Messages in this thread | | | Date | Thu, 22 Dec 2022 13:45:48 +0100 | From | Peter Zijlstra <> | Subject | Re: [bug-report] possible s64 overflow in max_vruntime() |
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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.
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