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Message-ID: <CAE4VaGDvBM20+VGbe+uCYOsuRX1bnQugHH=9DZCAuqAxRV5KiA@mail.gmail.com>
Date: Wed, 2 Jul 2025 15:24:46 +0200
From: Jirka Hladky <jhladky@...hat.com>
To: "Aithal, Srikanth" <sraithal@....com>
Cc: "Chen, Yu C" <yu.c.chen@...el.com>, Abhigyan ghosh <zscript.team.zs@...il.com>,
linux-kernel@...r.kernel.org, Suneeth D <Suneeth.D@....com>
Subject: Re: [BUG] Kernel panic in __migrate_swap_task() on 6.16-rc2 (NULL
pointer dereference)
Thank you, Aithal!
Chenyu, here is another debug log - the flag is essentially the same
as before, except that PF_NOFREEZE is also set, so it points to the
same race condition (task exit).
This happened on an AMD Bergamo 9754 single socket server.
cat /sys/kernel/debug/tracing/trace
# tracer: nop
#
# entries-in-buffer/entries-written: 1/1 #P:256
#
# _-----=> irqs-off/BH-disabled
# / _----=> need-resched
# | / _---=> hardirq/softirq
# || / _--=> preempt-depth
# ||| / _-=> migrate-disable
# |||| / delay
# TASK-PID CPU# ||||| TIMESTAMP FUNCTION
# | | | ||||| | |
migration/96-599 [096] dN.5. 84281.647548: __migrate_swap_task:
!! (2886754 sysbench-binary) flags=40804c
On Wed, Jul 2, 2025 at 2:19 PM Aithal, Srikanth <sraithal@....com> wrote:
>
> On 7/2/2025 5:23 PM, Jirka Hladky wrote:
> > Hi Aithal,
> >
> > thank you for the reproducer.
> >
> > With the patched kernel, after running a reproducer, please do
> >
> > cat /sys/kernel/debug/tracing/trace
> >
> > If the counter is nonzero like this:
> > entries-in-buffer/entries-written: 1/1
> >
> > please post the `cat /sys/kernel/debug/tracing/trace` here.
> >
> > It would be helpful to check if there are any other race conditions
> > besides the task exit.
> >
> > Thank you!
> > Jirka
> >
>
>
> None of the four machines running our workload [with the debug patch]
> showed any entries.
> Entries in Buffer/Entries Written: 0/0
>
>
> > On Wed, Jul 2, 2025 at 1:17 PM Chen, Yu C <yu.c.chen@...el.com> wrote:
> >>
> >> On 7/2/2025 3:36 PM, Aithal, Srikanth wrote:
> >>>
> >>>
> >>> On 7/2/2025 10:02 AM, Aithal, Srikanth wrote:
> >>>> On 6/27/2025 1:03 PM, Chen, Yu C wrote:
> >>>>> On 6/27/2025 3:16 PM, Chen, Yu C wrote:
> >>>>>> Hi Jirka,
> >>>>>>
> >>>>>> On 6/27/2025 5:46 AM, Jirka Hladky wrote:
> >>>>>>> Hi Chen and all,
> >>>>>>>
> >>>>>>> we have now verified that the following commit causes a kernel panic
> >>>>>>> discussed in this thread:
> >>>>>>>
> >>>>>>> ad6b26b6a0a79 sched/numa: add statistics of numa balance task
> >>>>>>>
> >>>>>>> Reverting this commit fixes the issue.
> >>>>>>>
> >>>>>>> I'm happy to help debug this further or test a proposed fix.
> >>>>>>>
> >>>>>>
> >>>>>> Thanks very much for your report, it seems that there is a
> >>>>>> race condition that when the swap task candidate was chosen,
> >>>>>> but its mm_struct get released due to task exit, then later
> >>>>>> when doing the task swaping, the p->mm is NULL which caused
> >>>>>> the problem:
> >>>>>>
> >>>>>> CPU0 CPU1
> >>>>>> :
> >>>>>> ...
