linux/kernel/time
Thomas Gleixner 07f4beb0b5 tick: Clear broadcast active bit when switching to oneshot
The first cpu which switches from periodic to oneshot mode switches
also the broadcast device into oneshot mode. The broadcast device
serves as a backup for per cpu timers which stop in deeper
C-states. To avoid starvation of the cpus which might be in idle and
depend on broadcast mode it marks the other cpus as broadcast active
and sets the brodcast expiry value of those cpus to the next tick.

The oneshot mode broadcast bit for the other cpus is sticky and gets
only cleared when those cpus exit idle. If a cpu was not idle while
the bit got set in consequence the bit prevents that the broadcast
device is armed on behalf of that cpu when it enters idle for the
first time after it switched to oneshot mode.

In most cases that goes unnoticed as one of the other cpus has usually
a timer pending which keeps the broadcast device armed with a short
timeout. Now if the only cpu which has a short timer active has the
bit set then the broadcast device will not be armed on behalf of that
cpu and will fire way after the expected timer expiry. In the case of
Christians bug report it took ~145 seconds which is about half of the
wrap around time of HPET (the limit for that device) due to the fact
that all other cpus had no timers armed which expired before the 145
seconds timeframe.

The solution is simply to clear the broadcast active bit
unconditionally when a cpu switches to oneshot mode after the first
cpu switched the broadcast device over. It's not idle at that point
otherwise it would not be executing that code.

[ I fundamentally hate that broadcast crap. Why the heck thought some
  folks that when going into deep idle it's a brilliant concept to
  switch off the last device which brings the cpu back from that
  state? ]

Thanks to Christian for providing all the valuable debug information!

Reported-and-tested-by: Christian Hoffmann <email@christianhoffmann.info>
Cc: John Stultz <johnstul@us.ibm.com>
Link: http://lkml.kernel.org/r/%3Calpine.LFD.2.02.1105161105170.3078%40ionos%3E
Cc: stable@kernel.org
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
2011-05-16 23:35:41 +02:00
..
clockevents.c time: Make do_timer() and xtime_lock local to kernel/time/ 2011-01-31 19:26:50 +01:00
clocksource.c clocksource: Install completely before selecting 2011-05-05 15:23:26 +02:00
jiffies.c Fix common misspellings 2011-03-31 11:26:23 -03:00
Kconfig time: Kill off CONFIG_GENERIC_TIME 2010-07-27 12:40:54 +02:00
Makefile posix clocks: Introduce dynamic clocks 2011-02-02 15:28:20 +01:00
ntp.c ntp: fix non privileged system time shifting 2011-04-04 08:31:23 -07:00
posix-clock.c posix clocks: Replace mutex with reader/writer semaphore 2011-04-18 10:39:38 +02:00
tick-broadcast.c tick: Clear broadcast active bit when switching to oneshot 2011-05-16 23:35:41 +02:00
tick-common.c Merge branch 'timers-core-for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/linux-2.6-tip 2011-03-15 18:53:35 -07:00
tick-internal.h Merge branch 'timers-core-for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/linux-2.6-tip 2011-03-15 18:53:35 -07:00
tick-oneshot.c time: Make do_timer() and xtime_lock local to kernel/time/ 2011-01-31 19:26:50 +01:00
tick-sched.c time: Make do_timer() and xtime_lock local to kernel/time/ 2011-01-31 19:26:50 +01:00
timecompare.c time: Use ARRAY_SIZE macro in timecompare.c 2010-10-21 17:30:06 +02:00
timeconv.c time: add function to convert between calendar time and broken-down time for universal use 2009-09-24 07:20:56 -07:00
timekeeping.c timekeeping: Use syscore_ops instead of sysdev class and sysdev 2011-03-23 22:16:04 +01:00
timer_list.c timer debug: Hide kernel addresses via %pK in /proc/timer_list 2011-02-12 14:11:56 +01:00
timer_stats.c Fix common misspellings 2011-03-31 11:26:23 -03:00