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Re: [Xen-devel] WARNING: at /home/konrad/linux-linus/kernel/time/tick-sched.c:935 tick_nohz_idle_exit+0x195/0x1b0() on v3.10-rc3

On Thu, 30 May 2013, Konrad Rzeszutek Wilk wrote:
> [   40.085841] WARNING: at 
> /home/konrad/linux-linus/kernel/time/tick-sched.c:935 
> tick_nohz_idle_exit+0x195/0x1b0()
> which I presume is b/c the code does not expect to be run _after_ it has
> offlined. However, under the PV code, the mechanism is that that a CPU
> that has been offlined, can resume (if it is onlined). If you look at:
> 445 static void __cpuinit xen_play_dead(void) /* used only with HOTPLUG_CPU 
> */      
> 446 {                                                                         
> 447         play_dead_common();                                               
> 448         HYPERVISOR_vcpu_op(VCPUOP_down, smp_processor_id(), NULL);        
> 449         cpu_bringup();                                                    
> 450 }                               
> That is called right after the CPU is put to sleep and the hypercall 
> VCPUOP_down blocks - until the CPU is brough back up. And which point
> we end up calling cpu_bringup - which sets up the clockevets, timers, etc.
> I am wondering if part of this is that the ts->inidle gets reset
> b/c we end up resetting all the timers but then when xen_play_dead
> exits, it ends up right back in the cpu_idle_loop() loop - and we
> call tick_nohz_idle_exit().
> Thoughts?

cpu_dead() is definitely not expected to return after the cpu has been
declared dead. I should have put a big fat warning into the generic
idle loop for this :)

The reason why you get that warning only now is commit 4b0c0f294
(tick: Cleanup NOHZ per cpu data on cpu down), which is btw. targeted
for stable as well.

We can't revert the above commit as it fixes a long standing
nastiness, so for now until I come around to make the idle loop return
on cpu down you probably need to call tick_nohz_idle_enter() before
returning from play_dead().




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