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Subject: Re: [Question] report a race condition between CPU hotplug state
 machine and hrtimer 'sched_cfs_period_timer' for cfs bandwidth throttling
To:     Thomas Gleixner <tglx@linutronix.de>, <vschneid@redhat.com>,
        Phil Auld <pauld@redhat.com>, <vdonnefort@google.com>
CC:     Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Wei Li <liwei391@huawei.com>,
        "liaoyu (E)" <liaoyu15@huawei.com>, <zhangqiao22@huawei.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Vincent Guittot <vincent.guittot@linaro.org>,
        Dietmar Eggemann <dietmar.eggemann@arm.com>,
        Ingo Molnar <mingo@kernel.org>
References: <8e785777-03aa-99e1-d20e-e956f5685be6@huawei.com>
 <87mt18it1y.ffs@tglx>
From:   Xiongfeng Wang <wangxiongfeng2@huawei.com>
Message-ID: <68baeac9-9fa7-5594-b5e7-4baf8ac86b77@huawei.com>
Date:   Mon, 12 Jun 2023 20:49:19 +0800
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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;
+       }

        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