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    Subject[PATCH/RFC] msleep() with hrtimers
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    The OLPC folks and I recently discovered something interesting: on a
    HZ=100 system, a call to msleep(1) will delay for about 20ms. The
    combination of jiffies timekeeping and rounding up means that the
    minimum delay from msleep will be two jiffies, never less. That led to
    multi-second delays in a driver which does a bunch of short msleep()
    calls and, in response, a change to mdelay(), which will come back in
    something closer to the requested time.

    Here's another approach: a reimplementation of msleep() and
    msleep_interruptible() using hrtimers. On a system without real
    hrtimers this code will at least drop down to single-jiffy delays much
    of the time (though not deterministically so). On my x86_64 system with
    Thomas's hrtimer/dyntick patch applied, msleep(1) gives almost exactly
    what was asked for.

    jon

    Use hrtimers for msleep() and msleep_interruptible()

    Signed-off-by: Jonathan Corbet <corbet@lwn.net>

    --- 2.6.22-vanilla/kernel/timer.c 2007-07-15 13:08:36.000000000 -0600
    +++ 2.6.22-timer/kernel/timer.c 2007-07-15 16:13:34.000000000 -0600
    @@ -1522,18 +1522,43 @@ unregister_time_interpolator(struct time
    }
    #endif /* CONFIG_TIME_INTERPOLATION */

    +
    +
    +
    +static void do_msleep(unsigned int msecs, struct hrtimer_sleeper *sleeper,
    + int sigs)
    +{
    + enum hrtimer_mode mode = HRTIMER_MODE_REL;
    + int state = sigs ? TASK_INTERRUPTIBLE : TASK_UNINTERRUPTIBLE;
    +
    + /*
    + * This is really just a reworked and simplified version
    + * of do_nanosleep().
    + */
    + hrtimer_init(&sleeper->timer, CLOCK_MONOTONIC, mode);
    + sleeper->timer.expires = ktime_set(0, msecs*NSEC_PER_MSEC);
    + hrtimer_init_sleeper(sleeper, current);
    +
    + do {
    + set_current_state(state);
    + hrtimer_start(&sleeper->timer, sleeper->timer.expires, mode);
    + if (sleeper->task)
    + schedule();
    + hrtimer_cancel(&sleeper->timer);
    + mode = HRTIMER_MODE_ABS;
    + } while (sleeper->task && !(sigs && signal_pending(current)));
    +}
    +
    /**
    * msleep - sleep safely even with waitqueue interruptions
    * @msecs: Time in milliseconds to sleep for
    */
    void msleep(unsigned int msecs)
    {
    - unsigned long timeout = msecs_to_jiffies(msecs) + 1;
    + struct hrtimer_sleeper sleeper;

    - while (timeout)
    - timeout = schedule_timeout_uninterruptible(timeout);
    + do_msleep(msecs, &sleeper, 0);
    }
    -
    EXPORT_SYMBOL(msleep);

    /**
    @@ -1542,11 +1567,15 @@ EXPORT_SYMBOL(msleep);
    */
    unsigned long msleep_interruptible(unsigned int msecs)
    {
    - unsigned long timeout = msecs_to_jiffies(msecs) + 1;
    + struct hrtimer_sleeper sleeper;
    + ktime_t left;

    - while (timeout && !signal_pending(current))
    - timeout = schedule_timeout_interruptible(timeout);
    - return jiffies_to_msecs(timeout);
    -}
    + do_msleep(msecs, &sleeper, 1);

    + if (! sleeper.task)
    + return 0;
    + left = ktime_sub(sleeper.timer.expires,
    + sleeper.timer.base->get_time());
    + return max(((long) ktime_to_ns(left))/NSEC_PER_MSEC, 1L);
    +}
    EXPORT_SYMBOL(msleep_interruptible);
    -
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