Messages in this thread Patch in this message | | | From | "Paul E. McKenney" <> | Subject | [PATCH v10 clocksource 6/7] clocksource: Forgive tsc_early pre-calibration drift | Date | Sun, 25 Apr 2021 15:47:07 -0700 |
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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.
Cc: John Stultz <john.stultz@linaro.org> Cc: Thomas Gleixner <tglx@linutronix.de> 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 | 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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