From d375b4e0fa3771343b370be0d876a1963c02e0a0 Mon Sep 17 00:00:00 2001 From: Peter Zijlstra Date: Wed, 22 Jan 2014 12:59:18 +0100 Subject: [PATCH] sched/clock: Fixup early initialization The code would assume sched_clock_stable() and switch to !stable later, this switch brings a discontinuity in time. The discontinuity on switching from stable to unstable was always present, but previously we would set stable/unstable before initializing TSC and usually stick to the one we start out with. So the static_key bits brought an extra switch where there previously wasn't one. Things are further complicated by the fact that we cannot use static_key as early as we usually call set_sched_clock_stable(). Fix things by tracking the stable state in a regular variable and only set the static_key to the right state on sched_clock_init(), which is ran right after late_time_init->tsc_init(). Before this we would not be using the TSC anyway. Reported-and-Tested-by: Sasha Levin Reported-by: dyoung@redhat.com Fixes: 35af99e646c7 ("sched/clock, x86: Use a static_key for sched_clock_stable") Cc: jacob.jun.pan@linux.intel.com Cc: Mike Galbraith Cc: hpa@zytor.com Cc: paulmck@linux.vnet.ibm.com Cc: John Stultz Cc: Andy Lutomirski Cc: Arjan van de Ven Cc: lenb@kernel.org Cc: rjw@rjwysocki.net Cc: Eliezer Tamir Cc: rui.zhang@intel.com Signed-off-by: Peter Zijlstra Link: http://lkml.kernel.org/r/20140122115918.GG3694@twins.programming.kicks-ass.net Signed-off-by: Ingo Molnar --- kernel/sched/clock.c | 53 ++++++++++++++++++++++++++++++++++++++++------------ 1 file changed, 41 insertions(+), 12 deletions(-) diff --git a/kernel/sched/clock.c b/kernel/sched/clock.c index 6bd6a67..43c2bcc 100644 --- a/kernel/sched/clock.c +++ b/kernel/sched/clock.c @@ -77,35 +77,50 @@ __read_mostly int sched_clock_running; #ifdef CONFIG_HAVE_UNSTABLE_SCHED_CLOCK static struct static_key __sched_clock_stable = STATIC_KEY_INIT; +static int __sched_clock_stable_early; int sched_clock_stable(void) { - if (static_key_false(&__sched_clock_stable)) - return false; - return true; + return static_key_false(&__sched_clock_stable); } -void set_sched_clock_stable(void) +static void __set_sched_clock_stable(void) { if (!sched_clock_stable()) - static_key_slow_dec(&__sched_clock_stable); + static_key_slow_inc(&__sched_clock_stable); +} + +void set_sched_clock_stable(void) +{ + __sched_clock_stable_early = 1; + + smp_mb(); /* matches sched_clock_init() */ + + if (!sched_clock_running) + return; + + __set_sched_clock_stable(); } static void __clear_sched_clock_stable(struct work_struct *work) { /* XXX worry about clock continuity */ if (sched_clock_stable()) - static_key_slow_inc(&__sched_clock_stable); + static_key_slow_dec(&__sched_clock_stable); } static DECLARE_WORK(sched_clock_work, __clear_sched_clock_stable); void clear_sched_clock_stable(void) { - if (keventd_up()) - schedule_work(&sched_clock_work); - else - __clear_sched_clock_stable(&sched_clock_work); + __sched_clock_stable_early = 0; + + smp_mb(); /* matches sched_clock_init() */ + + if (!sched_clock_running) + return; + + schedule_work(&sched_clock_work); } struct sched_clock_data { @@ -140,6 +155,20 @@ void sched_clock_init(void) } sched_clock_running = 1; + + /* + * Ensure that it is impossible to not do a static_key update. + * + * Either {set,clear}_sched_clock_stable() must see sched_clock_running + * and do the update, or we must see their __sched_clock_stable_early + * and do the update, or both. + */ + smp_mb(); /* matches {set,clear}_sched_clock_stable() */ + + if (__sched_clock_stable_early) + __set_sched_clock_stable(); + else + __clear_sched_clock_stable(NULL); } /* @@ -340,7 +369,7 @@ EXPORT_SYMBOL_GPL(sched_clock_idle_wakeup_event); */ u64 cpu_clock(int cpu) { - if (static_key_false(&__sched_clock_stable)) + if (!sched_clock_stable()) return sched_clock_cpu(cpu); return sched_clock(); @@ -355,7 +384,7 @@ u64 cpu_clock(int cpu) */ u64 local_clock(void) { - if (static_key_false(&__sched_clock_stable)) + if (!sched_clock_stable()) return sched_clock_cpu(raw_smp_processor_id()); return sched_clock(); -- 2.7.4