Re: [REGRESSION] workqueue/writeback: Severe CPU hang due to kworker proliferation during I/O flush and cgroup cleanup

[Chenglong Tang <chenglongtang@google.com>]

Just did more testing here. Confirmed that the system hang's still
there but less frequently(6/40) with the patches
http://lkml.kernel.org/r/20250912103522.2935-1-jack@suse.cz appied to
v6.17-rc7. In the bad instances, the kworker count climbed to over
600+ and caused the hang over 80+ seconds.

So I think the patches didn't fully solve the issue.




On Wed, Sep 24, 2025 at 5:52 PM Tejun Heo <tj@kernel.org> wrote:
>
> On Wed, Sep 24, 2025 at 05:24:15PM -0700, Chenglong Tang wrote:
> > The kernel v6.1 is good. The hang is reliably triggered(over 80% chance) on
> > kernels v6.6 and 6.12 and intermittently on mainline(6.17-rc7) with the
> > following steps:
> > -
> >
> > *Environment:* A machine with a fast SSD and a high core count (e.g.,
> > Google Cloud's N2-standard-128).
> > -
> >
> > *Workload:* Concurrently generate a large number of files (e.g., 2 million)
> > using multiple services managed by systemd-run. This creates significant
> > I/O and cgroup churn.
> > -
> >
> > *Trigger:* After the file generation completes, terminate the systemd-run
> > services.
> > -
> >
> > *Result:* Shortly after the services are killed, the system's CPU load
> > spikes, leading to a massive number of kworker/+inode_switch_wbs threads
> > and a system-wide hang/livelock where the machine becomes unresponsive (20s
> > - 300s).
>
> Sounds like:
>
>  http://lkml.kernel.org/r/20250912103522.2935-1-jack@suse.cz
>
> Can you see whether those patches resolve the problem?
>
> Thanks.
>
> --
> tejun