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Subject[BUG] CFS vs cpu hotplug
Hi Ingo, Peter,

I'm still seeing kernel crashes on cpu hotplug with Linus' current git tree.
All I have to do is to make all cpus busy (make -j4 of the kernel source is
sufficient) and then start cpu hotplug stress.
It usually takes below a minute to crash the system like this:

Unable to handle kernel pointer dereference at virtual kernel address 005a800000031000
Oops: 0038 [#1] PREEMPT SMP
Modules linked in:
CPU: 1 Not tainted 2.6.26-rc6-00232-g9bedbcb #356
Process swapper (pid: 0, task: 000000002fe7ccf8, ksp: 000000002fe93d78)
Krnl PSW : 0400e00180000000 0000000000032c6c (pick_next_task_fair+0x34/0xb0)
R:0 T:1 IO:0 EX:0 Key:0 M:0 W:0 P:0 AS:3 CC:2 PM:0 EA:3
Krnl GPRS: 00000000001ff000 0000000000030bd8 000000000075a380 000000002fe7ccf8
0000000000386690 0000000000000008 0000000000000000 000000002fe7cf58
0000000000000001 000000000075a300 0000000000000000 000000002fe93d40
005a800000031201 0000000000386010 000000002fe93d78 000000002fe93d40
Krnl Code: 0000000000032c5c: e3e0f0980024 stg %r14,152(%r15)
0000000000032c62: d507d000c010 clc 0(8,%r13),16(%r12)
0000000000032c68: a784003c brc 8,32ce0
>0000000000032c6c: d507d000c030 clc 0(8,%r13),48(%r12)
0000000000032c72: b904002c lgr %r2,%r12
0000000000032c76: a7a90000 lghi %r10,0
0000000000032c7a: a7840021 brc 8,32cbc
0000000000032c7e: c0e5ffffefe3 brasl %r14,30c44
Call Trace:
([<000000000075a300>] 0x75a300)
[<000000000037195a>] schedule+0x162/0x7f4
[<000000000001a2be>] cpu_idle+0x1ca/0x25c
[<000000000036f368>] start_secondary+0xac/0xb8
[<0000000000000000>] 0x0
[<0000000000000000>] 0x0
Last Breaking-Event-Address:
[<0000000000032cc6>] pick_next_task_fair+0x8e/0xb0
<4>---[ end trace 9bb55df196feedcc ]---
Kernel panic - not syncing: Attempted to kill the idle task!

Please note that the above call trace is from s390, however Avi reported the
same bug on x86_64.

I tried to bisect this and ended up somewhere at the beginning of 2.6.23 when
the CFS patches got merged. Unfortunately it got harder and harder to reproduce
so that I couldn't bisect this down to a single patch.

One observation however is that this always happens after cpu_up(), not
cpu_down().

I modified the kernel sources a bit (actually only added a single "noinline")
to get some sensible debug data and dumped a crashed system. These are the
contents of the scheduler data structures which cause the crash:

>> px *(cfs_rq *) 0x75a380
struct cfs_rq {
load = struct load_weight {
weight = 0x800
inv_weight = 0x0
}
nr_running = 0x1
exec_clock = 0x0
min_vruntime = 0xbf7e9776
tasks_timeline = struct rb_root {
rb_node = (nil)
}
rb_leftmost = (nil) <<<<<<<<<<<< shouldn't be NULL
tasks = struct list_head {
next = 0x759328
prev = 0x759328
}
balance_iterator = (nil)
curr = 0x759300
next = (nil)
nr_spread_over = 0x0
rq = 0x75a300
leaf_cfs_rq_list = struct list_head {
next = (nil)
prev = (nil)
}
tg = 0x564970
}

The sched_entity that belongs to the cfs_rq:

>> px *(sched_entity *) 0x759300
struct sched_entity {
load = struct load_weight {
weight = 0x800
inv_weight = 0x1ffc01
}
run_node = struct rb_node {
rb_parent_color = 0x1
rb_right = (nil)
rb_left = (nil)
}
group_node = struct list_head {
next = 0x75a3b8
prev = 0x75a3b8
}
on_rq = 0x1
exec_start = 0x189685acb4aa46
sum_exec_runtime = 0x188a2b84c
vruntime = 0xd036bd29
prev_sum_exec_runtime = 0x1672e3f62
last_wakeup = 0x0
avg_overlap = 0x0
parent = (nil)
cfs_rq = 0x75a380
my_q = 0x759400
}

And the rq:

