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

Vaidyanathan Srinivasan [off-list ref] writes:

* Michael Neuling [off-list ref] [2017-03-18 16:28:02]:

quoted

Vaidy,

Thanks for fixing this.

quoted

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@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