linux/arch/s390
Heiko Carstens 398812159e [S390] nohz/s390: fix arch_needs_cpu() return value on offline cpus
This fixes the same problem as described in the patch "nohz: fix
printk_needs_cpu() return value on offline cpus" for the arch_needs_cpu()
primitive:

arch_needs_cpu() may return 1 if called on offline cpus. When a cpu gets
offlined it schedules the idle process which, before killing its own cpu,
will call tick_nohz_stop_sched_tick().
That function in turn will call arch_needs_cpu() in order to check if the
local tick can be disabled. On offline cpus this function should naturally
return 0 since regardless if the tick gets disabled or not the cpu will be
dead short after. That is besides the fact that __cpu_disable() should already
have made sure that no interrupts on the offlined cpu will be delivered anyway.

In this case it prevents tick_nohz_stop_sched_tick() to call
select_nohz_load_balancer(). No idea if that really is a problem. However what
made me debug this is that on 2.6.32 the function get_nohz_load_balancer() is
used within __mod_timer() to select a cpu on which a timer gets enqueued.
If arch_needs_cpu() returns 1 then the nohz_load_balancer cpu doesn't get
updated when a cpu gets offlined. It may contain the cpu number of an offline
cpu. In turn timers get enqueued on an offline cpu and not very surprisingly
they never expire and cause system hangs.

This has been observed 2.6.32 kernels. On current kernels __mod_timer() uses
get_nohz_timer_target() which doesn't have that problem. However there might
be other problems because of the too early exit tick_nohz_stop_sched_tick()
in case a cpu goes offline.

This specific bug was indrocuded with 3c5d92a0 "nohz: Introduce
arch_needs_cpu".

In this case a cpu hotplug notifier is used to fix the issue in order to keep
the normal/fast path small. All we need to do is to clear the condition that
makes arch_needs_cpu() return 1 since it is just a performance improvement
which is supposed to keep the local tick running for a short period if a cpu
goes idle. Nothing special needs to be done except for clearing the condition.

Cc: stable@kernel.org
Acked-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>
2010-12-01 10:08:17 +01:00
..
appldata [S390] appldata: fix dev_get_stats 64 bit conversion 2010-08-13 10:06:55 +02:00
boot [S390] Add support for LZO-compressed kernels. 2010-05-26 23:27:10 +02:00
crypto [S390] cleanup facility list handling 2010-10-25 16:10:21 +02:00
hypfs Merge branch 'for-linus' of git://git390.marist.edu/pub/scm/linux-2.6 2010-10-29 08:49:18 -07:00
include/asm [S390] mm: add devmem_is_allowed() for STRICT_DEVMEM checking 2010-11-10 10:05:54 +01:00
kernel [S390] nohz/s390: fix arch_needs_cpu() return value on offline cpus 2010-12-01 10:08:17 +01:00
kvm [S390] cleanup facility list handling 2010-10-25 16:10:21 +02:00
lib [S390] nmi: fix clock comparator revalidation 2010-11-25 09:52:59 +01:00
math-emu tree-wide: fix assorted typos all over the place 2009-12-04 15:39:55 +01:00
mm [S390] fix get_user_pages_fast 2010-11-10 10:05:53 +01:00
oprofile Combine instrumentation menus in kernel/Kconfig.instrumentation 2007-10-19 11:53:54 -07:00
Kbuild [S390] standardize Kbuild rules 2010-10-25 16:10:14 +02:00
Kconfig [S390] ftrace: select HAVE_C_RECORDMCOUNT 2010-10-29 16:50:50 +02:00
Kconfig.debug [S390] mm: add devmem_is_allowed() for STRICT_DEVMEM checking 2010-11-10 10:05:54 +01:00
Makefile [S390] Add config option for z196 code generation. 2010-10-25 16:10:15 +02:00
defconfig defconfig reduction 2010-08-14 22:26:53 +02:00