>> px *(rq *) 0x75a300
struct rq {
lock = spinlock_t {
raw_lock = raw_spinlock_t {
owner_cpu = 0xfffffffe
}
break_lock = 0x1
magic = 0xdead4ead
owner_cpu = 0x1
owner = 0x2ef95350
}
nr_running = 0x1
cpu_load = {
[0] 0x3062
[1] 0x2bdf
[2] 0x20db
[3] 0x171e
[4] 0x1010
}
idle_at_tick = 0x0
last_tick_seen = 0x0
in_nohz_recently = 0x0
load = struct load_weight {
weight = 0xc31
inv_weight = 0x0
}
nr_load_updates = 0x95f
nr_switches = 0x3f68
cfs = struct cfs_rq {
load = struct load_weight {
weight = 0x800
inv_weight = 0x0
}
nr_running = 0x1
exec_clock = 0x0
min_vruntime = 0xbf7e9776
tasks_timeline = struct rb_root {
rb_node = (nil)
}
rb_leftmost = (nil)
tasks = struct list_head {
next = 0x759328
prev = 0x759328
}
balance_iterator = (nil)
curr = 0x759300
next = (nil)
nr_spread_over = 0x0
rq = 0x75a300
leaf_cfs_rq_list = struct list_head {
next = (nil)
prev = (nil)
}
tg = 0x564970
}
rt = struct rt_rq {
active = struct rt_prio_array {
bitmap = {
[0] 0x0
[1] 0x1000000000
}
queue = {
[0] struct list_head {
next = 0x75a418
prev = 0x75a418
}
[1] struct list_head {
next = 0x75a428
prev = 0x75a428
}
[2] struct list_head {
next = 0x75a438
prev = 0x75a438
}
[3] struct list_head {
next = 0x75a448
prev = 0x75a448
}
[4] struct list_head {
next = 0x75a458
prev = 0x75a458
}
[5] struct list_head {
next = 0x75a468
prev = 0x75a468
}
[6] struct list_head {
next = 0x75a478
prev = 0x75a478
}
[7] struct list_head {
next = 0x75a488
prev = 0x75a488
}
[8] struct list_head {
next = 0x75a498
prev = 0x75a498
}
[9] struct list_head {
next = 0x75a4a8
prev = 0x75a4a8
}
[10] struct list_head {
next = 0x75a4b8
prev = 0x75a4b8
}
[11] struct list_head {
next = 0x75a4c8
prev = 0x75a4c8
}
[12] struct list_head {
next = 0x75a4d8
prev = 0x75a4d8
}
[13] struct list_head {
next = 0x75a4e8
prev = 0x75a4e8
}
[14] struct list_head {
next = 0x75a4f8
prev = 0x75a4f8
}
[15] struct list_head {
next = 0x75a508
prev = 0x75a508
}
[16] struct list_head {
next = 0x75a518
prev = 0x75a518
}
[17] struct list_head {
next = 0x75a528
prev = 0x75a528
}
[18] struct list_head {
next = 0x75a538
prev = 0x75a538
}
[19] struct list_head {
next = 0x75a548
prev = 0x75a548
}
[20] struct list_head {
next = 0x75a558
prev = 0x75a558
}
[21] struct list_head {
next = 0x75a568
prev = 0x75a568
}
[22] struct list_head {
next = 0x75a578
prev = 0x75a578
}
[23] struct list_head {
next = 0x75a588
prev = 0x75a588
}
[24] struct list_head {
next = 0x75a598
prev = 0x75a598
}
[25] struct list_head {
next = 0x75a5a8
prev = 0x75a5a8
}
[26] struct list_head {
next = 0x75a5b8
prev = 0x75a5b8
}
[27] struct list_head {
next = 0x75a5c8
prev = 0x75a5c8
}
[28] struct list_head {
next = 0x75a5d8
prev = 0x75a5d8
}
[29] struct list_head {
next = 0x75a5e8
prev = 0x75a5e8
}
[30] struct list_head {
next = 0x75a5f8
prev = 0x75a5f8
}
[31] struct list_head {
next = 0x75a608
prev = 0x75a608
}
[32] struct list_head {
next = 0x75a618
prev = 0x75a618
}
[33] struct list_head {
next = 0x75a628
prev = 0x75a628
}
[34] struct list_head {
next = 0x75a638
prev = 0x75a638
}
[35] struct list_head {
next = 0x75a648
prev = 0x75a648
}
[36] struct list_head {
next = 0x75a658
prev = 0x75a658
}
[37] struct list_head {
next = 0x75a668
prev = 0x75a668
}
[38] struct list_head {
next = 0x75a678
prev = 0x75a678
}
[39] struct list_head {
next = 0x75a688
prev = 0x75a688
}
[40] struct list_head {
next = 0x75a698
prev = 0x75a698
}
[41] struct list_head {
next = 0x75a6a8
prev = 0x75a6a8
}
[42] struct list_head {
next = 0x75a6b8
prev = 0x75a6b8
}
[43] struct list_head {
next = 0x75a6c8
prev = 0x75a6c8
}
[44] struct list_head {
next = 0x75a6d8
prev = 0x75a6d8
}
[45] struct list_head {
next = 0x75a6e8
prev = 0x75a6e8
}
[46] struct list_head {
next = 0x75a6f8
prev = 0x75a6f8
}
[47] struct list_head {
next = 0x75a708
prev = 0x75a708
}
[48] struct list_head {
next = 0x75a718
prev = 0x75a718
}
[49] struct list_head {
next = 0x75a728
