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    SubjectRe: [RESEND, v3, 2/2] sched/deadline: Initialize cp->elements[].cpu to an invalid value
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    On 2018-06-01 15:18, Joel Fernandes wrote:
    > On Thu, May 31, 2018 at 11:12:55PM -0700, Joel Fernandes wrote:
    >> On Mon, Jan 08, 2018 at 03:14:41PM +0900, byungchul park wrote:
    >>> Currently, migrating tasks to cpu0 unconditionally happens when the
    >>> heap is empty, since cp->elements[].cpu was initialized to 0(=cpu0).
    >>> We have to distinguish between the empty case and cpu0 to avoid the
    >>> unnecessary migrations. Therefore, it has to return an invalid value
    >>> e.i. -1 in that case.
    >>>
    >>> Signed-off-by: Byungchul Park <byungchul.park@lge.com>
    >>> Acked-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
    >>> Acked-by: Daniel Bristot de Oliveira <bristot@redhat.com>
    >>> ---
    >>> kernel/sched/cpudeadline.c | 10 +++++++++-
    >>> 1 file changed, 9 insertions(+), 1 deletion(-)
    >>>
    >>> diff --git a/kernel/sched/cpudeadline.c b/kernel/sched/cpudeadline.c
    >>> index 9f02035..bcf903f 100644
    >>> --- a/kernel/sched/cpudeadline.c
    >>> +++ b/kernel/sched/cpudeadline.c
    >>> @@ -138,6 +138,12 @@ int cpudl_find(struct cpudl *cp, struct task_struct *p,
    >>> int best_cpu = cpudl_maximum_cpu(cp);
    >>> WARN_ON(best_cpu != -1 && !cpu_present(best_cpu));
    >>>
    >>> + /*
    >>> + * The heap tree is empty for now, just return.
    >>> + */
    >>> + if (best_cpu == -1)
    >>> + return 0;
    >>> +
    >>> if (cpumask_test_cpu(best_cpu, &p->cpus_allowed) &&
    >>> dl_time_before(dl_se->deadline, cpudl_maximum_dl(cp))) {
    >>> if (later_mask)
    >>> @@ -265,8 +271,10 @@ int cpudl_init(struct cpudl *cp)
    >>> return -ENOMEM;
    >>> }
    >>>
    >>> - for_each_possible_cpu(i)
    >>> + for_each_possible_cpu(i) {
    >>> + cp->elements[i].cpu = -1;
    >>> cp->elements[i].idx = IDX_INVALID;
    >>
    >> Shouldn't you also set cp->elements[cpu].cpu to -1 in cpudl_clear (when you
    >> set cp->elements[cpu].cpu to IDX_INVALID there)?
    >
    > I messed up my words, I meant : "when setting cp->elements[cpu].idx to
    > IDX_INVALID there". Which means I need to call it a day :-)

    Ah.. I agree with you. It might be a problem when removing the last
    element.. Then I think the following change should also be applied
    additionally. Right?

    ---
    diff --git a/kernel/sched/cpudeadline.c b/kernel/sched/cpudeadline.c
    index 8d9562d..44d4c88 100644
    --- a/kernel/sched/cpudeadline.c
    +++ b/kernel/sched/cpudeadline.c
    @@ -172,12 +172,14 @@ void cpudl_clear(struct cpudl *cp, int cpu)
    } else {
    new_cpu = cp->elements[cp->size - 1].cpu;
    cp->elements[old_idx].dl = cp->elements[cp->size - 1].dl;
    - cp->elements[old_idx].cpu = new_cpu;
    + cp->elements[old_idx].cpu = (new_cpu == cpu) ? -1 : new_cpu;
    cp->size--;
    - cp->elements[new_cpu].idx = old_idx;
    cp->elements[cpu].idx = IDX_INVALID;
    - cpudl_heapify(cp, old_idx);

    + if (new_cpu != cpu) {
    + cp->elements[new_cpu].idx = old_idx;
    + cpudl_heapify(cp, old_idx);
    + }
    cpumask_set_cpu(cpu, cp->free_cpus);
    }
    raw_spin_unlock_irqrestore(&cp->lock, flags);
    --
    Thanks,
    Byungchul

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