From b421b22b00b0011f6a2ce3561176c4e79e640c49 Mon Sep 17 00:00:00 2001 From: Peter Zijlstra Date: Fri, 21 Apr 2017 12:14:13 +0200 Subject: x86/tsc, sched/clock, clocksource: Use clocksource watchdog to provide stable sync points Currently we keep sched_clock_tick() active for stable TSC in order to keep the per-CPU state semi up-to-date. The (obvious) problem is that by the time we detect TSC is borked, our per-CPU state is also borked. So hook into the clocksource watchdog and call a method after we've found it to still be stable. There's the obvious race where the TSC goes wonky between finding it stable and us running the callback, but closing that is too much work and not really worth it, since we're already detecting TSC wobbles after the fact, so we cannot, per definition, fully avoid funny clock values. And since the watchdog runs less often than the tick, this is also an optimization. Signed-off-by: Peter Zijlstra (Intel) Cc: Linus Torvalds Cc: Mike Galbraith Cc: Peter Zijlstra Cc: Thomas Gleixner Cc: linux-kernel@vger.kernel.org Signed-off-by: Ingo Molnar --- kernel/sched/clock.c | 36 +++++++++++++++++++++++++++--------- 1 file changed, 27 insertions(+), 9 deletions(-) (limited to 'kernel/sched/clock.c') diff --git a/kernel/sched/clock.c b/kernel/sched/clock.c index dc650851935f..f861637f7fdc 100644 --- a/kernel/sched/clock.c +++ b/kernel/sched/clock.c @@ -366,20 +366,38 @@ void sched_clock_tick(void) { struct sched_clock_data *scd; + if (sched_clock_stable()) + return; + + if (unlikely(!sched_clock_running)) + return; + WARN_ON_ONCE(!irqs_disabled()); - /* - * Update these values even if sched_clock_stable(), because it can - * become unstable at any point in time at which point we need some - * values to fall back on. - * - * XXX arguably we can skip this if we expose tsc_clocksource_reliable - */ scd = this_scd(); __scd_stamp(scd); + sched_clock_local(scd); +} + +void sched_clock_tick_stable(void) +{ + u64 gtod, clock; - if (!sched_clock_stable() && likely(sched_clock_running)) - sched_clock_local(scd); + if (!sched_clock_stable()) + return; + + /* + * Called under watchdog_lock. + * + * The watchdog just found this TSC to (still) be stable, so now is a + * good moment to update our __gtod_offset. Because once we find the + * TSC to be unstable, any computation will be computing crap. + */ + local_irq_disable(); + gtod = ktime_get_ns(); + clock = sched_clock(); + __gtod_offset = (clock + __sched_clock_offset) - gtod; + local_irq_enable(); } /* -- cgit v1.2.3-59-g8ed1b