[PATCH] powerpc/powernv/cpuidle: Pass correct drv->cpumask for registration

[Michael Ellerman]

Vaidyanathan Srinivasan <svaidy at linux.vnet.ibm.com> writes:

> * Michael Neuling <mikey at neuling.org> [2017-03-18 16:28:02]:
>
>> Vaidy,
>> 
>> Thanks for fixing this.
>> 
>> > drv->cpumask defaults to cpu_possible_mask in __cpuidle_driver_init().
>> > This breaks cpuidle on powernv where sysfs files are not created for
>> > cpus in cpu_possible_mask that cannot be hot-added.
>> 
>> I think I prefer the longer description below than this.
>
> [PATCH] powerpc/powernv/cpuidle: Pass correct drv->cpumask for
>
> drv->cpumask defaults to cpu_possible_mask in __cpuidle_driver_init().
> This breaks cpuidle on powernv where sysfs files are not created for
> cpus in cpu_possible_mask that cannot be hot-added.

I'm confused.

> On powernv platform cpu_present could be less than cpu_possible
> in cases where firmware detects the cpu, but it is not available
> for OS.

It's entirely normal for present < possible, on my laptop for example,
so I don't see how that causes the bug.

> Such cpus are not hotplugable at runtime on powernv and
> hence we skip creating sysfs files for these cpus.

Why are they not hotpluggable? Looking at topology_init() they should be
hotpluggable as long as ppc_md.cpu_die is populated, which it is on
PowerNV AFAICS.

Is it really "creating sysfs files" that's important, or is that we
don't call register_cpu() for those CPUs?

> Trying cpuidle_register_device() on cpu without sysfs node will
> cause crash like:
>
> cpu 0xf: Vector: 380 (Data SLB Access) at [c000000ff1503490]
>     pc: c00000000022c8bc: string+0x34/0x60
>     lr: c00000000022ed78: vsnprintf+0x284/0x42c
>     sp: c000000ff1503710
>    msr: 9000000000009033
>    dar: 6000000060000000
>   current = 0xc000000ff1480000
>   paca    = 0xc00000000fe82d00   softe: 0        irq_happened: 0x01
>     pid   = 1, comm = swapper/8
> Linux version 4.11.0-rc2 (sv at sagarika) (gcc version 4.9.4 (Buildroot 2017.02-00004-gc28573e) ) #15 SMP Fri Mar 17 19:32:02 IST 2017
> enter ? for help
> [link register   ] c00000000022ed78 vsnprintf+0x284/0x42c
> [c000000ff1503710] c00000000022ebb8 vsnprintf+0xc4/0x42c (unreliable)
> [c000000ff1503800] c00000000022ef40 vscnprintf+0x20/0x44
> [c000000ff1503830] c0000000000ab61c vprintk_emit+0x94/0x2cc
> [c000000ff15038a0] c0000000000acc9c vprintk_func+0x60/0x74
> [c000000ff15038c0] c000000000619694 printk+0x38/0x4c
> [c000000ff15038e0] c000000000224950 kobject_get+0x40/0x60
> [c000000ff1503950] c00000000022507c kobject_add_internal+0x60/0x2c4
> [c000000ff15039e0] c000000000225350 kobject_init_and_add+0x70/0x78
> [c000000ff1503a60] c00000000053c288 cpuidle_add_sysfs+0x9c/0xe0
> [c000000ff1503ae0] c00000000053aeac cpuidle_register_device+0xd4/0x12c
> [c000000ff1503b30] c00000000053b108 cpuidle_register+0x98/0xcc
> [c000000ff1503bc0] c00000000085eaf0 powernv_processor_idle_init+0x140/0x1e0
> [c000000ff1503c60] c00000000000cd60 do_one_initcall+0xc0/0x15c
> [c000000ff1503d20] c000000000833e84 kernel_init_freeable+0x1a0/0x25c
> [c000000ff1503dc0] c00000000000d478 kernel_init+0x24/0x12c
> [c000000ff1503e30] c00000000000b564 ret_from_kernel_thread+0x5c/0x78

I really don't understand how a CPU not being present leads to a crash
in printf()? Something in that call chain should have checked that the
CPU was registered before crashing in printf() - surely?

cheers