[Xen-devel] patch "x86/cpufreq: relocate the driver register function" breaks cpu hot(un)plug

[Dario Faggioli <dario.faggioli@xxxxxxxxxx>]

Hey,

As far as my bisection goes, commit
49388f11d512bb92706ce046643bfbb3c1d963c9 "x86/cpufreq: relocate the
driver register function" prevents me from hot unplugging pCPUs.

Xen does not crash or anything, but dom0 is stalled. In fact, with
current staging, here's what I see:

root@Zhaman:~# echo 0 > /sys/devices/system/xen_cpu/xen_cpu6/online 
[   81.583001] INFO: rcu_sched detected stalls on CPUs/tasks: { 12} (detected 
by 3, t=5252 jiffies, g=1691, c=1690, q=76)
[   81.583036] Task dump for CPU 12:
[   81.583044] bash            R  running task        0  1347   1094 0x00000008
[   81.583056]  ffffffff00000000 0000000000000000 0000000000000000 
ffff8800192c2e38
[   81.583070]  ffff8800008472e8 0000000000000002 ffff8800008472e8 
ffff880013817858
[   81.583082]  0000000000000000 00000000000081a4 ffffffff811e8137 
ffff8800192c2e38
[   81.583095] Call Trace:
[   81.583110]  [<ffffffff811e8137>] ? notify_change+0x2f7/0x390
[   81.583148]  [<ffffffff811c8c74>] ? do_truncate+0x74/0x90
[   81.583158]  [<ffffffff811e2866>] ? dput+0x26/0x230
[   81.583167]  [<ffffffff811d53c5>] ? terminate_walk+0x35/0x40
[   81.583176]  [<ffffffff811d92b1>] ? do_last+0x621/0x12c0
[   81.583188]  [<ffffffff8139f0e7>] ? xen_pcpu_down+0x47/0x70
[   81.583199]  [<ffffffff8156c64d>] ? store_online+0x9d/0xb0
[   81.583210]  [<ffffffff81240bfc>] ? kernfs_fop_write+0x12c/0x180
[   81.583220]  [<ffffffff811ca513>] ? __vfs_write+0x23/0xf0
[   81.583230]  [<ffffffff811cd142>] ? __sb_start_write+0x42/0xf0
[   81.583241]  [<ffffffff8125f711>] ? security_file_permission+0x21/0xa0
[   81.583250]  [<ffffffff811caea1>] ? vfs_write+0xa1/0x1c0
[   81.583259]  [<ffffffff811c828f>] ? filp_close+0x4f/0x70
[   81.583268]  [<ffffffff811cbb12>] ? SyS_write+0x42/0xb0
[   81.583277]  [<ffffffff811e9031>] ? __close_fd+0x71/0xb0
[   81.583287]  [<ffffffff815780f2>] ? system_call_fastpath+0x16/0x75
[  144.555020] INFO: rcu_sched detected stalls on CPUs/tasks: { 12} (detected 
by 4, t=21007 jiffies, g=1691, c=1690, q=244)
[  144.555046] Task dump for CPU 12:
[  144.555051] bash            R  running task        0  1347   1094 0x00000008
[  144.555059]  ffffffff00000000 0000000000000000 0000000000000000 
ffff8800192c2e38
[  144.555068]  ffff8800008472e8 0000000000000002 ffff8800008472e8 
ffff880013817858
[  144.555076]  0000000000000000 00000000000081a4 ffffffff811e8137 
ffff8800192c2e38
[  144.555084] Call Trace:
[  144.555096]  [<ffffffff811e8137>] ? notify_change+0x2f7/0x390
[  144.555105]  [<ffffffff811c8c74>] ? do_truncate+0x74/0x90
[  144.555112]  [<ffffffff811e2866>] ? dput+0x26/0x230
[  144.555118]  [<ffffffff811d53c5>] ? terminate_walk+0x35/0x40
[  144.555124]  [<ffffffff811d92b1>] ? do_last+0x621/0x12c0
[  144.555164]  [<ffffffff8139f0e7>] ? xen_pcpu_down+0x47/0x70
[  144.555172]  [<ffffffff8156c64d>] ? store_online+0x9d/0xb0
[  144.555179]  [<ffffffff81240bfc>] ? kernfs_fop_write+0x12c/0x180
[  144.555186]  [<ffffffff811ca513>] ? __vfs_write+0x23/0xf0
[  144.555192]  [<ffffffff811cd142>] ? __sb_start_write+0x42/0xf0
[  144.555200]  [<ffffffff8125f711>] ? security_file_permission+0x21/0xa0
[  144.555206]  [<ffffffff811caea1>] ? vfs_write+0xa1/0x1c0
[  144.555212]  [<ffffffff811c828f>] ? filp_close+0x4f/0x70
[  144.555217]  [<ffffffff811cbb12>] ? SyS_write+0x42/0xb0
[  144.555223]  [<ffffffff811e9031>] ? __close_fd+0x71/0xb0
[  144.555230]  [<ffffffff815780f2>] ? system_call_fastpath+0x16/0x75

If I revert that patch, the issue goes away.

Any ideas? 

Regards,
Dario

PS. yes, I'll implement a cpu hotplug/unplug testcase ASAP. :-)

-- 
<<This happens because I choose it to happen!>> (Raistlin Majere)
-----------------------------------------------------------------
Dario Faggioli, Ph.D, http://about.me/dario.faggioli
Senior Software Engineer, Citrix Systems R&D Ltd., Cambridge (UK)

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