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SubjectRe: [BUG] circular lock dependency in tip

On Thu, 19 Mar 2009, Rusty Russell wrote:
>
> We use the same trick in load_balance() and load_balance_newidle(): the
> cpumask is a temporary so we can eliminate busy queues with all tasks pinned.
>
> This actually only addresses a subset of the problem though: if we have a
> heap of tasks which are pinned to two cpus, this logic doesn't help us.
>
> We could keep separate cpu load measuring only movable processes, but that
> seems like optimizing for the wrong case. In fact, allocating here seems
> like optimizing for the correct case. Since we can't do that, a per-cpu
> scratch mask seems in order. It's messier tho.
>
> Does this solve it? (Untested).

It compiled and booted, and I dont see the lockdep bug. But that did not
show up all the time. I guess it took a special case to cause the kmalloc
lock -> run queue lock to happen. But since we do not allocate here
anymore, this should be safe.

-- Steve

>
> diff --git a/kernel/sched.c b/kernel/sched.c
> index 5dabd80..48862d4 100644
> --- a/kernel/sched.c
> +++ b/kernel/sched.c
> @@ -3448,19 +3448,23 @@ find_busiest_queue(struct sched_group *group, enum cpu_idle_type idle,
> */
> #define MAX_PINNED_INTERVAL 512
>
> +/* Working cpumask for load_balance and load_balance_newidle. */
> +static DEFINE_PER_CPU(cpumask_var_t, load_balance_tmpmask);
> +
> /*
> * Check this_cpu to ensure it is balanced within domain. Attempt to move
> * tasks if there is an imbalance.
> */
> static int load_balance(int this_cpu, struct rq *this_rq,
> struct sched_domain *sd, enum cpu_idle_type idle,
> - int *balance, struct cpumask *cpus)
> + int *balance)
> {
> int ld_moved, all_pinned = 0, active_balance = 0, sd_idle = 0;
> struct sched_group *group;
> unsigned long imbalance;
> struct rq *busiest;
> unsigned long flags;
> + struct cpumask *cpus = __get_cpu_var(load_balance_tmpmask);
>
> cpumask_setall(cpus);
>
> @@ -3615,8 +3619,7 @@ out:
> * this_rq is locked.
> */
> static int
> -load_balance_newidle(int this_cpu, struct rq *this_rq, struct sched_domain *sd,
> - struct cpumask *cpus)
> +load_balance_newidle(int this_cpu, struct rq *this_rq, struct sched_domain *sd)
> {
> struct sched_group *group;
> struct rq *busiest = NULL;
> @@ -3624,6 +3627,7 @@ load_balance_newidle(int this_cpu, struct rq *this_rq, struct sched_domain *sd,
> int ld_moved = 0;
> int sd_idle = 0;
> int all_pinned = 0;
> + struct cpumask *cpus = __get_cpu_var(load_balance_tmpmask);
>
> cpumask_setall(cpus);
>
> @@ -3764,10 +3768,6 @@ static void idle_balance(int this_cpu, struct rq *this_rq)
> struct sched_domain *sd;
> int pulled_task = 0;
> unsigned long next_balance = jiffies + HZ;
> - cpumask_var_t tmpmask;
> -
> - if (!alloc_cpumask_var(&tmpmask, GFP_ATOMIC))
> - return;
>
> for_each_domain(this_cpu, sd) {
> unsigned long interval;
> @@ -3778,7 +3778,7 @@ static void idle_balance(int this_cpu, struct rq *this_rq)
> if (sd->flags & SD_BALANCE_NEWIDLE)
> /* If we've pulled tasks over stop searching: */
> pulled_task = load_balance_newidle(this_cpu, this_rq,
> - sd, tmpmask);
> + sd);
>
> interval = msecs_to_jiffies(sd->balance_interval);
> if (time_after(next_balance, sd->last_balance + interval))
> @@ -3793,7 +3793,6 @@ static void idle_balance(int this_cpu, struct rq *this_rq)
> */
> this_rq->next_balance = next_balance;
> }
> - free_cpumask_var(tmpmask);
> }
>
> /*
> @@ -3943,11 +3942,6 @@ static void rebalance_domains(int cpu, enum cpu_idle_type idle)
> unsigned long next_balance = jiffies + 60*HZ;
> int update_next_balance = 0;
> int need_serialize;
> - cpumask_var_t tmp;
> -
> - /* Fails alloc? Rebalancing probably not a priority right now. */
> - if (!alloc_cpumask_var(&tmp, GFP_ATOMIC))
> - return;
>
> for_each_domain(cpu, sd) {
> if (!(sd->flags & SD_LOAD_BALANCE))
> @@ -3972,7 +3966,7 @@ static void rebalance_domains(int cpu, enum cpu_idle_type idle)
> }
>
> if (time_after_eq(jiffies, sd->last_balance + interval)) {
> - if (load_balance(cpu, rq, sd, idle, &balance, tmp)) {
> + if (load_balance(cpu, rq, sd, idle, &balance)) {
> /*
> * We've pulled tasks over so either we're no
> * longer idle, or one of our SMT siblings is
> @@ -4006,8 +4000,6 @@ out:
> */
> if (likely(update_next_balance))
> rq->next_balance = next_balance;
> -
> - free_cpumask_var(tmp);
> }
>
> /*
> @@ -8304,6 +8296,9 @@ void __init sched_init(void)
> #ifdef CONFIG_USER_SCHED
> alloc_size *= 2;
> #endif
> +#ifdef CONFIG_CPUMASK_OFFSTACK
> + alloc_size *= num_possible_cpus() * cpumask_size();
> +#endif
> /*
> * As sched_init() is called before page_alloc is setup,
> * we use alloc_bootmem().
> @@ -8341,6 +8336,12 @@ void __init sched_init(void)
> ptr += nr_cpu_ids * sizeof(void **);
> #endif /* CONFIG_USER_SCHED */
> #endif /* CONFIG_RT_GROUP_SCHED */
> +#ifdef CONFIG_CPUMASK_OFFSTACK
> + for_each_possible_cpu(i) {
> + per_cpu(load_balance_tmpmask, i) = (void *)ptr;
> + ptr += cpumask_size();
> + }
> +#endif /* CONFIG_CPUMASK_OFFSTACK */
> }
>
> #ifdef CONFIG_SMP
>
>
>


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