Re: [RFC PATCH]nohz: Use raw_smp_processor_id() in tick_nohz_task_switch()

[Li Zhong <zhong@linux.vnet.ibm.com>]

On Sat, 2013-04-27 at 15:40 +0200, Frederic Weisbecker wrote:
> 2013/4/27 Li Zhong <zhong@linux.vnet.ibm.com>:
> > I saw following error when testing the latest nohz code on Power:
> >
> > [   85.295384] BUG: using smp_processor_id() in preemptible [00000000] code: rsyslogd/3493
> > [   85.295396] caller is .tick_nohz_task_switch+0x1c/0xb8
> > [   85.295402] Call Trace:
> > [   85.295408] [c0000001fababab0] [c000000000012dc4] .show_stack+0x110/0x25c (unreliable)
> > [   85.295420] [c0000001fababba0] [c0000000007c4b54] .dump_stack+0x20/0x30
> > [   85.295430] [c0000001fababc10] [c00000000044eb74] .debug_smp_processor_id+0xf4/0x124
> > [   85.295438] [c0000001fababca0] [c0000000000d7594] .tick_nohz_task_switch+0x1c/0xb8
> > [   85.295447] [c0000001fababd20] [c0000000000b9748] .finish_task_switch+0x13c/0x160
> > [   85.295455] [c0000001fababdb0] [c0000000000bbe50] .schedule_tail+0x50/0x124
> > [   85.295463] [c0000001fababe30] [c000000000009dc8] .ret_from_fork+0x4/0x54
> >
> > It seems to me that we could just use raw_smp_processor_id() here. Even
> > if the tick_nohz_full_cpu() check is done on a !nohz_full cpu, then the
> > task is moved to another nohz_full cpu, it seems the context switching
> > because of the task moving would call tick_nohz_task_switch() again to
> > evaluate the need for tick.
> >
> > I don't know whether I missed something here.
> 
> You're right it looks safe to do so. But I suggest we rather move the
> test inside local_irq_save()/restore section to avoid confusion on
> reviewers minds.

OK, I'll send an updated version, using local_irq_save() to protect it. 
I tried using raw_* because seems it could avoid some unnecessary irq
disabling...

Thanks, Zhong

> Thanks!
>