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    Subject[RFC][patch 2/5] cleanup clocksource selection
    From: Martin Schwidefsky <schwidefsky@de.ibm.com>

    Introduce clocksource_dequeue & clocksource_update and move spinlock
    calls. clocksource_update does nothing for GENERIC_TIME=n since
    change_clocksource does nothing as well.

    Cc: Ingo Molnar <mingo@elte.hu>
    Cc: Thomas Gleixner <tglx@linutronix.de>
    Cc: john stultz <johnstul@us.ibm.com>
    Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>
    ---
    kernel/time/clocksource.c | 123 ++++++++++++++++++++++++++--------------------
    1 file changed, 70 insertions(+), 53 deletions(-)

    Index: linux-2.6/kernel/time/clocksource.c
    ===================================================================
    --- linux-2.6.orig/kernel/time/clocksource.c
    +++ linux-2.6/kernel/time/clocksource.c
    @@ -341,46 +341,25 @@ struct clocksource *clocksource_get_next
    return curr_clocksource;
    }

    -/**
    - * select_clocksource - Selects the best registered clocksource.
    - *
    - * Private function. Must hold clocksource_lock when called.
    - *
    - * Select the clocksource with the best rating, or the clocksource,
    - * which is selected by userspace override.
    - */
    -static struct clocksource *select_clocksource(void)
    -{
    - struct clocksource *next;
    -
    - if (list_empty(&clocksource_list))
    - return NULL;
    -
    - if (clocksource_override)
    - next = clocksource_override;
    - else
    - next = list_entry(clocksource_list.next, struct clocksource,
    - list);
    -
    - if (next == curr_clocksource)
    - return NULL;
    -
    - return next;
    -}
    -
    /*
    * Enqueue the clocksource sorted by rating
    */
    static int clocksource_enqueue(struct clocksource *c)
    {
    struct list_head *tmp, *entry = &clocksource_list;
    + unsigned long flags;
    + int rc;

    + spin_lock_irqsave(&clocksource_lock, flags);
    + rc = 0;
    list_for_each(tmp, &clocksource_list) {
    struct clocksource *cs;

    cs = list_entry(tmp, struct clocksource, list);
    - if (cs == c)
    - return -EBUSY;
    + if (cs == c) {
    + rc = -EBUSY;
    + goto out;
    + }
    /* Keep track of the place, where to insert */
    if (cs->rating >= c->rating)
    entry = tmp;
    @@ -390,8 +369,23 @@ static int clocksource_enqueue(struct cl
    if (strlen(c->name) == strlen(override_name) &&
    !strcmp(c->name, override_name))
    clocksource_override = c;
    +out:
    + spin_unlock_irqrestore(&clocksource_lock, flags);
    + return rc;
    +}
    +
    +/*
    + * Dequeue a clocksource
    + */
    +static void clocksource_dequeue(struct clocksource *cs)
    +{
    + unsigned long flags;

    - return 0;
    + spin_lock_irqsave(&clocksource_lock, flags);
    + list_del(&cs->list);
    + if (clocksource_override == cs)
    + clocksource_override = NULL;
    + spin_unlock_irqrestore(&clocksource_lock, flags);
    }

    /**
    @@ -539,6 +533,25 @@ void clocksource_forward_now(void)
    }

    /**
    + * select_clocksource - Selects the best registered clocksource.
    + *
    + * Private function. Must hold clocksource_lock when called.
    + *
    + * Select the clocksource with the best rating, or the clocksource,
    + * which is selected by userspace override.
    + */
    +static struct clocksource *select_clocksource(void)
    +{
    + if (list_empty(&clocksource_list))
    + return NULL;
    +
    + if (clocksource_override)
    + return clocksource_override;
    +
    + return list_entry(clocksource_list.next, struct clocksource, list);
    +}
    +
    +/**
    * change_clocksource - Swaps clocksources if a new one is available
    *
    * Accumulates current time interval and initializes new clocksource
    @@ -577,6 +590,23 @@ void change_clocksource(void)
    clock->name);
    */
    }
    +
    +/**
    + * clocksource_update - Check if a better clocksource is available
    + */
    +static void clocksource_update(void)
    +{
    + struct clocksource *new;
    + unsigned long flags;
    +
    + spin_lock_irqsave(&clocksource_lock, flags);
    + new = select_clocksource();
    + if (new)
    + next_clocksource = new;
    + spin_unlock_irqrestore(&clocksource_lock, flags);
    +}
    +#else /* CONFIG_GENERIC_TIME */
    +static inline void clocksource_update(void) { }
    #endif

    /**
    @@ -587,16 +617,13 @@ void change_clocksource(void)
    */
    int clocksource_register(struct clocksource *c)
    {
    - unsigned long flags;
    int ret;

    - spin_lock_irqsave(&clocksource_lock, flags);
    ret = clocksource_enqueue(c);
    - if (!ret)
    - next_clocksource = select_clocksource();
    - spin_unlock_irqrestore(&clocksource_lock, flags);
    - if (!ret)
    + if (!ret) {
    + clocksource_update();
    clocksource_check_watchdog(c);
    + }
    return ret;
    }
    EXPORT_SYMBOL(clocksource_register);
    @@ -607,14 +634,10 @@ EXPORT_SYMBOL(clocksource_register);
    */
    void clocksource_change_rating(struct clocksource *cs, int rating)
    {
    - unsigned long flags;
    -
    - spin_lock_irqsave(&clocksource_lock, flags);
    - list_del(&cs->list);
    + clocksource_dequeue(cs);
    cs->rating = rating;
    clocksource_enqueue(cs);
    - next_clocksource = select_clocksource();
    - spin_unlock_irqrestore(&clocksource_lock, flags);
    + clocksource_update();
    }

    /**
    @@ -622,14 +645,8 @@ void clocksource_change_rating(struct cl
    */
    void clocksource_unregister(struct clocksource *cs)
    {
    - unsigned long flags;
    -
    - spin_lock_irqsave(&clocksource_lock, flags);
    - list_del(&cs->list);
    - if (clocksource_override == cs)
    - clocksource_override = NULL;
    - next_clocksource = select_clocksource();
    - spin_unlock_irqrestore(&clocksource_lock, flags);
    + clocksource_dequeue(cs);
    + clocksource_update();
    }

    /**
    @@ -721,13 +738,13 @@ static ssize_t sysfs_override_clocksourc
    }

    /* Reselect, when the override name has changed */
    - if (ovr != clocksource_override) {
    + if (ovr != clocksource_override)
    clocksource_override = ovr;
    - next_clocksource = select_clocksource();
    - }

    spin_unlock_irq(&clocksource_lock);

    + clocksource_update();
    +
    return ret;
    }

    --
    blue skies,
    Martin.

    "Reality continues to ruin my life." - Calvin.



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