Messages in this thread | | | Date | Wed, 24 Sep 2014 17:24:40 +0900 | From | Yasuaki Ishimatsu <> | Subject | Re: [PATCH v6] sched: fix llc shared map unreleased during cpu hotplug |
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(2014/09/24 17:13), 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 repeatly. > > 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. > > Yasuaki also reported this can happen on their real hardware. > https://lkml.org/lkml/2014/7/22/1018 > > His 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.
Please remove it. The description is not explanation for your patch.
Thanks, Yasuaki Ishimatsu
> > Reviewed-by: Borislav Petkov <bp@suse.de> > Reviewed-by: Toshi Kani <toshi.kani@hp.com> > Reviewed-by: Yasuaki Ishimatsu <isimatu.yasuaki@jp.fujitsu.com> > Tested-by: Linn Crosetto <linn@hp.com> > Signed-off-by: Wanpeng Li <wanpeng.li@linux.intel.com> > --- > v5 -> v6: > * add the real-hardware reports to the changelog > v4 -> v5: > * add the description when the bug can occur > v3 -> v4: > * simplify backtrace > v2 -> v3: > * simplify backtrace > v1 -> v2: > * fix subject line > > arch/x86/kernel/smpboot.c | 3 +++ > 1 file changed, 3 insertions(+) > > diff --git a/arch/x86/kernel/smpboot.c b/arch/x86/kernel/smpboot.c > index 5492798..0134ec7 100644 > --- a/arch/x86/kernel/smpboot.c > +++ b/arch/x86/kernel/smpboot.c > @@ -1292,6 +1292,9 @@ static void remove_siblinginfo(int cpu) > > for_each_cpu(sibling, cpu_sibling_mask(cpu)) > cpumask_clear_cpu(cpu, cpu_sibling_mask(sibling)); > + for_each_cpu(sibling, cpu_llc_shared_mask(cpu)) > + cpumask_clear_cpu(cpu, cpu_llc_shared_mask(sibling)); > + cpumask_clear(cpu_llc_shared_mask(cpu)); > cpumask_clear(cpu_sibling_mask(cpu)); > cpumask_clear(cpu_core_mask(cpu)); > c->phys_proc_id = 0; >
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