(2014/09/17 16:17), Wanpeng Li wrote: > BUG: unable to handle kernel NULL pointer dereference at 0000000000000004 > IP: [..] find_busiest_group > PGD 5a9d5067 PUD 13067 PMD 0 > Oops: 0000 [#3] SMP > [...] > Call Trace: > load_balance > ? _raw_spin_unlock_irqrestore > idle_balance > __schedule > schedule > schedule_timeout > ? lock_timer_base > schedule_timeout_uninterruptible > msleep > lock_device_hotplug_sysfs > online_store > dev_attr_store > sysfs_write_file > vfs_write > SyS_write > system_call_fastpath > > This bug can be triggered by hot add and remove large number of xen > domain0's vcpus repeatedly. > > Last level cache shared map is built during cpu up and build sched domain > routine takes advantage of it to setup sched domain cpu topology, however, > llc shared map is unreleased during cpu disable which lead to invalid sched > domain cpu topology. This patch fix it by release llc shared map correctly > during cpu disable. > > Reviewed-by: Toshi Kani <toshi.k...@hp.com> > Reviewed-by: Yasuaki Ishimatsu <isimatu.yasu...@jp.fujitsu.com> > Tested-by: Linn Crosetto <l...@hp.com> > Signed-off-by: Wanpeng Li <wanpeng...@linux.intel.com>
Yasuaki reported this can happen on our real hardware. https://lkml.org/lkml/2014/7/22/1018 Our case is here. == Here is a example on my system. My system has 4 sockets and each socket has 15 cores and HT is enabled. In this case, each core of sockes is numbered as follows: | CPU# Socket#0 | 0-14 , 60-74 Socket#1 | 15-29, 75-89 Socket#2 | 30-44, 90-104 Socket#3 | 45-59, 105-119 Then llc_shared_mask of CPU#30 has 0x3fff80000001fffc0000000. It means that last level cache of Socket#2 is shared with CPU#30-44 and 90-104. When hot-removing socket#2 and #3, each core of sockets is numbered as follows: | CPU# Socket#0 | 0-14 , 60-74 Socket#1 | 15-29, 75-89 But llc_shared_mask is not cleared. So llc_shared_mask of CPU#30 remains having 0x3fff80000001fffc0000000. After that, when hot-adding socket#2 and #3, each core of sockets is numbered as follows: | CPU# Socket#0 | 0-14 , 60-74 Socket#1 | 15-29, 75-89 Socket#2 | 30-59 Socket#3 | 90-119 Then llc_shared_mask of CPU#30 becomes 0x3fff8000fffffffc0000000. It means that last level cache of Socket#2 is shared with CPU#30-59 and 90-104. So the mask has wrong value. At first, I cleared hot-removed CPU number's bit from llc_shared_map when hot removing CPU. But Borislav suggested that the problem will disappear if readded CPU is assigned same CPU number. And llc_shared_map must not be changed. == So, please. Thanks, -Kame -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
