On 02/03/2014 05:00 AM, Mike Galbraith wrote:

On Sun, 2014-02-02 at 21:10 +0100, Sebastian Andrzej Siewior wrote:

quoted

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..

spanless-alien is?
BUG_ON() is actually _very_ cheap. It shouldn't even create any kind of
compiler barrier which would reload variables / registers. It should
evaluate sd and "spanless-alien", do the compare and then go on.

                        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.

my copy of build_sched_groups() always returns 0 so it never goes to
the error marker. Did you consider a compiler bug? I could try to
rebuild your source + config on two different compilers just to see if
it makes a difference.

Sebastian