Re: [bug-report] possible s64 overflow in max_vruntime()

[Peter Zijlstra <peterz@infradead.org>]

On Wed, Dec 21, 2022 at 11:19:31PM +0800, Zhang Qiao wrote:
> hi folks,
> 
>     I found problem about s64 overflow in max_vruntime().
> 
>     I create a task group GROUPA (path: /system.slice/xxx/yyy/CGROUPA) and run a task in this
> group on each cpu, these tasks is while loop and 100% cpu usage.
> 
>     When unregister net devices, will queue a kwork on system_highpri_wq at flush_all_backlogs()
> and wake up a high-priority kworker thread on each cpu. However, the kworker thread has been
> waiting on the queue and has not been scheduled.
> 
>     After parsing the vmcore, the vruntime of the kworker is 0x918fdb05287da7c3 and the
> cfs_rq->min_vruntime is 0x124b17fd59db8d02.
> 
>     why the difference between the cfs_rq->min_vruntime and kworker's vruntime is so large?
>     1) the kworker of the system_highpri_wq sleep for long long time(about 300 days).
>     2) cfs_rq->curr is the ancestor of the GROUPA, cfs->curr->load.weight is 2494, so when
> the task belonging to the GROUPA run for a long time, its vruntime will increase by 420
> times, cfs_rq->min_vruntime will also grow rapidly.
>     3) when wakeup kworker thread, kworker will be set the maximum value between kworker's
> vruntime and cfs_rq->min_vruntime. But at max_vruntime(), there will be a s64 overflow issue,
> as follow:
> 
> ---------
> 
> static inline u64 min_vruntime(u64 min_vruntime, u64 vruntime)
> {
> 	/*
> 	 * vruntime=0x124b17fd59db8d02
> 	 * min_vruntime=0x918fdb05287da7c3
> 	 * vruntime - min_vruntime = 9276074894177461567 > s64_max, will s64 overflow
> 	 */
> 	s64 delta = (s64)(vruntime - min_vruntime);
> 	if (delta < 0)
> 		min_vruntime = vruntime;
> 
> 	return min_vruntime;
> }
> 
> ----------
> 
> max_vruntime() will return the kworker's old vruntime, it is incorrect and the correct result
> shoud be cfs_rq->minvruntime. This incorrect result is greater than cfs_rq->min_vruntime and
> will cause kworker thread starved.
> 
>     Does anyone have a good suggestion for slove this problem? or bugfix patch.

I don't understand what you tihnk the problem is. Signed overflow is
perfectly fine and works as designed here.