Date:	Tue, 26 May 2015 10:38:55 -0700
From:	bsegall@...gle.com
To:	Kirill Tkhai <tkhai@...dex.ru>
Cc:	Mohammed Naser <mnaser@...xhost.com>,
	Peter Zijlstra <peterz@...radead.org>,
	"linux-kernel\@vger.kernel.org" <linux-kernel@...r.kernel.org>,
	"mingo\@redhat.com" <mingo@...hat.com>,
	Konstantin Khlebnikov <khlebnikov@...dex-team.ru>
Subject: Re: [PATCH] sched/fair: Fix null pointer dereference of empty queues

Kirill Tkhai <tkhai@...dex.ru> writes:

> Hi,
>
> 25.05.2015, 23:53, "Mohammed Naser" <mnaser@...xhost.com>:
>> Hi Peter,
>>
>> (resending as plain text, my bad)
>>
>> Thank you for reply.
>>
>> Would you have any ideas on why this would have occurred or other
>> steps to look at?  It's my first time attempting to help fix a problem
>> like this.
>>
>> I have a crashdump of the kernel since this issue repeated itself a
>> few times on a loaded KVM host (it's 12GB however).  I can also
>> provide the values of cfs_rq before the kernel crash.
>>
>> ===================================
>> [146055.357476] BUG: unable to handle kernel NULL pointer dereference
>> at 0000000000000038
>> [146055.359620] IP: [<ffffffff810a7d31>] set_next_entity+0x11/0xb0
>> [146055.361890] PGD 0
>> [146055.364131] Oops: 0000 [#1] SMP
>> [146055.366475] Modules linked in: vhost_net vhost macvtap macvlan
>> act_police cls_u32 sch_ingress ipmi_si xt_multiport nf_conntrack_ipv6
>> nf_defrag_ipv6 xt_mac xt_physdev xt_set iptable_raw ip_set_hash_ip
>> ip_set nfnetlink mpt3sas mpt2sas raid_class scsi_transport_sas mptctl
>> mptbase veth xt_CHECKSUM iptable_mangle ipt_MASQUERADE iptable_nat
>> nf_nat_ipv4 nf_nat nf_conntrack_ipv4 nf_defrag_ipv4 xt_conntrack
>> nf_conntrack ipt_REJECT xt_tcpudp dell_rbu bridge stp llc
>> ebtable_filter ebtables ip6table_filter ip6_tables iptable_filter
>> ip_tables x_tables nbd openvswitch gre vxlan libcrc32c ib_iser rdma_cm
>> iw_cm ib_cm ib_sa ib_mad ib_core ib_addr iscsi_tcp libiscsi_tcp
>> libiscsi scsi_transport_iscsi ipmi_devintf intel_rapl
>> x86_pkg_temp_thermal intel_powerclamp coretemp kvm_intel dcdbas kvm
>> crct10dif_pclmul crc32_pclmul
>> [146055.388889]  ghash_clmulni_intel aesni_intel aes_x86_64 lrw
>> gf128mul dm_multipath glue_helper ablk_helper scsi_dh cryptd mei_me
>> mei lpc_ich ipmi_msghandler shpchp wmi acpi_power_meter mac_hid lp
>> parport nls_iso8859_1 igb ixgbe i2c_algo_bit dca ptp ahci pps_core
>> megaraid_sas libahci mdio [last unloaded: ipmi_si]
>> [146055.404208] CPU: 31 PID: 67922 Comm: qemu-system-x86 Not tainted
>> 3.16.0-37-generic #51~14.04.1-Ubuntu
>> [146055.409906] Hardware name: Dell Inc. PowerEdge R630/0CNCJW, BIOS
>> 1.0.4 08/28/2014
>> [146055.415754] task: ffff883fcab69e90 ti: ffff883a1c168000 task.ti:
>> ffff883a1c168000
>> [146055.421817] RIP: 0010:[<ffffffff810a7d31>]  [<ffffffff810a7d31>]
>> set_next_entity+0x11/0xb0
>> [146055.428079] RSP: 0018:ffff883a1c16bce8  EFLAGS: 00010092
