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SubjectRe: [PATCH] Resurect proper handling of maxcpus= kernel option wrote:
> From: Max Krasnyansky <>
> For some reason we had redundant parsers registered for maxcpus=.
> One in init/main.c and another in arch/x86/smpboot.c
> So I nuked the one in arch/x86.
> Also 64-bit kernels used to handle maxcpus= as documented in
> Documentation/cpu-hotplug.txt. CPUs with 'id > maxcpus' are initialized
> but not booted. 32-bit version for some reason ignored them even though
> all the infrastructure for booting them later is there.
> In the current mainline both 64 and 32 bit versions are broken. I'm
> too lazy to look through git history but I'm guessing it happened as
> part of the i386 and x86_64 unification.
> This patch restores the correct behaviour. I've tested x86_64 version on
> 4- and 8- way Core2 and 2-way Opteron based machines. Various config
> combinations SMP, !SMP, CPU_HOTPLUG, !CPU_HOTPLUG.
> Booted with maxcpus=1 and maxcpus=4, etc. Everything is working as expected.
> I cannot test 32-bit version (no 32-bit machines here).

I booted my 2-core x86_32 box with maxcpus=1, and saw cpu1 was offline,
and then I got softlockup BUG immediately when I onlined cpu1:

SMP alternatives: switching to SMP code
CPU 1 irqstacks, hard=c078c000 soft=c076c000
Booting processor 1/1 ip 6000
Initializing CPU#1
Calibrating delay using timer specific routine.. 5600.37 BogoMIPS (lpj=2800188)
CPU: Trace cache: 12K uops, L1 D cache: 16K
CPU: L2 cache: 1024K
CPU: Physical Processor ID: 0
CPU: Processor Core ID: 1
Intel machine check architecture supported.
Intel machine check reporting enabled on CPU#1.
CPU1: Intel P4/Xeon Extended MCE MSRs (24) available
CPU1: Thermal monitoring enabled
CPU1: Intel(R) Pentium(R) D CPU 2.80GHz stepping 04
checking TSC synchronization [CPU#0 -> CPU#1]: passed.
Switched to high resolution mode on CPU 1
BUG: soft lockup - CPU#1 stuck for 216s! [events/0:0]
Modules linked in: bridge stp llc autofs4 dm_mirror dm_log dm_mod snd_intel8x0 snd_ac97_codec ac97_bus snd_seq_dummy snd_seq_oss snd_seq_midi_event snd_seq snd_seq_device snd_pcm_oss snd_pcm snd_timer snd soundcore r8169 snd_page_alloc sg button sata_sis pata_sis ata_generic libata sd_mod scsi_mod ext3 jbd mbcache uhci_hcd ohci_hcd ehci_hcd [last unloaded: scsi_wait_scan]
irq event stamp: 156
hardirqs last enabled at (155): [<c044407f>] trace_hardirqs_on+0xb/0xd
hardirqs last disabled at (156): [<c04eee88>] trace_hardirqs_off_thunk+0xc/0x10
softirqs last enabled at (152): [<c042c2f3>] __do_softirq+0xe3/0xe9
softirqs last disabled at (95): [<c04058eb>] do_softirq+0x65/0xb4

Pid: 0, comm: events/0 Not tainted (2.6.27-rc1 #224)
EIP: 0060:[<c04088ba>] EFLAGS: 00000246 CPU: 1
EIP is at mwait_idle+0x3c/0x4a
EAX: 00000000 EBX: e3e48008 ECX: 00000000 EDX: 00000000
ESI: 00000000 EDI: 00000000 EBP: e3e48f9c ESP: e3e48f98
DS: 007b ES: 007b FS: 00d8 GS: 0000 SS: 0068
CR0: 8005003b CR2: 00000000 CR3: 00768000 CR4: 000006d0
DR0: 00000000 DR1: 00000000 DR2: 00000000 DR3: 00000000
DR6: ffff0ff0 DR7: 00000400
[<c0402591>] cpu_idle+0xbf/0xdf
[<c05fb737>] start_secondary+0x16b/0x170

216s should be the time since the machine booted up.

(maybe off-topic)
I never succeed to offline cpu1, it caused the kernel to hang
whenver I offlined cpu1

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