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Date: Mon, 26 Apr 2021 23:01:27 +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 v10 clocksource 6/7] clocksource: Forgive tsc_early
pre-calibration drift
Hi Paul,
On Sun, Apr 25, 2021 at 03:47:07PM -0700, Paul E. McKenney wrote:
> Because the x86 tsc_early clocksource is given a quick and semi-accurate
> calibration (by design!), it might have drift rates well in excess of
> the 0.1% limit that is in the process of being adopted.
>
> Therefore, add a max_drift field to the clocksource structure that, when
> non-zero, specifies the maximum allowable drift rate in nanoseconds over
> a half-second period. The tsc_early clocksource initializes this to five
> miliseconds, which corresponds to the 1% drift rate limit suggested by
> Xing Zhengjun. This max_drift field is intended only for early boot,
> so clocksource_watchdog() splats if it encounters a non-zero value in
> this field more than 60 seconds after boot, inspired by a suggestion by
> Thomas Gleixner.
>
> This was tested by setting the clocksource_tsc ->max_drift field to 1,
> which, as expected, resulted in a clock-skew event.
We've run the same last for this v10, and those 'unstable' thing [1] can
not be reproduced!
We've reported one case that tsc can be wrongly judged as 'unstable'
by 'refined-jiffies' watchdog [1], while reducing the threshold could
make it easier to be triggered.
It could be reproduced on the a plaform with a 115200 serial console,
and hpet been disabled (several x86 platforms has this), add
'initcall_debug' cmdline parameter to get more debug message, we can
see:
[ 1.134197] clocksource: timekeeping watchdog on CPU1: Marking clocksource 'tsc-early' as unstable because the skew is too large:
[ 1.134214] clocksource: 'refined-jiffies' wd_nesc: 500000000 wd_now: ffff8b35 wd_last: ffff8b03 mask: ffffffff
[ 1.134217] clocksource: 'tsc-early' cs_nsec: 507537855 cs_now: 4e63c9d09 cs_last: 4bebd81f5 mask: ffffffffffffffff
[ 1.134220] clocksource: No current clocksource.
[ 1.134222] tsc: Marking TSC unstable due to clocksource watchdog
[1]. https://lore.kernel.org/lkml/20210420064934.GE31773@xsang-OptiPlex-9020/
[2]. https://lore.kernel.org/lkml/20201126012421.GA92582@shbuild999.sh.intel.com/
Thanks,
Feng
> Cc: John Stultz <john.stultz@...aro.org>
> Cc: Thomas Gleixner <tglx@...utronix.de>
> 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 | 5 +++++
> kernel/time/clocksource.c | 19 ++++++++++++++++++-
> 3 files changed, 24 insertions(+), 1 deletion(-)
>
> diff --git a/arch/x86/kernel/tsc.c b/arch/x86/kernel/tsc.c
> index 56289170753c..7192b8950322 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,
> + .max_drift = 5 * 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..07d25a158d7a 100644
> --- a/include/linux/clocksource.h
> +++ b/include/linux/clocksource.h
> @@ -42,6 +42,10 @@ 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%)
> + * @max_drift: Maximum drift rate in nanoseconds per half second.
> + * Zero says to use default WATCHDOG_THRESHOLD.
> + * A non-zero value for ->max_drift more than 60 seconds after boot
> + * will result in a splat.
> * @archdata: Optional arch-specific data
> * @max_cycles: Maximum safe cycle value which won't overflow on
> * multiplication
> @@ -93,6 +97,7 @@ struct clocksource {
> u32 shift;
> u64 max_idle_ns;
> u32 maxadj;
> + u32 max_drift;
> #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..070b09dab815 100644
> --- a/kernel/time/clocksource.c
> +++ b/kernel/time/clocksource.c
> @@ -126,6 +126,7 @@ static void __clocksource_change_rating(struct clocksource *cs, int rating);
> */
> #define WATCHDOG_INTERVAL (HZ >> 1)
> #define WATCHDOG_THRESHOLD (NSEC_PER_SEC >> 4)
> +#define WATCHDOG_SYNC_FORGIVENESS (HZ * 60UL)
>
> /*
> * Maximum permissible delay between two readouts of the watchdog
> @@ -377,6 +378,7 @@ 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 +425,22 @@ static void clocksource_watchdog(struct timer_list *unused)
> continue;
>
> /* Check the deviation from the watchdog clocksource. */
> - if (abs(cs_nsec - wd_nsec) > WATCHDOG_THRESHOLD) {
> + if (!cs->max_drift) {
> + md = WATCHDOG_THRESHOLD;
> + } else {
> + static unsigned long first_jiffies;
> + static bool beenhere;
> +
> + if (beenhere) {
> + WARN_ON_ONCE(time_after(jiffies,
> + first_jiffies + WATCHDOG_SYNC_FORGIVENESS));
> + } else {
> + beenhere = true;
> + first_jiffies = jiffies;
> + }
> + md = cs->max_drift;
> + }
> + 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",
> --
> 2.31.1.189.g2e36527f23
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