linux-kernel - Re: 回复: INFO: rcu detected stall in tc_modify

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Date:   Thu, 30 Jul 2020 14:01:05 -0700
From:   Vinicius Costa Gomes <vinicius.gomes@...el.com>
To:     Eric Dumazet <eric.dumazet@...il.com>,
        Dmitry Vyukov <dvyukov@...gle.com>
Cc:     "Zhang\, Qiang" <Qiang.Zhang@...driver.com>,
        syzbot <syzbot+9f78d5c664a8c33f4cce@...kaller.appspotmail.com>,
        "davem\@davemloft.net" <davem@...emloft.net>,
        "fweisbec\@gmail.com" <fweisbec@...il.com>,
        "jhs\@mojatatu.com" <jhs@...atatu.com>,
        "jiri\@resnulli.us" <jiri@...nulli.us>,
        "linux-kernel\@vger.kernel.org" <linux-kernel@...r.kernel.org>,
        "mingo\@kernel.org" <mingo@...nel.org>,
        "netdev\@vger.kernel.org" <netdev@...r.kernel.org>,
        "syzkaller-bugs\@googlegroups.com" <syzkaller-bugs@...glegroups.com>,
        "tglx\@linutronix.de" <tglx@...utronix.de>,
        "xiyou.wangcong\@gmail.com" <xiyou.wangcong@...il.com>
Subject: Re: 回复: INFO: rcu detected stall in
 tc_modify_qdisc

Hi Eric,

Eric Dumazet <eric.dumazet@...il.com> writes:

>> I admit that I am on the fence on that argument: do not let even root
>> crash the system (the point that my code is crashing the system gives
>> weight to this side) vs. root has great powers, they need to know what
>> they are doing.
>> 
>> The argument that I used to convince myself was: root can easily create
>> a bunch of processes and give them the highest priority and do
>> effectively the same thing as this issue, so I went with a the "they
>> need to know what they are doing side".
>> 
>> A bit more on the specifics here:
>> 
>>   - Using a small interval size, is only a limitation of the taprio
>>   software mode, when using hardware offloads (which I think most users
>>   do), any interval size (supported by the hardware) can be used;
>> 
>>   - Choosing a good lower limit for this seems kind of hard: something
>>   below 1us would never work well, I think, but things 1us < x < 100us
>>   will depend on the hardware/kernel config/system load, and this is the
>>   range includes "useful" values for many systems.
>> 
>> Perhaps a middle ground would be to impose a limit based on the link
>> speed, the interval can never be smaller than the time it takes to send
>> the minimum ethernet frame (for 1G links this would be ~480ns, should be
>> enough to catch most programming mistakes). I am going to add this and
>> see how it looks like.
>> 
>> Sorry for the brain dump :-)
>
>
> I do not know taprio details, but do you really need a periodic timer
> ?

As we can control the transmission time of packets, you are right, I
don't.

Just a bit more detail about the current implementation taprio,
basically it has a sequence of { Traffic Classes that are open; Interval
} that repeats cyclicly, it uses an hrtimer to advance the pointer for
the current element, so during dequeue I can check if a traffic class is
"open" or "closed".

But again, if I calculate the 'skb->tstamp' of each packet during
enqueue, I don't need the hrtimer. What we have in the txtime-assisted
mode is half way there.

I think this is what you had in mind.


Cheers,
-- 
Vinicius