linux-kernel - Re: [Question] The system may be stuck if there is a cpu cgroup cpu.cfs_quato

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Date:   Fri, 1 Jul 2022 15:34:41 +0800
From:   Zhang Qiao <zhangqiao22@...wei.com>
To:     Tejun Heo <tj@...nel.org>
CC:     <mingo@...hat.com>, <peterz@...radead.org>,
        Juri Lelli <juri.lelli@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        <lizefan.x@...edance.com>, <hannes@...xchg.org>,
        <cgroups@...r.kernel.org>, lkml <linux-kernel@...r.kernel.org>,
        <vschneid@...hat.com>, <dietmar.eggemann@....com>,
        <bristot@...hat.com>, <bsegall@...gle.com>,
        Steven Rostedt <rostedt@...dmis.org>, <mgorman@...e.de>
Subject: Re: [Question] The system may be stuck if there is a cpu cgroup
 cpu.cfs_quato_us is very low


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.

Thanks.
    Qiao

> So, right now, it's rather broken and can easily be used as an dos
> attack vector.
> 
> Thanks.
>