Re: [BUG] CFS vs cpu hotplug

[Dmitry Adamushko]

2008/6/19 Peter Zijlstra <a.p.zijlstra@xxxxxxxxx>:
> On Thu, 2008-06-19 at 18:19 +0200, Heiko Carstens wrote:
>> 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)
>
> I presume this is:
>
>                se = pick_next_entity(cfs_rq);
>
>>            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
>> }
>
> Right, this cfs_rq is buggered. rb_leftmost may be null when the tree is
> empty (as is the case here).
>
> However cfs_rq->curr != NULL and cfs_rq->nr_running != 0.
>
> So this hints at a missing put_prev_entity() - we keep current out of
> the tree, and put it back in right before we schedule(). The advantage
> is that we don't need to reposition (dequeue/enqueue) curr in the tree
> every time we update its virtual timeline.
>
> So what races so that we can miss put_prev_entity() and how is cpu_up()
> special..
>

hum, I'd rather suppose that something weird happened at the time of
cpu_down() and some per-cpu data is already inconsistent by the time
of cpu_up().

Is it with CONFIG_USER_SCHED?

Maybe we can write a small function that does a 'sanety' check of :

for all sched_groups (task_groups's) : check 'sanity' of
group->cfs_rq[CPU] and group->se[CPU] somewhere early in cpu_up().

So we can verify whether it's legacy of cpu_down() or something
related to cpu_up().

hm?


-- 
Best regards,
Dmitry Adamushko
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