Re: [PATCH v14 clocksource 4/6] clocksource: Reduce clocksource-skew threshold for TSC

Feng Tang
2021-05-12T02:19:00Z

Hi Paul,

On Tue, May 11, 2021 at 04:34:53PM -0700, Paul E. McKenney wrote:
> 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/202104291438.PuHsxRkl-lkp@intel.com/
> Link: https://lore.kernel.org/lkml/20210429140440.GT975577@paulmck-ThinkPad-P17-Gen-1
> 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@linutronix.de>
> Cc: John Stultz <john.stultz@linaro.org>
> Cc: Stephen Boyd <sboyd@kernel.org>
> Cc: Jonathan Corbet <corbet@lwn.net>
> Cc: Mark Rutland <Mark.Rutland@arm.com>
> Cc: Marc Zyngier <maz@kernel.org>
> Cc: Andi Kleen <ak@linux.intel.com>
> Cc: Xing Zhengjun <zhengjun.xing@linux.intel.com>
> Cc: Feng Tang <feng.tang@intel.com>
> Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
> ---
> arch/x86/kernel/tsc.c | 1 +
> include/linux/clocksource.h | 3 +++
> kernel/time/clocksource.c | 48 +++++++++++++++++++++++++++++--------
> kernel/time/jiffies.c | 15 ++++++------
> 4 files changed, 50 insertions(+), 17 deletions(-)
>
> diff --git a/arch/x86/kernel/tsc.c b/arch/x86/kernel/tsc.c
> index 6eb1b097e97e..2e076a459a0c 100644
> --- a/arch/x86/kernel/tsc.c
> +++ b/arch/x86/kernel/tsc.c
> @@ -1128,6 +1128,7 @@ 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 7f83d51c0fd7..895203727cb5 100644
> --- a/include/linux/clocksource.h
> +++ b/include/linux/clocksource.h
> @@ -43,6 +43,8 @@ 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
> @@ -98,6 +100,7 @@ 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 66243da2dadb..9ebf9931f3d6 100644
> --- a/kernel/time/clocksource.c
> +++ b/kernel/time/clocksource.c
> @@ -95,6 +95,20 @@ static char override_name[CS_NAME_LEN];
> static int finished_booting;
> static u64 suspend_start;
>
> +/*
> + * Threshold: 0.0312s, when doubled: 0.0625s.
> + * Also a default for cs->uncertainty_margin when registering clocks.
> + */
> +#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. Used as
> + * a lower bound for cs->uncertainty_margin values when registering clocks.
> + */
> +#define WATCHDOG_MAX_SKEW (50 * NSEC_PER_USEC)
> +
> #ifdef CONFIG_CLOCKSOURCE_WATCHDOG
> static void clocksource_watchdog_work(struct work_struct *work);
> static void clocksource_select(void);
> @@ -121,17 +135,9 @@ 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.
> */
> #define WATCHDOG_INTERVAL (HZ >> 1)
> -#define WATCHDOG_THRESHOLD (NSEC_PER_SEC >> 4)
> -
> -/*
> - * 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)
>
> static void clocksource_watchdog_work(struct work_struct *work)
> {
> @@ -347,6 +353,7 @@ 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)
> @@ -393,7 +400,8 @@ 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",
> @@ -1046,6 +1054,26 @@ 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 a492e4da69ba..b3f608c2b936 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,

'jiffies' is known to be very bad as a watchdog ("worse bandaid" in
Thomas' words :)), and TICK_NSEC just turns to 1ms for HZ=1000 case.
Maybe we should give it a bigger margin, like the 32ms margin for
'tsc-early'?

Other than this, it looks good to me, thanks!

- Feng

> + .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