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

[Thomas Gleixner <tglx@linutronix.de>]

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