Re: [RFC][PATCH] Dynamic Tick: Allow 32-bit machines to sleep formore than 2.15 seconds
From: Jon Hunter
Date: Tue Apr 21 2009 - 19:21:15 EST
john stultz wrote:
The concern is many clocksources wrap after a handful of seconds. The
acpi_pm is the best example (its only 24 bits wide).
I brought this issue up earlier, and provided some example code that
could be used to limit the idle time appropriately for the current
clocksource here:
http://lkml.indiana.edu/hypermail/linux/kernel/0901.3/02693.html
Jon: Did you see that mail, or is there a reason you didn't adapt this
code into your patch?
Hi John, yes I did read this email and thanks for bringing this up.
As I looked at this more I noticed that for 64-bit machines that the
max_delta_ns would be a 64-bit integer already and so this change would
only have an impact for 32-bit machines. I understand that there are
more 32-bit machines that 64-bit. However, I was trying to understand
how the wrapping of clocksources, such as the one you mention above, is
handled today for 64-bit machines that could theoretically sleep for
longer periods.
In addition to this as I was reviewing the "tick_nohz_stop_sched_tick()"
function that is configuring the dynamic tick and I noticed that this
code would actually stop the timer altogether if the time for the next
timer event is greater than NEXT_TIMER_MAX_DELTA jiffies. See code
snippet below. This is very unlikely, however, if this scenario was to
occur what would be the impact on the clocksource?
/*
* delta_jiffies >= NEXT_TIMER_MAX_DELTA signals that
* there is no timer pending or at least extremly far
* into the future (12 days for HZ=1000). In this case
* we simply stop the tick timer:
*/
if (unlikely(delta_jiffies >= NEXT_TIMER_MAX_DELTA)) {
ts->idle_expires.tv64 = KTIME_MAX;
if (ts->nohz_mode == NOHZ_MODE_HIGHRES)
hrtimer_cancel(&ts->sched_timer);
goto out;
}
I understand that clocksources need to be handled correctly, but as I
looked into this more I wanted to clarify how this is handled today for
64-bit machines. I appreciate your comments and feedback.
Cheers
Jon
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