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    SubjectRe: [PATCH] sched: sched_clock() clocksource handling.
    On Tue, Jun 02, 2009 at 09:41:35AM +0200, Peter Zijlstra wrote:
    > On Tue, 2009-06-02 at 16:35 +0900, Paul Mundt wrote:
    > >
    > > We already do via select_clocksource(), if we are unregistering the
    > > current one then a new one with the flag set is selected. Before that,
    > > the override is likewise given preference, and we fall back on jiffies if
    > > there is nothing else. I suppose we could try and find the "best" one,
    > > but I think the override and manual clocksource selection should be fine
    > > for this.
    >
    > Ah, ok. So unregister calls select_clocksource again? That does leave us
    > a small window with jiffies, but I guess that's ok.
    >
    A synchronize_rcu() would fix that up, but I think a small window with
    jiffies is less painful than sorting out RCU ordering and synchronization
    for a corner case of a corner case ;-)

    > > Now that you mention it though, the sched_clocksource() assignment within
    > > select_clocksource() happens underneath the clocksource_lock, but is not
    > > using rcu_assign_pointer().
    >
    > Right, that would want fixing indeed.
    >
    > > If the assignment there needs to use
    > > rcu_assign_pointer() then presumably all of the unlock paths that do
    > > select_clocksource() will have to synchronize_rcu()?
    >
    > No, you only have to do sync_rcu() when stuff that could have referenced
    > is going away and you cannot use call_rcu().
    >
    > So when selecting a new clocksource, you don't need synchonization
    > because stuff doesn't go away (I think :-)

    Ok, that keeps things more simplified then. How does this look?

    ---

    include/linux/clocksource.h | 4 +++-
    kernel/sched_clock.c | 13 +++++++++++--
    kernel/time/clocksource.c | 19 +++++++++++++++++++
    kernel/time/jiffies.c | 2 +-
    4 files changed, 34 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..b51d48d 100644
    --- a/kernel/sched_clock.c
    +++ b/kernel/sched_clock.c
    @@ -30,6 +30,8 @@
    #include <linux/percpu.h>
    #include <linux/ktime.h>
    #include <linux/sched.h>
    +#include <linux/clocksource.h>
    +#include <linux/rcupdate.h>

    /*
    * Scheduler clock - returns current time in nanosec units.
    @@ -38,8 +40,15 @@
    */
    unsigned long long __attribute__((weak)) sched_clock(void)
    {
    - return (unsigned long long)(jiffies - INITIAL_JIFFIES)
    - * (NSEC_PER_SEC / HZ);
    + unsigned long long time;
    + struct clocksource *clock;
    +
    + rcu_read_lock();
    + clock = rcu_dereference(sched_clocksource);
    + time = cyc2ns(clock, clocksource_read(clock));
    + rcu_read_unlock();
    +
    + return time;
    }

    static __read_mostly int sched_clock_running;
    diff --git a/kernel/time/clocksource.c b/kernel/time/clocksource.c
    index 80189f6..f7243f2 100644
    --- a/kernel/time/clocksource.c
    +++ b/kernel/time/clocksource.c
    @@ -25,6 +25,7 @@
    */

    #include <linux/clocksource.h>
    +#include <linux/rcupdate.h>
    #include <linux/sysdev.h>
    #include <linux/init.h>
    #include <linux/module.h>
    @@ -109,6 +110,7 @@ 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 +364,9 @@ static struct clocksource *select_clocksource(void)
    if (next == curr_clocksource)
    return NULL;

    + if (next->flags & CLOCK_SOURCE_USE_FOR_SCHED_CLOCK)
    + rcu_assign_pointer(sched_clocksource, next);
    +
    return next;
    }

    @@ -440,7 +445,21 @@ void clocksource_unregister(struct clocksource *cs)
    list_del(&cs->list);
    if (clocksource_override == cs)
    clocksource_override = NULL;
    +
    next_clocksource = select_clocksource();
    +
    + /*
    + * If select_clocksource() fails to find another suitable
    + * clocksource for sched_clocksource and we are unregistering
    + * it, switch back to jiffies.
    + */
    + if (sched_clocksource == cs) {
    + rcu_assign_pointer(sched_clocksource, &clocksource_jiffies);
    + spin_unlock_irqrestore(&clocksource_lock, flags);
    + synchronize_rcu();
    + return;
    + }
    +
    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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