Messages in this thread Patch in this message |  | | | Date | Thu, 28 May 2009 20:09:02 +0900 | | From | Paul Mundt <> | | Subject | Re: [PATCH] sched: Support current clocksource handling in fallback sched_clock(). |
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On Thu, May 28, 2009 at 11:34:41AM +0200, Peter Zijlstra wrote: > That's a single assignment, vs two reads on use. Should we be worried > about the SMP race where we observe two different sched_clocksource > pointers on read? > > > I would suggest we write it as: > > u64 __weak sched_clock(void) > { > struct clocksource *clock = ACCESS_ONCE(sched_clocksource); > > return cyc2ns(clock, clocksource_read(clock)); > }
Here's an updated version, also with an added sanity check in the unregister path..
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include/linux/clocksource.h | 4 +++- kernel/sched_clock.c | 6 ++++-- kernel/time/clocksource.c | 13 +++++++++++++ kernel/time/jiffies.c | 2 +- 4 files changed, 21 insertions(+), 4 deletions(-) diff --git a/include/linux/clocksource.h b/include/linux/clocksource.h index c56457c..2109940 100644 --- a/include/linux/clocksource.h +++ b/include/linux/clocksource.h @@ -202,7 +202,8 @@ struct clocksource { #endif }; -extern struct clocksource *clock; /* current clocksource */ +extern struct clocksource *clock; /* current clocksource */ +extern struct clocksource *sched_clocksource; /* sched_clock() clocksource */ /* * Clock source flags bits:: @@ -212,6 +213,7 @@ extern struct clocksource *clock; /* current clocksource */ #define CLOCK_SOURCE_WATCHDOG 0x10 #define CLOCK_SOURCE_VALID_FOR_HRES 0x20 +#define CLOCK_SOURCE_USE_FOR_SCHED_CLOCK 0x40 /* simplify initialization of mask field */ #define CLOCKSOURCE_MASK(bits) (cycle_t)((bits) < 64 ? ((1ULL<<(bits))-1) : -1) diff --git a/kernel/sched_clock.c b/kernel/sched_clock.c index e1d16c9..b91190e 100644 --- a/kernel/sched_clock.c +++ b/kernel/sched_clock.c @@ -30,6 +30,7 @@ #include <linux/percpu.h> #include <linux/ktime.h> #include <linux/sched.h> +#include <linux/clocksource.h> /* * Scheduler clock - returns current time in nanosec units. @@ -38,8 +39,9 @@ */ unsigned long long __attribute__((weak)) sched_clock(void) { - return (unsigned long long)(jiffies - INITIAL_JIFFIES) - * (NSEC_PER_SEC / HZ); + struct clocksource *clock = ACCESS_ONCE(sched_clocksource); + + return cyc2ns(clock, clocksource_read(clock)); } static __read_mostly int sched_clock_running; diff --git a/kernel/time/clocksource.c b/kernel/time/clocksource.c index 80189f6..57b011a 100644 --- a/kernel/time/clocksource.c +++ b/kernel/time/clocksource.c @@ -109,6 +109,7 @@ EXPORT_SYMBOL(timecounter_cyc2time); /* XXX - Would like a better way for initializing curr_clocksource */ extern struct clocksource clocksource_jiffies; +struct clocksource *sched_clocksource = &clocksource_jiffies; /*[Clocksource internal variables]--------- * curr_clocksource: @@ -362,6 +363,9 @@ static struct clocksource *select_clocksource(void) if (next == curr_clocksource) return NULL; + if (next->flags & CLOCK_SOURCE_USE_FOR_SCHED_CLOCK) + sched_clocksource = next; + return next; } @@ -437,10 +441,19 @@ void clocksource_unregister(struct clocksource *cs) unsigned long flags; spin_lock_irqsave(&clocksource_lock, flags); + + if (sched_clocksource == cs) { + printk(KERN_WARNING "Refusing to unregister " + "scheduler clocksource %s", cs->name); + goto out; + } + list_del(&cs->list); if (clocksource_override == cs) clocksource_override = NULL; next_clocksource = select_clocksource(); + +out: spin_unlock_irqrestore(&clocksource_lock, flags); } diff --git a/kernel/time/jiffies.c b/kernel/time/jiffies.c index c3f6c30..727d881 100644 --- a/kernel/time/jiffies.c +++ b/kernel/time/jiffies.c @@ -52,7 +52,7 @@ static cycle_t jiffies_read(struct clocksource *cs) { - return (cycle_t) jiffies; + return (cycle_t) (jiffies - INITIAL_JIFFIES); } struct clocksource clocksource_jiffies = {
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