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    SubjectRe: [PATCH v8 -tip 02/26] sched: Introduce sched_class::pick_task()
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    On 2020/10/24 5:47, Joel Fernandes wrote:
    > On Fri, Oct 23, 2020 at 01:25:38PM +0800, Li, Aubrey wrote:
    >>>>> @@ -2517,6 +2528,7 @@ const struct sched_class dl_sched_class
    >>>>>
    >>>>> #ifdef CONFIG_SMP
    >>>>> .balance = balance_dl,
    >>>>> + .pick_task = pick_task_dl,
    >>>>> .select_task_rq = select_task_rq_dl,
    >>>>> .migrate_task_rq = migrate_task_rq_dl,
    >>>>> .set_cpus_allowed = set_cpus_allowed_dl,
    >>>>> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
    >>>>> index dbd9368a959d..bd6aed63f5e3 100644
    >>>>> --- a/kernel/sched/fair.c
    >>>>> +++ b/kernel/sched/fair.c
    >>>>> @@ -4450,7 +4450,7 @@ pick_next_entity(struct cfs_rq *cfs_rq, struct sched_entity *curr)
    >>>>> * Avoid running the skip buddy, if running something else can
    >>>>> * be done without getting too unfair.
    >>>>> */
    >>>>> - if (cfs_rq->skip == se) {
    >>>>> + if (cfs_rq->skip && cfs_rq->skip == se) {
    >>>>> struct sched_entity *second;
    >>>>>
    >>>>> if (se == curr) {
    >>>>> @@ -6976,6 +6976,35 @@ static void check_preempt_wakeup(struct rq *rq, struct task_struct *p, int wake_
    >>>>> set_last_buddy(se);
    >>>>> }
    >>>>>
    >>>>> +#ifdef CONFIG_SMP
    >>>>> +static struct task_struct *pick_task_fair(struct rq *rq)
    >>>>> +{
    >>>>> + struct cfs_rq *cfs_rq = &rq->cfs;
    >>>>> + struct sched_entity *se;
    >>>>> +
    >>>>> + if (!cfs_rq->nr_running)
    >>>>> + return NULL;
    >>>>> +
    >>>>> + do {
    >>>>> + struct sched_entity *curr = cfs_rq->curr;
    >>>>> +
    >>>>> + se = pick_next_entity(cfs_rq, NULL);
    >>>>> +
    >>>>> + if (curr) {
    >>>>> + if (se && curr->on_rq)
    >>>>> + update_curr(cfs_rq);
    >>>>> +
    >>>>> + if (!se || entity_before(curr, se))
    >>>>> + se = curr;
    >>>>> + }
    >>>>> +
    >>>>> + cfs_rq = group_cfs_rq(se);
    >>>>> + } while (cfs_rq);
    >>>>> ++
    >>>>> + return task_of(se);
    >>>>> +}
    >>>>> +#endif
    >>>>
    >>>> One of my machines hangs when I run uperf with only one message:
    >>>> [ 719.034962] BUG: kernel NULL pointer dereference, address: 0000000000000050
    >>>>
    >>>> Then I replicated the problem on my another machine(no serial console),
    >>>> here is the stack by manual copy.
    >>>>
    >>>> Call Trace:
    >>>> pick_next_entity+0xb0/0x160
    >>>> pick_task_fair+0x4b/0x90
    >>>> __schedule+0x59b/0x12f0
    >>>> schedule_idle+0x1e/0x40
    >>>> do_idle+0x193/0x2d0
    >>>> cpu_startup_entry+0x19/0x20
    >>>> start_secondary+0x110/0x150
    >>>> secondary_startup_64_no_verify+0xa6/0xab
    >>>
    >>> Interesting. Wondering if we screwed something up in the rebase.
    >>>
    >>> Questions:
    >>> 1. Does the issue happen if you just apply only up until this patch,
    >>> or the entire series?
    >>
    >> I applied the entire series and just find a related patch to report the
    >> issue.
    >
    > Ok.
    >
    >>> 2. Do you see the issue in v7? Not much if at all has changed in this
    >>> part of the code from v7 -> v8 but could be something in the newer
    >>> kernel.
    >>>
    >>
    >> IIRC, I can run uperf successfully on v7.
    >> I'm on tip/master 2d3e8c9424c9 (origin/master) "Merge branch 'linus'."
    >> Please let me know if this is a problem, or you have a repo I can pull
    >> for testing.
    >
    > Here is a repo with v8 series on top of v5.9 release:
    > https://git.kernel.org/pub/scm/linux/kernel/git/jfern/linux.git/log/?h=coresched-v5.9

    I didn't see NULL pointer dereference BUG of this repo, will post performance
    data later.

    Thanks,
    -Aubrey

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