RE: [Question] softlockup in run_timer_softirq

["liujian (CE)" <liujian56@huawei.com>]

> -----Original Message-----
> From: John Stultz [mailto:jstultz@google.com]
> Sent: Tuesday, February 14, 2023 4:01 AM
> To: liujian (CE) <liujian56@huawei.com>
> Cc: tglx@linutronix.de; sboyd@kernel.org; linux-kernel@vger.kernel.org;
> peterz@infradead.org; Paul E. McKenney <paulmck@kernel.org>
> Subject: Re: [Question] softlockup in run_timer_softirq
> 
> On Fri, Feb 10, 2023 at 1:51 AM liujian (CE) <liujian56@huawei.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?
> 
Okay,   I'll learn this framework and do this thing.
> (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=co
> re/softirq
> 
> Maybe it could extend that series for the timer softirq as well?
> 
Thank you. Yes.
Base on the patchset and the extended patch for timer [1], the soft lockup problem does not occur.

By the way, I see this is a very old patchset?  Will this patchset push the main line? @John @Peter


 [1]
Author: Liu Jian <liujian56@huawei.com>
Date:   Tue Feb 14 09:53:46 2023 +0800

    softirq, timer: Use softirq_needs_break()
    
    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.
    
    To prevent this, use the timeout/break logic provided by SoftIRQs.If the
    running time exceeds the limit, break the loop and an additional
    TIMER_SOFTIRQ is triggered.
    
    Signed-off-by: Liu Jian <liujian56@huawei.com>

diff --git a/kernel/time/timer.c b/kernel/time/timer.c
index 63a8ce7177dd..70744a469a39 100644
--- a/kernel/time/timer.c
+++ b/kernel/time/timer.c
@@ -1992,7 +1992,7 @@ void timer_clear_idle(void)
  * __run_timers - run all expired timers (if any) on this CPU.
  * @base: the timer vector to be processed.
  */
-static inline void __run_timers(struct timer_base *base)
+static inline void __run_timers(struct timer_base *base, struct softirq_action *h)
 {
        struct hlist_head heads[LVL_DEPTH];
        int levels;
@@ -2020,6 +2020,12 @@ static inline void __run_timers(struct timer_base *base)
 
                while (levels--)
                        expire_timers(base, heads + levels);
+
+               if (softirq_needs_break(h)) {
+                       if (time_after_eq(jiffies, base->next_expiry))
+                               __raise_softirq_irqoff(TIMER_SOFTIRQ);
+                       break;
+               }
        }
        raw_spin_unlock_irq(&base->lock);
        timer_base_unlock_expiry(base);
@@ -2032,9 +2038,9 @@ static __latent_entropy void run_timer_softirq(struct softirq_action *h)
 {
        struct timer_base *base = this_cpu_ptr(&timer_bases[BASE_STD]);
 
-       __run_timers(base);
+       __run_timers(base, h);
        if (IS_ENABLED(CONFIG_NO_HZ_COMMON))
-               __run_timers(this_cpu_ptr(&timer_bases[BASE_DEF]));
+               __run_timers(this_cpu_ptr(&timer_bases[BASE_DEF]), h);
 }
 
 /*
> thanks
> -john