Message-ID: <20210512021849.GB78351@shbuild999.sh.intel.com>
Date:   Wed, 12 May 2021 10:18:49 +0800
From:   Feng Tang <feng.tang@...el.com>
To:     "Paul E. McKenney" <paulmck@...nel.org>
Cc:     tglx@...utronix.de, 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,
        zhengjun.xing@...el.com,
        Xing Zhengjun <zhengjun.xing@...ux.intel.com>
Subject: Re: [PATCH v14 clocksource 4/6] clocksource: Reduce clocksource-skew
 threshold for TSC

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@...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   | 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

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