>> [146055.434377] RAX: 0000000000000000 RBX: 0000000000000000 RCX:
>> 00000000044aa200
>> [146055.440913] RDX: 0000000000000000 RSI: 0000000000000000 RDI:
>> ffff883ffedf3140
>> [146055.447474] RBP: ffff883a1c16bd00 R08: 0000000000000000 R09:
>> 0000000000000001
>> [146055.454181] R10: 0000000000000004 R11: 0000000000000206 R12:
>> ffff883ffedf3140
>> [146055.460968] R13: 000000000000001f R14: 0000000000000001 R15:
>> ffff883ffedf30c0
>> [146055.467722] FS:  00007f404919d700(0000) GS:ffff883ffede0000(0000)
>> knlGS:ffff880002380000
>> [146055.474756] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
>> [146055.481830] CR2: 0000000000000038 CR3: 0000003a1c45b000 CR4:
>> 00000000001427e0
>> [146055.489134] Stack:
>> [146055.496412]  0000000000000000 ffff883ffedf3140 000000000000001f
>> ffff883a1c16bd68
>> [146055.504053]  ffffffff810af2f8 ffff883ffedf3140 00000000000130c0
>> ffff883fcab69e90
>> [146055.511786]  ffffffff8101c3b9 ffff883a1c16bd50 ffffffff810a4895
>> ffff883fcab6a3c8
>> [146055.519551] Call Trace:
>> [146055.527330]  [<ffffffff810af2f8>] pick_next_task_fair+0x78/0x880
>> [146055.535292]  [<ffffffff8101c3b9>] ? sched_clock+0x9/0x10
>> [146055.543379]  [<ffffffff810a4895>] ? sched_clock_cpu+0x85/0xc0
>> [146055.551519]  [<ffffffff81768afb>] __schedule+0x11b/0x7a0
>> [146055.559722]  [<ffffffff81769579>] _cond_resched+0x29/0x40
>> [146055.568020]  [<ffffffffc0338289>] kvm_arch_vcpu_ioctl_run+0x3e9/0x460 [kvm]
>> [146055.576509]  [<ffffffffc0321ce2>] kvm_vcpu_ioctl+0x2a2/0x5e0 [kvm]
>> [146055.585045]  [<ffffffff81156952>] ? perf_event_context_sched_in+0xa2/0xc0
>> [146055.593771]  [<ffffffff811e7250>] do_vfs_ioctl+0x2e0/0x4c0
>> [146055.602531]  [<ffffffff8109dec8>] ? finish_task_switch+0x108/0x180
>> [146055.611413]  [<ffffffffc032bcd4>] ? kvm_on_user_return+0x74/0x80 [kvm]
>> [146055.620339]  [<ffffffff811e74b1>] SyS_ioctl+0x81/0xa0
>> [146055.629396]  [<ffffffff8176d20d>] system_call_fastpath+0x1a/0x1f
>> [146055.638500] Code: 83 c4 10 4c 89 f2 4c 89 ee ff d0 49 8b 04 24 48
>> 85 c0 75 e6 eb 99 0f 1f 40 00 0f 1f 44 00 00 55 48 89 e5 41 55 41 54
>> 49 89 fc 53 <8b> 46 38 48 89 f3 85 c0 75 5d 49 8b 84 24 b0 00 00 00 48
>> 8b 80
>> [146055.657833] RIP  [<ffffffff810a7d31>] set_next_entity+0x11/0xb0
>> [146055.667524]  RSP <ffff883a1c16bce8>
>> [146055.677082] CR2: 0000000000000038
>> ===================================
>
> This looks like https://lkml.org/lkml/2015/4/3/231


If you're using cfs_bandwidth, it's possible (though I can't tell just
from that). If so and you can reproduce this, could you test
https://lkml.org/lkml/2015/4/6/396 (assuming it still applies at all
anyway)?

If you want to provide a bunch of debugging info, rq->clock,
rq->clock_task, *cfs_rq and *se for all rqs and cgroups would be ideal,
but getting all that is kinda annoying. The most basic info would be
*cfs_rq of the crashing cpu and *cfs_rq->tg.
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