Re: [Question] The system may be stuck if there is a cpu cgroup cpu.cfs_quato_us is very low

[Benjamin Segall]

Zhang Qiao <zhangqiao22@xxxxxxxxxx> writes:

> Hi, tejun
>
> Thanks for your reply.
>
> 在 2022/6/27 16:32, Tejun Heo 写道:
>> Hello,
>> 
>> On Mon, Jun 27, 2022 at 02:50:25PM +0800, Zhang Qiao wrote:
>>> Becuase the task cgroup's cpu.cfs_quota_us is very small and
>>> test_fork's load is very heavy, the test_fork may be throttled long
>>> time, therefore, the cgroup_threadgroup_rw_sem read lock is held for
>>> a long time, other processes will get stuck waiting for the lock:
>> 
>> Yeah, this is a known problem and can happen with other locks too. The
>> solution prolly is only throttling while in or when about to return to
>> userspace. There is one really important and wide-spread assumption in
>> the kernel:
>> 
>>   If things get blocked on some shared resource, whatever is holding
>>   the resource ends up using more of the system to exit the critical
>>   section faster and thus unblocks others ASAP. IOW, things running in
>>   kernel are work-conserving.
>> 
>> The cpu bw controller gives the userspace a rather easy way to break
>> this assumption and thus is rather fundamentally broken. This is
>> basically the same problem we had with the old cgroup freezer
>> implementation which trapped threads in random locations in the
>> kernel.
>> 
>
> so, if we want to completely slove this problem, is the best way to
> change the cfs bw controller throttle mechanism? for example, throttle
> tasks in a safe location.

Yes, fixing (kernel) priority inversion due to CFS_BANDWIDTH requires a
serious reworking of how it works, because it would need to dequeue
tasks individually rather than doing the entire cfs_rq at a time (and
would require some effort to avoid pinging every throttling task to get
it into the kernel).