> >>>>>> task_numa_migrate
> >>>>>> task_numa_find_cpu
> >>>>>> task_numa_compare
> >>>>>> # a normal task p is chosen
> >>>>>> env->best_task = p
> >>>>>>
> >>>>>> # p exit:
> >>>>>> exit_signals(p);
> >>>>>> p->flags |= PF_EXITING
> >>>>>> exit_mm
> >>>>>> p->mm = NULL;
> >>>>>>
> >>>>>> migrate_swap_stop
> >>>>>> __migrate_swap_task((arg->src_task, arg->dst_cpu)
> >>>>>> count_memcg_event_mm(p->mm, NUMA_TASK_SWAP)# p->mm is NULL
> >>>>>>
> >>>>>> Could you please help check if the following debug patch works,
> >>>>>
> >>>>> Attached the patch:
> >>>>>
> >>>>> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> >>>>> index 8988d38d46a3..82fc966b390c 100644
> >>>>> --- a/kernel/sched/core.c
> >>>>> +++ b/kernel/sched/core.c
> >>>>> @@ -3364,7 +3364,12 @@ static void __migrate_swap_task(struct
> >>>>> task_struct *p, int cpu)
> >>>>> {
> >>>>> __schedstat_inc(p->stats.numa_task_swapped);
> >>>>> count_vm_numa_event(NUMA_TASK_SWAP);
> >>>>> - count_memcg_event_mm(p->mm, NUMA_TASK_SWAP);
> >>>>> + if (unlikely(!p->mm)) {
> >>>>> + trace_printk("!! (%d %s) flags=%lx\n", p->pid, p->comm,
> >>>>> + p->flags);
> >>>>> + } else {
> >>>>> + count_memcg_event_mm(p->mm, NUMA_TASK_SWAP);
> >>>>> + }
> >>>>>
> >>>>> if (task_on_rq_queued(p)) {
> >>>>> struct rq *src_rq, *dst_rq;
> >>>>
> >>>> I was encountering the same issue as mentioned earlier in this thread,
> >>>> which has been recurring in our daily linux-next CI builds within our
> >>>> virtualization CI stream where we observed this BUG appearing randomly
> >>>> during the runs.
> >>>>
> >>>> Additionally, we were able to reproduce this issue while running the
> >>>> autonuma benchmark. As mentioned earlier, the BUG would occur randomly
> >>>> across iterations, typically between the 5th and 10th iterations.
> >>>>
> >>>> We consistently encountered this issue up to the 6.16.0-rc4-
> >>>> next-20250630 build [https://git.kernel.org/pub/scm/linux/kernel/git/
> >>>> next/linux-next.git].
> >>>>
> >>>> After applying the aforementioned patch ontop of next-20250630 build,
> >>>> I tested it in our virtualization CI and with the autonuma benchmark
> >>>> reproducer described below, and the issue no longer occurred. The
> >>>> patch appears to have resolved the reported problem.
> >>>>
> >>>>
> >>>> git clone https://github.com/pholasek/autonuma-benchmark.git
> >>>> cd autonuma-benchmark
> >>>> for i in $(seq 1 80); do bash ./start_bench.sh -s -t; done
> >>>> Note: The server running the autonuma-benchmark must have at least two
> >>>> nodes.
> >>>>
> >>>> If the provided fix is final, please feel free to include the
> >>>> following Tested-by tag:
> >>>>
> >>>> Tested-by: Srikanth Aithal <Srikanth.Aithal@....com>
> >>>> Tested-by: Suneeth D <Suneeth.D@....com>
> >>>>
> >>>
> >>>
> >>> I apologize for overlooking that this was a debug patch. With this debug
> >>> patch applied on top of 6.16.0-rc4-next-20250630, I am unable to
> >>> reproduce the issue. However, when I revert this debug patch, I
> >>> encounter the issue again on 6.16.0-rc4-next-20250630.
> >>>
> >>>
> >>
> >> Thank you Aithal, this is a debug + fix patch. And Jirka has confirmed
> >> that this helps somehow. I'll post a formal one and Cc you.
> >>
> >> Thanks,
> >> Chenyu
> >>
> >
> >
>
--
-Jirka
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