FYI, we noticed the following commit:

commit: cc639db4acfeb459f3dcec080c6cfe11e36266e0 ("kernel/fork: use vfree_atomic() to free thread stack")
https://git.kernel.org/pub/scm/linux/kernel/git/next/linux-next.git master

in testcase: iperf
with following parameters:

	runtime: 300s
	cluster: cs-localhost
	protocol: tcp

test-description: iperf is a tool for active measurements of the maximum achievable bandwidth on IP networks.
test-url: https://iperf.fr/


on test machine: qemu-system-x86_64 -enable-kvm -cpu Nehalem -smp 2 -m 1G

caused below changes:


+-------------------------------------------------+------------+------------+
|                                                 | 1d2bc599c4 | cc639db4ac |
+-------------------------------------------------+------------+------------+
| boot_successes                                  | 48         | 41         |
| boot_failures                                   | 24         | 29         |
| invoked_oom-killer:gfp_mask=0x                  | 21         | 19         |
| Mem-Info                                        | 21         | 19         |
| BUG:kernel_reboot-without-warning_in_test_stage | 3          | 2          |
| BUG:using_smp_processor_id()in_preemptible      | 0          | 13         |
| calltrace:SyS_clone                             | 0          | 8          |
+-------------------------------------------------+------------+------------+



[   36.609372] 
[   37.466792] skip http request: cgi-bin/lkp-jobfile-append-var?job_file=/lkp/scheduled/vm-vp-1G-7/iperf-300s-cs-localhost-tcp-debian-x86_64-2016-08-31.cgz-cc639db4acfeb459f3dcec080c6cfe11e36266e0-20161209-43576-1ch14l2-25.yaml&job_state=running -o /dev/null
[   37.470889] 
[   38.354206] BUG: using smp_processor_id() in preemptible [00000000] code: iperf-300s-cs-l/277
[   38.364617] caller is debug_smp_processor_id+0x17/0x19
[   38.369836] CPU: 1 PID: 277 Comm: iperf-300s-cs-l Not tainted 4.9.0-rc8-00140-gcc639db #2
[   38.371241]  ffffc900003f3cf0 ffffffff8123ae6f 0000000000000001 ffffffff818181da
[   38.372656]  ffffc900003f3d20 ffffffff81252f41 0000000000012de0 00000000fffffdff
[   38.375556]  ffff880009328f40 ffff88000592c400 ffffc900003f3d30 ffffffff81252f6a
[   38.381015] Call Trace:
[   38.381439]  [<ffffffff8123ae6f>] dump_stack+0x9a/0xd0
[   38.382302]  [<ffffffff81252f41>] check_preemption_disabled+0xdd/0xef
[   38.383365]  [<ffffffff81252f6a>] debug_smp_processor_id+0x17/0x19
[   38.384389]  [<ffffffff811796df>] __vfree_deferred+0x16/0x4c
[   38.389428]  [<ffffffff8117b584>] vfree_atomic+0x22/0x24
[   38.390334]  [<ffffffff81094f5d>] free_thread_stack+0xc2/0x106
[   38.391330]  [<ffffffff810951be>] put_task_stack+0x4c/0x62
[   38.392239]  [<ffffffff81095f81>] copy_process+0x7e0/0x16e8
[   38.397439]  [<ffffffff8109702d>] _do_fork+0xbb/0x2d3
[   38.398285]  [<ffffffff810465e8>] ? __do_page_fault+0x2e1/0x384
[   38.399275]  [<ffffffff8112633f>] ? trace_hardirqs_off_caller+0x12/0x24
[   38.400382]  [<ffffffff810972cb>] SyS_clone+0x19/0x1b
[   38.405409]  [<ffffffff81003800>] do_syscall_64+0x143/0x173
[   38.406340]  [<ffffffff81507289>] entry_SYSCALL64_slow_path+0x25/0x25
[   39.757101] {
[   39.757377] 


To reproduce:

        git clone git://git.kernel.org/pub/scm/linux/kernel/git/wfg/lkp-tests.git
        cd lkp-tests
        bin/lkp qemu -k <bzImage> job-script  # job-script is attached in this email



Thanks,
Ying Huang