delay between two readouts of the watchdog
* clocksource surrounding a read of the clocksource being validated.
* This delay could be due to SMIs, NMIs, or to VCPU preemptions.
*/
-#define WATCHDOG_MAX_SKEW (100 * NSEC_PER_USEC)
+#define WATCHDOG_MAX_SKEW (50 * NSEC_PER_USEC)
static void clocksource_watchdog_work(struct work_struct *work)
{
@@ -377,6 +377,7 @@ static void clocksource_watchdog(struct timer_list *unused)
int next_cpu, reset_pending;
int64_t wd_nsec, cs_nsec;
struct clocksource *cs;
+ u32 md;
spin_lock(&watchdog_lock);
if (!watchdog_running)
@@ -423,7 +424,8 @@ static void clocksource_watchdog(struct timer_list *unused)
continue;
/* Check the deviation from the watchdog clocksource. */
- if (abs(cs_nsec - wd_nsec) > WATCHDOG_THRESHOLD) {
+ md = cs->uncertainty_margin + watchdog->uncertainty_margin;
+ if (abs(cs_nsec - wd_nsec) > md) {
pr_warn("timekeeping watchdog on CPU%d: Marking clocksource '%s' as unstable because the skew is too large:\n",
smp_processor_id(), cs->name);
pr_warn(" '%s' wd_now: %llx wd_last: %llx mask: %llx\n",
@@ -1076,6 +1078,26 @@ void __clocksource_update_freq_scale(struct clocksource *cs, u32 scale, u32 freq
clocks_calc_mult_shift(&cs->mult, &cs->shift, freq,
NSEC_PER_SEC / scale, sec * scale);
}
+
+ /*
+ * If the uncertainty margin is not specified, calculate it.
+ * If both scale and freq are non-zero, calculate the clock
+ * period, but bound below at 2*WATCHDOG_MAX_SKEW. However,
+ * if either of scale or freq is zero, be very conservative and
+ * take the tens-of-milliseconds WATCHDOG_THRESHOLD value for the
+ * uncertainty margin. Allow stupidly small uncertainty margins
+ * to be specified by the caller for testing purposes, but warn
+ * to discourage production use of this capability.
+ */
+ if (scale && freq && !cs->uncertainty_margin) {
+ cs->uncertainty_margin = NSEC_PER_SEC / (scale * freq);
+ if (cs->uncertainty_margin < 2 * WATCHDOG_MAX_SKEW)
+ cs->uncertainty_margin = 2 * WATCHDOG_MAX_SKEW;
+ } else if (!cs->uncertainty_margin) {
+ cs->uncertainty_margin = WATCHDOG_THRESHOLD;
+ }
+ WARN_ON_ONCE(cs->uncertainty_margin < 2 * WATCHDOG_MAX_SKEW);
+
/*
* Ensure clocksources that have large 'mult' values don't overflow
* when adjusted.
diff --git a/kernel/time/jiffies.c b/kernel/time/jiffies.c
index a5cffe2a1770..165b85bcdf29 100644
--- a/kernel/time/jiffies.c
+++ b/kernel/time/jiffies.c
@@ -49,13 +49,14 @@ static u64 jiffies_read(struct clocksource *cs)
* for "tick-less" systems.
*/
static struct clocksource clocksource_jiffies = {
- .name = "jiffies",
- .rating = 1, /* lowest valid rating*/
- .read = jiffies_read,
- .mask = CLOCKSOURCE_MASK(32),
- .mult = TICK_NSEC << JIFFIES_SHIFT, /* details above */
- .shift = JIFFIES_SHIFT,
- .max_cycles = 10,
+ .name = "jiffies",
+ .rating = 1, /* lowest valid rating*/
+ .uncertainty_margin = TICK_NSEC,
+ .read = jiffies_read,
+ .mask = CLOCKSOURCE_MASK(32),
+ .mult = TICK_NSEC << JIFFIES_SHIFT, /* details above */
+ .shift = JIFFIES_SHIFT,
+ .max_cycles = 10,
};
__cacheline_aligned_in_smp DEFINE_RAW_SPINLOCK(jiffies_lock);
--
2.31.1.189.g2e36527f23
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