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    SubjectRe: [PATCH] timers: Introduce the concept of timer slack for legacy timers
    On Sat, 9 Jan 2010 13:37:44 -0800
    Arjan van de Ven <arjan@infradead.org> wrote:

    > While HR timers have had the concept of timer slack for quite some time
    > now, the legacy timers lacked this concept, and had to make do with
    > round_jiffies() and friends.
    >
    > Timer slack is important for power management; grouping timers reduces
    > the number of wakeups which in turn reduces power consumption.
    >
    > This patch introduces timer slack to the legacy timers using the following
    > pieces:
    > * A slack field in the timer struct
    > * An api (set_timer_slack) that callers can use to set explicit timer slack
    > * A default slack of 0.4% of the requested delay for callers that do not set
    > any explicit slack
    > * Rounding code that is part of mod_timer() that tries to
    > group timers around jiffies values every 'power of two'
    > (so quick timers will group around every 2, but longer timers
    > will group around every 4, 8, 16, 32 etc)
    >
    > ...
    >
    > +/**
    > + * set_timer_slack - set the allowed slack for a timer
    > + * @slack_hz: the amount of time (in jiffies) allowed for rounding
    > + *
    > + * Set the amount of time, in jiffies, that a certain timer has
    > + * in terms of slack. By setting this value, the timer subsystem
    > + * will schedule the actual timer somewhere between
    > + * the time mod_timer() asks for, and that time plus the slack.
    > + *
    > + * By setting the slack to -1, a percentage of the delay is used
    > + * instead.
    > + */
    > +void set_timer_slack(struct timer_list *timer, int slack_hz)
    > +{
    > + timer->slack = slack_hz;
    > +}
    > +EXPORT_SYMBOL_GPL(set_timer_slack);

    I suppose this could be inlined.

    > }
    > EXPORT_SYMBOL(mod_timer_pending);
    >
    > +/*
    > + * Decide where to put the timer while taking the slack into account
    > + *
    > + * Algorithm:
    > + * 1) calculate the maximum (absolute) time
    > + * 2) calculate the highest bit where the expires and new max are different
    > + * 3) use this bit to make a mask
    > + * 4) use the bitmask to round down the maximum time, so that all last
    > + * bits are zeros
    > + */
    > +static inline
    > +unsigned long apply_slack(struct timer_list *timer, unsigned long expires)
    > +{
    > + unsigned long expires_limit, mask;
    > + int bit;
    > +
    > + expires_limit = expires + timer->slack;
    > +
    > + if (timer->slack < 0) /* auto slack: use 0.4% */
    > + expires_limit = expires + (expires - jiffies)/256;
    > +
    > + mask = expires ^ expires_limit;
    > +
    > + if (mask == 0)
    > + return expires;
    > +
    > + bit = find_last_bit(&mask, BITS_PER_LONG);
    > +
    > + mask = (1 << bit) - 1;
    > +
    > + expires_limit = expires_limit & ~(mask);
    > +
    > + return expires_limit;
    > +}

    OK, so by default this causes every timer in the system to have a bit
    of slack (unless they're really short-term?), so the feature does get
    runtime tested.

    But the set_timer_slack() interface has no callers. Perhaps it should?


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