Re: [PATCH v3] cgroup: avoid flushing global workqueue in cgroup1_pidlist_destroy_all

[Junnan Zhang]

Hi Ridong,

> Shouldn't we first figure out why flush_workqueue() waited 120s? Was
> it because there were too many pids, or because someone held
> pidlist_mutex for too long?

That's the same question Michal raised; please see my reply to him in
this thread for the full analysis. The short version: it's neither a
single long mutex holder nor oversized pidlists per se -- the wait is
backlog x per-work latency. flush_workqueue() waits for every work
already queued on the shared wq, which drains serially
(WQ_PERCPU, max_active=1). Container churn keeps queueing destroy
works, and each work must take the owner's pidlist_mutex behind
readers whose pidlist_array_load() runs entirely under that mutex.
A few thousand queued works each delayed by tens of ms is enough to
exceed 120s.

Unfortunately the guest memory dump captured at the incident couldn't
be analyzed with crash, so exact queue depths aren't available; I've
offered to build a synthetic reproducer with measured latency data if
that's needed to move this forward.

Thanks,
Junnan