Re: [Question] report a race condition between CPU hotplug state machine and hrtimer 'sched_cfs_period_timer' for cfs bandwidth throttling

[Vincent Guittot]

On Tue, 27 Jun 2023 at 18:46, Vincent Guittot
<vincent.guittot@xxxxxxxxxx> wrote:
>
> On Mon, 26 Jun 2023 at 10:23, Xiongfeng Wang <wangxiongfeng2@xxxxxxxxxx> wrote:
> >
> > Hi,
> >
> > Kindly ping~
> > Could you please take a look at this issue and the below temporary fix ?
> >
> > Thanks,
> > Xiongfeng
> >
> > On 2023/6/12 20:49, Xiongfeng Wang wrote:
> > >
> > >
> > > On 2023/6/9 22:55, Thomas Gleixner wrote:
> > >> On Fri, Jun 09 2023 at 19:24, Xiongfeng Wang wrote:
> > >>
> > >> Cc+ scheduler people, leave context intact
> > >>
> > >>> Hello,
> > >>>  When I do some low power tests, the following hung task is printed.
> > >>>
> > >>>   Call trace:
> > >>>    __switch_to+0xd4/0x160
> > >>>    __schedule+0x38c/0x8c4
> > >>>    __cond_resched+0x24/0x50
> > >>>    unmap_kernel_range_noflush+0x210/0x240
> > >>>    kretprobe_trampoline+0x0/0xc8
> > >>>    __vunmap+0x70/0x31c
> > >>>    __vfree+0x34/0x8c
> > >>>    vfree+0x40/0x58
> > >>>    free_vm_stack_cache+0x44/0x74
> > >>>    cpuhp_invoke_callback+0xc4/0x71c
> > >>>    _cpu_down+0x108/0x284
> > >>>    kretprobe_trampoline+0x0/0xc8
> > >>>    suspend_enter+0xd8/0x8ec
> > >>>    suspend_devices_and_enter+0x1f0/0x360
> > >>>    pm_suspend.part.1+0x428/0x53c
> > >>>    pm_suspend+0x3c/0xa0
> > >>>    devdrv_suspend_proc+0x148/0x248 [drv_devmng]
> > >>>    devdrv_manager_set_power_state+0x140/0x680 [drv_devmng]
> > >>>    devdrv_manager_ioctl+0xcc/0x210 [drv_devmng]
> > >>>    drv_ascend_intf_ioctl+0x84/0x248 [drv_davinci_intf]
> > >>>    __arm64_sys_ioctl+0xb4/0xf0
> > >>>    el0_svc_common.constprop.0+0x140/0x374
> > >>>    do_el0_svc+0x80/0xa0
> > >>>    el0_svc+0x1c/0x28
> > >>>    el0_sync_handler+0x90/0xf0
> > >>>    el0_sync+0x168/0x180
> > >>>
> > >>> After some analysis, I found it is caused by the following race condition.
> > >>>
> > >>> 1. A task running on CPU1 is throttled for cfs bandwidth. CPU1 starts the
> > >>> hrtimer cfs_bandwidth 'period_timer' and enqueue the hrtimer on CPU1's rbtree.
> > >>> 2. Then the task is migrated to CPU2 and starts to offline CPU1. CPU1 starts
> > >>> CPUHP AP steps, and then the hrtimer 'period_timer' expires and re-enqueued on CPU1.
> > >>> 3. CPU1 runs to take_cpu_down() and disable irq. After CPU1 finished CPUHP AP
> > >>> steps, CPU2 starts the rest CPUHP step.
> > >>> 4. When CPU2 runs to free_vm_stack_cache(), it is sched out in __vunmap()
> > >>> because it run out of CPU quota. start_cfs_bandwidth() does not restart the
> > >>> hrtimer because 'cfs_b->period_active' is set.
> > >>> 5. The task waits the hrtimer 'period_timer' to expire to wake itself up, but
> > >>> CPU1 has disabled irq and the hrtimer won't expire until it is migrated to CPU2
> > >>> in hrtimers_dead_cpu(). But the task is blocked and cannot proceed to
> > >>> hrtimers_dead_cpu() step. So the task hungs.
> > >>>
> > >>>     CPU1                                                     CPU2
> > >>> Task set cfs_quota
> > >>> start hrtimer cfs_bandwidth 'period_timer'
> > >>>                                             start to offline CPU1
> > >>> CPU1 start CPUHP AP step
> > >>> ...
> > >>> 'period_timer' expired and re-enqueued on CPU1
> > >>> ...
> > >>> disable irq in take_cpu_down()
> > >>> ...
> > >>>                                             CPU2 start the rest CPUHP steps
> > >>>                                             ...
> > >>>                                           sched out in free_vm_stack_cache()
> > >>>                                             wait for 'period_timer' expires
> > >>>
> > >>>
> > >>> Appreciate it a lot if anyone can give some suggestion on how fix this problem !
> > >>>
> > >>> Thanks,
> > >>> Xiongfeng
> > >> .
> > >>
> > >
> > > Test script:
> > > taskset -cp 1 $$
> > > mkdir /sys/fs/cgroup/cpu/test
> > > echo $$ > /sys/fs/cgroup/cpu/test/tasks
> > > echo 80000 > /sys/fs/cgroup/cpu/test/cpu.cfs_quota_us
> > > echo 100000 > /sys/fs/cgroup/cpu/test/cpu.cfs_period_us
> > > taskset -cp 2 $$
> > > echo 0 > /sys/devices/system/cpu/cpu1/online
> > >
> > >
> > > Tests show that the following modification can solve the problem of above test
> > > scripts. But I am not sure if there exists some other issues.
> > >
> > > diff --cc kernel/sched/fair.c
> > > index d9d6519fae01,bd6624353608..000000000000
> > > --- a/kernel/sched/fair.c
> > > +++ b/kernel/sched/fair.c
> > > @@@ -5411,10 -5411,16 +5411,15 @@@ void start_cfs_bandwidth(struct cfs_ban
> > >   {
> > >         lockdep_assert_held(&cfs_b->lock);
> > >
> > > -       if (cfs_b->period_active)
> > > +       if (cfs_b->period_active) {
> > > +               struct hrtimer_clock_base *clock_base = cfs_b->period_timer.base;
> > > +               int cpu = clock_base->cpu_base->cpu;
> > > +               if (!cpu_active(cpu) && cpu != smp_processor_id())
> > > +                       hrtimer_start_expires(&cfs_b->period_timer,
> > > HRTIMER_MODE_ABS_PINNED);
> > >                 return;
> > > +       }
>
> I have been able to reproduce your problem and run your fix on top. I
> still wonder if there is a

Looks like I have been preempted and never finished the sentence. The
full sentence is:
I still wonder if there is a race condition where the hang can still
happen but i haven't been able to find one so far

> Could we have a helper from hrtimer to get the cpu of the clock_base ?
>
>
> > >
> > >         cfs_b->period_active = 1;
> > >  -
> > >         hrtimer_forward_now(&cfs_b->period_timer, cfs_b->period);
> > >         hrtimer_start_expires(&cfs_b->period_timer, HRTIMER_MODE_ABS_PINNED);
> > >   }
> > >
> > > Thanks,
> > > Xiongfeng
> > >
> > > .
> > >