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    Subject[PATCH] nohz/cpuset: enable addition&removal of cpus while in adaptive nohz mode
    Currently modifying cpuset.cpus mask of a cgroup does not
    update the reference counters for adaptive nohz mode if the
    cpuset already had cpuset.adaptive_nohz == 1. Fix it so that
    cpus can be added or removed from a adaptive_nohz cpuset.

    Apply to Frederic's nohz/cpuset-v2 kernel 3.3.0-rc7nohz/cpuset-v2+
    (commit abd48ecb412ff023582e97442a02a7cb162a1c4e)

    Signed-off-by: Hakan Akkan <hakanakkan@gmail.com>
    ---
    kernel/cpuset.c | 107 ++++++++++++++++++++++++++++++++++++-------------------
    1 files changed, 70 insertions(+), 37 deletions(-)

    diff --git a/kernel/cpuset.c b/kernel/cpuset.c
    index aa8304d..1446210 100644
    --- a/kernel/cpuset.c
    +++ b/kernel/cpuset.c
    @@ -862,6 +862,8 @@ static void update_tasks_cpumask(struct cpuset *cs, struct ptr_heap *heap)
    cgroup_scan_tasks(&scan);
    }

    +static void update_nohz_cpus(struct cpuset *old_cs, struct cpuset *cs);
    +
    /**
    * update_cpumask - update the cpus_allowed mask of a cpuset and all tasks in it
    * @cs: the cpuset to consider
    @@ -902,6 +904,11 @@ static int update_cpumask(struct cpuset *cs, struct cpuset *trialcs,
    if (cpumask_equal(cs->cpus_allowed, trialcs->cpus_allowed))
    return 0;

    + /*
    + * Update adaptive nohz bits.
    + */
    + update_nohz_cpus(cs,trialcs);
    +
    retval = heap_init(&heap, PAGE_SIZE, GFP_KERNEL, NULL);
    if (retval)
    return retval;
    @@ -1247,51 +1254,74 @@ static void cpu_exit_nohz(int cpu)
    preempt_enable();
    }

    -static void update_nohz_cpus(struct cpuset *old_cs, struct cpuset *cs)
    +static void update_cpu_nohz_flag(int cpu, int adjust)
    +{
    + int ref = (per_cpu(cpu_adaptive_nohz_ref, cpu) += adjust);
    + printk("cpu %d, adjust %d, ref %d\n", cpu, adjust, ref);
    +
    + if (ref == 1 && adjust > 0) {
    + cpumask_set_cpu(cpu, &nohz_cpuset_mask);
    + /*
    + * The mask update needs to be visible right away
    + * so that this CPU is part of the cputime IPI
    + * update right now.
    + */
    + smp_mb();
    + } else if (!ref) {
    + /*
    + * The update to cpu_adaptive_nohz_ref must be
    + * visible right away. So that once we restart the tick
    + * from the IPI, it won't be stopped again due to cache
    + * update lag.
    + * FIXME: We probably need more to ensure this value is really
    + * visible right away.
    + */
    + smp_mb();
    + cpu_exit_nohz(cpu);
    + /*
    + * Now that the tick has been restarted and cputimes
    + * flushed, we don't need anymore to be part of the
    + * cputime flush IPI.
    + */
    + cpumask_clear_cpu(cpu, &nohz_cpuset_mask);
    + }
    +}
    +
    +static void update_nohz_flag(struct cpuset *old_cs, struct cpuset *cs)
    {
    int cpu;
    - int val;
    + int adjust;

    if (is_adaptive_nohz(old_cs) == is_adaptive_nohz(cs))
    return;

    + adjust = is_adaptive_nohz(cs) ? 1 : -1;
    for_each_cpu(cpu, cs->cpus_allowed) {
    - if (is_adaptive_nohz(cs))
    - per_cpu(cpu_adaptive_nohz_ref, cpu) += 1;
    - else
    - per_cpu(cpu_adaptive_nohz_ref, cpu) -= 1;
    -
    - val = per_cpu(cpu_adaptive_nohz_ref, cpu);
    -
    - if (val == 1) {
    - cpumask_set_cpu(cpu, &nohz_cpuset_mask);
    - /*
    - * The mask update needs to be visible right away
    - * so that this CPU is part of the cputime IPI
    - * update right now.
    - */
    - smp_mb();
    - } else if (!val) {
    - /*
    - * The update to cpu_adaptive_nohz_ref must be
    - * visible right away. So that once we restart the tick
    - * from the IPI, it won't be stopped again due to cache
    - * update lag.
    - * FIXME: We probably need more to ensure this value is really
    - * visible right away.
    - */
    - smp_mb();
    - cpu_exit_nohz(cpu);
    - /*
    - * Now that the tick has been restarted and cputimes
    - * flushed, we don't need anymore to be part of the
    - * cputime flush IPI.
    - */
    - cpumask_clear_cpu(cpu, &nohz_cpuset_mask);
    - }
    + update_cpu_nohz_flag(cpu, adjust);
    }
    }
    +
    +static void update_nohz_cpus(struct cpuset *old_cs, struct cpuset *cs)
    +{
    + int cpu;
    + cpumask_t cpus;
    +
    + /*
    + * Only bother if the cpuset has adaptive nohz
    + */
    + if (!is_adaptive_nohz(cs))
    + return;
    +
    + cpumask_xor(&cpus, old_cs->cpus_allowed, cs->cpus_allowed);
    +
    + for_each_cpu(cpu, &cpus)
    + update_cpu_nohz_flag(cpu,
    + cpumask_test_cpu(cpu, cs->cpus_allowed) ? 1 : -1);
    +}
    #else
    +static inline void update_nohz_flag(struct cpuset *old_cs, struct cpuset *cs)
    +{
    +}
    static inline void update_nohz_cpus(struct cpuset *old_cs, struct cpuset *cs)
    {
    }
    @@ -1362,7 +1392,7 @@ static int update_flag(cpuset_flagbits_t bit, struct cpuset *cs,
    spread_flag_changed = ((is_spread_slab(cs) != is_spread_slab(trialcs))
    || (is_spread_page(cs) != is_spread_page(trialcs)));

    - update_nohz_cpus(cs, trialcs);
    + update_nohz_flag(cs, trialcs);

    mutex_lock(&callback_mutex);
    cs->flags = trialcs->flags;
    @@ -2006,7 +2036,8 @@ static struct cgroup_subsys_state *cpuset_create(
    /*
    * If the cpuset being removed has its flag 'sched_load_balance'
    * enabled, then simulate turning sched_load_balance off, which
    - * will call async_rebuild_sched_domains().
    + * will call async_rebuild_sched_domains(). Also update adaptive
    + * nohz flag.
    */

    static void cpuset_destroy(struct cgroup_subsys *ss, struct cgroup *cont)
    @@ -2016,6 +2047,8 @@ static void cpuset_destroy(struct cgroup_subsys *ss, struct cgroup *cont)
    if (is_sched_load_balance(cs))
    update_flag(CS_SCHED_LOAD_BALANCE, cs, 0);

    + update_flag(CS_ADAPTIVE_NOHZ, cs, 0);
    +
    number_of_cpusets--;
    free_cpumask_var(cs->cpus_allowed);
    kfree(cs);


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