RE: v5.10: sched_cpu_dying() hits BUG_ON during hibernation: kernel BUG at kernel/sched/core.c:7596!

[Dexuan Cui]

> From: Valentin Schneider <valentin.schneider@xxxxxxx>
> Sent: Tuesday, December 22, 2020 5:40 AM
> To: Dexuan Cui <decui@xxxxxxxxxxxxx>
> Cc: mingo@xxxxxxxxxx; peterz@xxxxxxxxxxxxx; juri.lelli@xxxxxxxxxx;
> vincent.guittot@xxxxxxxxxx; dietmar.eggemann@xxxxxxx;
> rostedt@xxxxxxxxxxx; bsegall@xxxxxxxxxx; mgorman@xxxxxxx;
> bristot@xxxxxxxxxx; x86@xxxxxxxxxx; linux-pm@xxxxxxxxxxxxxxx;
> linux-kernel@xxxxxxxxxxxxxxx; linux-hyperv@xxxxxxxxxxxxxxx; Michael Kelley
> <mikelley@xxxxxxxxxxxxx>
> Subject: Re: v5.10: sched_cpu_dying() hits BUG_ON during hibernation: kernel
> BUG at kernel/sched/core.c:7596!
> 
> 
> Hi,
> 
> On 22/12/20 09:13, Dexuan Cui wrote:
> > Hi,
> > I'm running a Linux VM with the recent mainline (48342fc07272, 12/20/2020)
> on Hyper-V.
> > When I test hibernation, the VM can easily hit the below BUG_ON during the
> resume
> > procedure (I estimate this can repro about 1/5 of the time). BTW, my VM has
> 40 vCPUs.
> >
> > I can't repro the BUG_ON with v5.9.0, so I suspect something in v5.10.0 may
> be broken?
> >
> > In v5.10.0, when the BUG_ON happens, rq->nr_running==2, and
> rq->nr_pinned==0:
> >
> > 7587 int sched_cpu_dying(unsigned int cpu)
> > 7588 {
> > 7589         struct rq *rq = cpu_rq(cpu);
> > 7590         struct rq_flags rf;
> > 7591
> > 7592         /* Handle pending wakeups and then migrate everything off
> */
> > 7593         sched_tick_stop(cpu);
> > 7594
> > 7595         rq_lock_irqsave(rq, &rf);
> > 7596         BUG_ON(rq->nr_running != 1 || rq_has_pinned_tasks(rq));
> > 7597         rq_unlock_irqrestore(rq, &rf);
> > 7598
> > 7599         calc_load_migrate(rq);
> > 7600         update_max_interval();
> > 7601         nohz_balance_exit_idle(rq);
> > 7602         hrtick_clear(rq);
> > 7603         return 0;
> > 7604 }
> >
> > The last commit that touches the BUG_ON line is the commit
> > 3015ef4b98f5 ("sched/core: Make migrate disable and CPU hotplug
> cooperative")
> > but the commit looks good to me.
> >
> > Any idea?
> >
> 
> I'd wager this extra task is a kworker; could you give this series a try?
> 
> 
> https ://lore.kernel.org/lkml/20201218170919.2950-1-jiangshanlai@xxxxxxxxx/

Thanks, Valentin! It looks like the patchset can fix the BUG_ON, though I see
a warning, which I reported here: https://lkml.org/lkml/2020/12/22/648

Thanks,
-- Dexuan