Subject: Re: -next: Nov 12 - kernel BUG at kernel/sched.c:7359!

From: Sachin Sant <[email-masked]>
Date: Mon, 23 Nov 2009 15:23:43 +0530

Peter Zijlstra wrote:
> Well, it boots for me, but then, I've not been able to reproduce any
> issues anyway :/
>
> /me goes try a PREEMPT=n kernel, since that is what Mike reports boot
> funnies with..
>
> Full running diff against -tip:
>   
Peter i still can recreate this issue with today's next(20091123).
Looks like the following patch haven't been merged yet.

Thanks
-Sachin

> ---
> diff --git a/kernel/sched.c b/kernel/sched.c
> index 1f2e99d..7089063 100644
> --- a/kernel/sched.c
> +++ b/kernel/sched.c
> @@ -2374,17 +2374,24 @@ static int try_to_wake_up(struct task_struct *p, unsigned int state,
>  	if (task_contributes_to_load(p))
>  		rq->nr_uninterruptible--;
>  	p->state = TASK_WAKING;
> -	task_rq_unlock(rq, &flags);
> +	__task_rq_unlock(rq);
>
> +again:
>  	cpu = p->sched_class->select_task_rq(p, SD_BALANCE_WAKE, wake_flags);
> +	if (!cpu_online(cpu))
> +		cpu = cpumask_any_and(&p->cpus_allowed, cpu_online_mask);
> +	if (cpu >= nr_cpu_ids) {
> +		printk(KERN_ERR "Breaking affinity on %d/%s\n", p->pid, p->comm);
> +		cpuset_cpus_allowed_locked(p, &p->cpus_allowed);
> +		goto again;
> +	}
> +
>  	if (cpu != orig_cpu) {
> -		local_irq_save(flags);
>  		rq = cpu_rq(cpu);
>  		update_rq_clock(rq);
>  		set_task_cpu(p, cpu);
> -		local_irq_restore(flags);
>  	}
> -	rq = task_rq_lock(p, &flags);
> +	rq = __task_rq_lock(p);
>
>  	WARN_ON(p->state != TASK_WAKING);
>  	cpu = task_cpu(p);
> @@ -7620,6 +7627,8 @@ migration_call(struct notifier_block *nfb, unsigned long action, void *hcpu)
>  	unsigned long flags;
>  	struct rq *rq;
>
> +	printk(KERN_ERR "migration call\n");
> +
>  	switch (action) {
>
>  	case CPU_UP_PREPARE:
> @@ -9186,6 +9195,8 @@ int __init sched_create_sysfs_power_savings_entries(struct sysdev_class *cls)
>  static int update_sched_domains(struct notifier_block *nfb,
>  				unsigned long action, void *hcpu)
>  {
> +	printk(KERN_ERR "update_sched_domains\n");
> +
>  	switch (action) {
>  	case CPU_ONLINE:
>  	case CPU_ONLINE_FROZEN:
> diff --git a/kernel/sched_fair.c b/kernel/sched_fair.c
> index 5488a5d..0ff21af 100644
> --- a/kernel/sched_fair.c
> +++ b/kernel/sched_fair.c
> @@ -1345,6 +1345,37 @@ find_idlest_cpu(struct sched_group *group, struct task_struct *p, int this_cpu)
>  }
>
>  /*
> + * Try and locate an idle CPU in the sched_domain.
> + */
> +static int
> +select_idle_sibling(struct task_struct *p, struct sched_domain *sd, int target)
> +{
> +	int cpu = smp_processor_id();
> +	int prev_cpu = task_cpu(p);
> +	int i;
> +
> +	/*
> +	 * If this domain spans both cpu and prev_cpu (see the SD_WAKE_AFFINE
> +	 * test in select_task_rq_fair) and the prev_cpu is idle then that's
> +	 * always a better target than the current cpu.
> +	 */
> +	if (target == cpu && !cpu_rq(prev_cpu)->cfs.nr_running)
> +		return prev_cpu;
> +
> +	/*
> +	 * Otherwise, iterate the domain and find an elegible idle cpu.
> +	 */
> +	for_each_cpu_and(i, sched_domain_span(sd), &p->cpus_allowed) {
> +		if (!cpu_rq(i)->cfs.nr_running) {
> +			target = i;
> +			break;
> +		}
> +	}
> +
> +	return target;
> +}
> +
> +/*
>   * sched_balance_self: balance the current task (running on cpu) in domains
>   * that have the 'flag' flag set. In practice, this is SD_BALANCE_FORK and
>   * SD_BALANCE_EXEC.
> @@ -1398,37 +1429,34 @@ static int select_task_rq_fair(struct task_struct *p, int sd_flag, int wake_flag
>  				want_sd = 0;
>  		}
>
> -		if (want_affine && (tmp->flags & SD_WAKE_AFFINE)) {
> -			int candidate = -1, i;
> +		/*
> +		 * While iterating the domains looking for a spanning
> +		 * WAKE_AFFINE domain, adjust the affine target to any idle cpu
> +		 * in cache sharing domains along the way.
> +		 */
> +		if (want_affine) {
> +			int target = -1;
>
> +			/*
> +			 * If both cpu and prev_cpu are part of this domain,
> +			 * cpu is a valid SD_WAKE_AFFINE target.
> +			 */
>  			if (cpumask_test_cpu(prev_cpu, sched_domain_span(tmp)))
> -				candidate = cpu;
> +				target = cpu;
>
>  			/*
> -			 * Check for an idle shared cache.
> +			 * If there's an idle sibling in this domain, make that
> +			 * the wake_affine target instead of the current cpu.
>  			 */
> -			if (tmp->flags & SD_PREFER_SIBLING) {
> -				if (candidate == cpu) {
> -					if (!cpu_rq(prev_cpu)->cfs.nr_running)
> -						candidate = prev_cpu;
> -				}
> +			if (tmp->flags & SD_PREFER_SIBLING)
> +				target = select_idle_sibling(p, tmp, target);
>
> -				if (candidate == -1 || candidate == cpu) {
> -					for_each_cpu(i, sched_domain_span(tmp)) {
> -						if (!cpumask_test_cpu(i, &p->cpus_allowed))
> -							continue;
> -						if (!cpu_rq(i)->cfs.nr_running) {
> -							candidate = i;
> -							break;
> -						}
> -					}
> +			if (target >= 0) {
> +				if (tmp->flags & SD_WAKE_AFFINE) {
> +					affine_sd = tmp;
> +					want_affine = 0;
>  				}
> -			}
> -
> -			if (candidate >= 0) {
> -				affine_sd = tmp;
> -				want_affine = 0;
> -				cpu = candidate;
> +				cpu = target;
>  			}
>  		}
>
>
>
>   

-- 

---------------------------------
Sachin Sant
IBM Linux Technology Center
India Systems and Technology Labs
Bangalore, India
---------------------------------