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Subject: Re: CPU hotplug broken in 2.6.8-rc2 ?
From: Nathan Lynch <nathanl@austin.ibm.com>
To: dipankar@in.ibm.com
Cc: V Srivatsa <vatsa@in.ibm.com>, Joel Schopp <jschopp@austin.ibm.com>,
       Rusty Russell <rusty@rustcorp.com.au>,
       lkml <linux-kernel@vger.kernel.org>,
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On Mon, 2004-08-02 at 15:26, Nathan Lynch wrote:
> On Mon, 2004-08-02 at 14:38, Nathan Lynch wrote:
> > Could you try on 2.6.8-rc2-mm2 along with this patch?  Vatsa had a patch
> > go in that should prevent the crash you are seeing -- the patch below is
> > needed to prevent the same crash in the offline case.  This check used
> > to be in load_balance and some other scheduler functions, iirc; does
> > anyone know why they were removed?
> 
> Er, I meant to put the check in rebalance_tick, not load_balance.
> 
> However, after a few minutes with this, I hit the BUG_ON in the CPU_DEAD
> case in migration_call; not sure whether this is a separate issue.

So, with the cpu_is_offline check in rebalance_tick on top of
2.6.8-rc2-mm2, this is the BUG_ON in migration_call I tend to hit while
hotplugging cpus as quickly as possible while running make -j 40:

        case CPU_DEAD:
                migrate_all_tasks(cpu);
                rq = cpu_rq(cpu);
                kthread_stop(rq->migration_thread);
                rq->migration_thread = NULL;
                /* Idle task back to normal (off runqueue, low prio) */
                rq = task_rq_lock(rq->idle, &flags);
                deactivate_task(rq->idle, rq);
                rq->idle->static_prio = MAX_PRIO;
                __setscheduler(rq->idle, SCHED_NORMAL, 0);
                task_rq_unlock(rq, &flags);
                BUG_ON(rq->nr_running != 0);

I can reproduce this on both ppc64 and i386.  Does anyone know why this
is happening?

If I remove the BUG_ON, things seem to go ok, but I doubt that's the
right thing to do.

Nathan