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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. Suggested-by: Thomas Gleixner <tglx@linutronix.de> Signed-off-by: Paul E. McKenney <paulmck@kernel.org> Signed-off-by: Thomas Gleixner <tglx@linutronix.de> Acked-by: Feng Tang <feng.tang@intel.com> Link: https://lore.kernel.org/r/20210527190124.440372-4-paulmck@kernel.org
124 lines
3.2 KiB
C
124 lines
3.2 KiB
C
// SPDX-License-Identifier: GPL-2.0+
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/*
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* This file contains the jiffies based clocksource.
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*
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* Copyright (C) 2004, 2005 IBM, John Stultz (johnstul@us.ibm.com)
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*/
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#include <linux/clocksource.h>
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#include <linux/jiffies.h>
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#include <linux/module.h>
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#include <linux/init.h>
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#include "timekeeping.h"
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/* Since jiffies uses a simple TICK_NSEC multiplier
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* conversion, the .shift value could be zero. However
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* this would make NTP adjustments impossible as they are
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* in units of 1/2^.shift. Thus we use JIFFIES_SHIFT to
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* shift both the nominator and denominator the same
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* amount, and give ntp adjustments in units of 1/2^8
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*
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* The value 8 is somewhat carefully chosen, as anything
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* larger can result in overflows. TICK_NSEC grows as HZ
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* shrinks, so values greater than 8 overflow 32bits when
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* HZ=100.
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*/
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#if HZ < 34
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#define JIFFIES_SHIFT 6
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#elif HZ < 67
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#define JIFFIES_SHIFT 7
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#else
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#define JIFFIES_SHIFT 8
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#endif
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static u64 jiffies_read(struct clocksource *cs)
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{
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return (u64) jiffies;
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}
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/*
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* The Jiffies based clocksource is the lowest common
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* denominator clock source which should function on
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* all systems. It has the same coarse resolution as
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* the timer interrupt frequency HZ and it suffers
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* inaccuracies caused by missed or lost timer
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* interrupts and the inability for the timer
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* interrupt hardware to accurately tick at the
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* requested HZ value. It is also not recommended
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* for "tick-less" systems.
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*/
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static struct clocksource clocksource_jiffies = {
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.name = "jiffies",
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.rating = 1, /* lowest valid rating*/
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.uncertainty_margin = 32 * NSEC_PER_MSEC,
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.read = jiffies_read,
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.mask = CLOCKSOURCE_MASK(32),
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.mult = TICK_NSEC << JIFFIES_SHIFT, /* details above */
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.shift = JIFFIES_SHIFT,
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.max_cycles = 10,
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};
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__cacheline_aligned_in_smp DEFINE_RAW_SPINLOCK(jiffies_lock);
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__cacheline_aligned_in_smp seqcount_raw_spinlock_t jiffies_seq =
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SEQCNT_RAW_SPINLOCK_ZERO(jiffies_seq, &jiffies_lock);
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#if (BITS_PER_LONG < 64)
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u64 get_jiffies_64(void)
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{
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unsigned int seq;
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u64 ret;
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do {
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seq = read_seqcount_begin(&jiffies_seq);
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ret = jiffies_64;
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} while (read_seqcount_retry(&jiffies_seq, seq));
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return ret;
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}
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EXPORT_SYMBOL(get_jiffies_64);
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#endif
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EXPORT_SYMBOL(jiffies);
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static int __init init_jiffies_clocksource(void)
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{
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return __clocksource_register(&clocksource_jiffies);
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}
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core_initcall(init_jiffies_clocksource);
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struct clocksource * __init __weak clocksource_default_clock(void)
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{
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return &clocksource_jiffies;
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}
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static struct clocksource refined_jiffies;
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int register_refined_jiffies(long cycles_per_second)
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{
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u64 nsec_per_tick, shift_hz;
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long cycles_per_tick;
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refined_jiffies = clocksource_jiffies;
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refined_jiffies.name = "refined-jiffies";
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refined_jiffies.rating++;
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/* Calc cycles per tick */
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cycles_per_tick = (cycles_per_second + HZ/2)/HZ;
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/* shift_hz stores hz<<8 for extra accuracy */
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shift_hz = (u64)cycles_per_second << 8;
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shift_hz += cycles_per_tick/2;
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do_div(shift_hz, cycles_per_tick);
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/* Calculate nsec_per_tick using shift_hz */
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nsec_per_tick = (u64)NSEC_PER_SEC << 8;
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nsec_per_tick += (u32)shift_hz/2;
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do_div(nsec_per_tick, (u32)shift_hz);
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refined_jiffies.mult = ((u32)nsec_per_tick) << JIFFIES_SHIFT;
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__clocksource_register(&refined_jiffies);
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return 0;
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}
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