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    SubjectRe: [PATCH] sched: Support current clocksource handling in fallback sched_clock().
    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..

    --

    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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