On Sun, 2014-02-02 at 21:10 +0100, Sebastian Andrzej Siewior wrote:
> According to the backtrace both of them are trying to access the
> per-cpu hrtimer (sched_timer) in order to cancel but they seem to fail
> to get the timer lock here. They shouldn't spin there for minutes, I
> have no idea why they did so…
Hm. per-cpu...
I've been chasing an rt hotplug heisenbug that is pointing to per-cpu
oddness. During sched domain re-construction while running Steven's
stress script on 64 core box, we hit a freshly constructed domain with
_no span_, build_sched_groups()->get_group() explodes when we meeting
it. But if you try to watch the thing appear... it just doesn't.
static int build_sched_domains(const struct cpumask *cpu_map,
struct sched_domain_attr *attr)
{
enum s_alloc alloc_state;
struct sched_domain *sd;
struct s_data d;
int i, ret = -ENOMEM;
alloc_state = __visit_domain_allocation_hell(&d, cpu_map);
if (alloc_state != sa_rootdomain)
goto error;
/* Set up domains for cpus specified by the cpu_map. */
for_each_cpu(i, cpu_map) {
struct sched_domain_topology_level *tl;
sd = NULL;
for_each_sd_topology(tl) {
sd = build_sched_domain(tl, cpu_map, attr, sd, i);
BUG_ON(sd == spanless-alien) here..
if (tl == sched_domain_topology)
*per_cpu_ptr(d.sd, i) = sd;
if (tl->flags & SDTL_OVERLAP || sched_feat(FORCE_SD_OVERLAP))
sd->flags |= SD_OVERLAP;
if (cpumask_equal(cpu_map, sched_domain_span(sd)))
break;
}
}
/* Build the groups for the domains */
for_each_cpu(i, cpu_map) {
for (sd = *per_cpu_ptr(d.sd, i); sd; sd = sd->parent) {
sd->span_weight = cpumask_weight(sched_domain_span(sd));
if (sd->flags & SD_OVERLAP) {
if (build_overlap_sched_groups(sd, i))
goto error;
} else {
if (build_sched_groups(sd, i))
..prevents meeting that alien here.. while hotplug locked.
static int get_group(int cpu, struct sd_data *sdd, struct sched_group **sg)
{
struct sched_domain *sd = *per_cpu_ptr(sdd->sd, cpu);
struct sched_domain *child = sd->child;
if (child)
cpu = cpumask_first(sched_domain_span(child));
^^^nr_cpus
if (sg) {
*sg = *per_cpu_ptr(sdd->sg, cpu); BOOM
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