linux-kernel - Re: [Question] softlockup in run_timer

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Date:   Mon, 13 Feb 2023 12:00:56 -0800
From:   John Stultz <jstultz@...gle.com>
To:     "liujian (CE)" <liujian56@...wei.com>
Cc:     "tglx@...utronix.de" <tglx@...utronix.de>,
        "sboyd@...nel.org" <sboyd@...nel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        peterz@...radead.org, "Paul E. McKenney" <paulmck@...nel.org>
Subject: Re: [Question] softlockup in run_timer_softirq

On Fri, Feb 10, 2023 at 1:51 AM liujian (CE) <liujian56@...wei.com> wrote:
>
> During the syz test, we encountered many problems with various timer handler
> functions softlockup.
>
> We analyze __run_timers() and find the following problem.
>
> In the while loop of __run_timers(), because there are too many timers or
> improper timer handler functions, if the processing time of the expired
> timers is always greater than the time wheel's next_expiry, the function
> will loop infinitely.
>
> The following extreme test case can be used to reproduce the problem.
> An extreme test case[1] is constructed to reproduce the problem.

Thanks for reporting and sending out this data:

First, any chance you might submit this as a in-kernel-stress test?
Maybe utilizing the kernel/torture.c framework?

(Though the test may need to occasionally take a break so the system
can eventually catch up)

> Is this a problem or an unreasonable use?
>
> Can we limit the running time of __run_timers() [2]?
>
> Does anyone have a good idea to solve this problem?

So your patch reminds me of Peter's softirq_needs_break() logic:
  https://git.kernel.org/pub/scm/linux/kernel/git/peterz/queue.git/log/?h=core/softirq

Maybe it could extend that series for the timer softirq as well?

thanks
-john