prev = 0x75a728
}
[50] struct list_head {
next = 0x75a738
prev = 0x75a738
}
[51] struct list_head {
next = 0x75a748
prev = 0x75a748
}
[52] struct list_head {
next = 0x75a758
prev = 0x75a758
}
[53] struct list_head {
next = 0x75a768
prev = 0x75a768
}
[54] struct list_head {
next = 0x75a778
prev = 0x75a778
}
[55] struct list_head {
next = 0x75a788
prev = 0x75a788
}
[56] struct list_head {
next = 0x75a798
prev = 0x75a798
}
[57] struct list_head {
next = 0x75a7a8
prev = 0x75a7a8
}
[58] struct list_head {
next = 0x75a7b8
prev = 0x75a7b8
}
[59] struct list_head {
next = 0x75a7c8
prev = 0x75a7c8
}
[60] struct list_head {
next = 0x75a7d8
prev = 0x75a7d8
}
[61] struct list_head {
next = 0x75a7e8
prev = 0x75a7e8
}
[62] struct list_head {
next = 0x75a7f8
prev = 0x75a7f8
}
[63] struct list_head {
next = 0x75a808
prev = 0x75a808
}
[64] struct list_head {
next = 0x75a818
prev = 0x75a818
}
[65] struct list_head {
next = 0x75a828
prev = 0x75a828
}
[66] struct list_head {
next = 0x75a838
prev = 0x75a838
}
[67] struct list_head {
next = 0x75a848
prev = 0x75a848
}
[68] struct list_head {
next = 0x75a858
prev = 0x75a858
}
[69] struct list_head {
next = 0x75a868
prev = 0x75a868
}
[70] struct list_head {
next = 0x75a878
prev = 0x75a878
}
[71] struct list_head {
next = 0x75a888
prev = 0x75a888
}
[72] struct list_head {
next = 0x75a898
prev = 0x75a898
}
[73] struct list_head {
next = 0x75a8a8
prev = 0x75a8a8
}
[74] struct list_head {
next = 0x75a8b8
prev = 0x75a8b8
}
[75] struct list_head {
next = 0x75a8c8
prev = 0x75a8c8
}
[76] struct list_head {
next = 0x75a8d8
prev = 0x75a8d8
}
[77] struct list_head {
next = 0x75a8e8
prev = 0x75a8e8
}
[78] struct list_head {
next = 0x75a8f8
prev = 0x75a8f8
}
[79] struct list_head {
next = 0x75a908
prev = 0x75a908
}
[80] struct list_head {
next = 0x75a918
prev = 0x75a918
}
[81] struct list_head {
next = 0x75a928
prev = 0x75a928
}
[82] struct list_head {
next = 0x75a938
prev = 0x75a938
}
[83] struct list_head {
next = 0x75a948
prev = 0x75a948
}
[84] struct list_head {
next = 0x75a958
prev = 0x75a958
}
[85] struct list_head {
next = 0x75a968
prev = 0x75a968
}
[86] struct list_head {
next = 0x75a978
prev = 0x75a978
}
[87] struct list_head {
next = 0x75a988
prev = 0x75a988
}
[88] struct list_head {
next = 0x75a998
prev = 0x75a998
}
[89] struct list_head {
next = 0x75a9a8
prev = 0x75a9a8
}
[90] struct list_head {
next = 0x75a9b8
prev = 0x75a9b8
}
[91] struct list_head {
next = 0x75a9c8
prev = 0x75a9c8
}
[92] struct list_head {
next = 0x75a9d8
prev = 0x75a9d8
}
[93] struct list_head {
next = 0x75a9e8
prev = 0x75a9e8
}
[94] struct list_head {
next = 0x75a9f8
prev = 0x75a9f8
}
[95] struct list_head {
next = 0x75aa08
prev = 0x75aa08
}
[96] struct list_head {
next = 0x75aa18
prev = 0x75aa18
}
[97] struct list_head {
next = 0x75aa28
prev = 0x75aa28
}
[98] struct list_head {
next = 0x75aa38
prev = 0x75aa38
}
[99] struct list_head {
next = 0x75aa48
prev = 0x75aa48
}
}
}
rt_nr_running = 0x0
highest_prio = 0x64
rt_nr_migratory = 0x0
overloaded = 0x0
rt_throttled = 0x0
rt_time = 0x123a999
rt_runtime = 0x389fd980
rt_runtime_lock = spinlock_t {
raw_lock = raw_spinlock_t {
owner_cpu = 0x0
}
break_lock = 0x0
magic = 0xdead4ead
owner_cpu = 0xffffffff
owner = 0xffffffffffffffff
}
}
leaf_cfs_rq_list = struct list_head {
next = 0x2f5a8970
prev = 0x759470
}
nr_uninterruptible = 0xfffffffffffffffe
curr = 0x2ef95350
idle = 0x2fe7ccf8
next_balance = 0x10000093b
prev_mm = (nil)
clock = 0x189685acb4d536
nr_iowait = atomic_t {
counter = 0x0
}
rd = 0x564a58
sd = (nil)
active_balance = 0x0
push_cpu = 0x0
cpu = 0x1
migration_thread = 0x2ef95350
migration_queue = struct list_head {
next = 0x75ab10
prev = 0x75ab10
}
rq_lock_key = struct lock_class_key {
}
}

Hopefully all of this debug data is of any use. If you need more, just let me
know.

Thanks!


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