On Mon, Oct 18, 2021 at 03:08:53PM +0800, Youfu Zhang wrote:

Hi,

I ran into a kernel bug related to blk-throttle on CentOS 7 AltArch for i386.
Userspace programs may panic the kernel if they hit the I/O limit
within 5 minutes after startup.

Root cause:
1. jiffies was initialized to -300HZ during boot on 32bit machines
2. enable blkio cgroup hierarchy
   __DEVEL__sane_behavior for cgroup v1 or default hierarchy for cgroup v2
   EL7 kernel modified throtl_pd_init and always enable hierarchical throttling
3. enable & trigger blkio throttling within 5 minutes after startup
   bio propagated from child tg to parent
4. enter throtl_start_new_slice_with_credit
   if(time_after_eq(start, tg->slice_start[rw]))
   aka. time_after_eq(0xFFFxxxxx, 0) does not hold
   parent tg->slice_start[rw] was zero-initialized and not updated
5. enter throtl_trim_slice
   BUG_ON(time_before(tg->slice_end[rw], tg->slice_start[rw]))
   aka. time_before(0xFFFxxxxx, 0) triggers a panic

This doesn't reproduce on 5.14.

Thanks.

-- 
tejun