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    Subject[tip: sched/core] sched/fair: Default to false in test_idle_cores()
    The following commit has been merged into the sched/core branch of tip:

    Commit-ID: 398ba2b0cc0a43964fe3d2dd19cb2a478f1f220b
    Gitweb: https://git.kernel.org/tip/398ba2b0cc0a43964fe3d2dd19cb2a478f1f220b
    Author: Abel Wu <wuyun.abel@bytedance.com>
    AuthorDate: Wed, 07 Sep 2022 19:19:59 +08:00
    Committer: Peter Zijlstra <peterz@infradead.org>
    CommitterDate: Wed, 07 Sep 2022 21:53:47 +02:00

    sched/fair: Default to false in test_idle_cores()

    It's uncertain whether idle cores exist or not if shared sched-
    domains are not ready, so returning "no idle cores" usually
    makes sense.

    While __update_idle_core() is an exception, it checks status
    of this core and set hint to shared sched-domain if necessary.
    So the whole logic of this function depends on the existence
    of shared sched-domain, and can certainly bail out early if
    it is not available.

    It's somehow a little tricky, and as Josh suggested that it
    should be transient while the domain isn't ready. So remove
    the self-defined default value to make things more clearer.

    Signed-off-by: Abel Wu <wuyun.abel@bytedance.com>
    Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
    Reviewed-by: Josh Don <joshdon@google.com>
    Acked-by: Mel Gorman <mgorman@techsingularity.net>
    Link: https://lore.kernel.org/r/20220907112000.1854-5-wuyun.abel@bytedance.com
    ---
    kernel/sched/fair.c | 16 ++++++++--------
    1 file changed, 8 insertions(+), 8 deletions(-)

    diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
    index 03ce650..23b020c 100644
    --- a/kernel/sched/fair.c
    +++ b/kernel/sched/fair.c
    @@ -1588,11 +1588,11 @@ numa_type numa_classify(unsigned int imbalance_pct,

    #ifdef CONFIG_SCHED_SMT
    /* Forward declarations of select_idle_sibling helpers */
    -static inline bool test_idle_cores(int cpu, bool def);
    +static inline bool test_idle_cores(int cpu);
    static inline int numa_idle_core(int idle_core, int cpu)
    {
    if (!static_branch_likely(&sched_smt_present) ||
    - idle_core >= 0 || !test_idle_cores(cpu, false))
    + idle_core >= 0 || !test_idle_cores(cpu))
    return idle_core;

    /*
    @@ -6271,7 +6271,7 @@ static inline void set_idle_cores(int cpu, int val)
    WRITE_ONCE(sds->has_idle_cores, val);
    }

    -static inline bool test_idle_cores(int cpu, bool def)
    +static inline bool test_idle_cores(int cpu)
    {
    struct sched_domain_shared *sds;

    @@ -6279,7 +6279,7 @@ static inline bool test_idle_cores(int cpu, bool def)
    if (sds)
    return READ_ONCE(sds->has_idle_cores);

    - return def;
    + return false;
    }

    /*
    @@ -6295,7 +6295,7 @@ void __update_idle_core(struct rq *rq)
    int cpu;

    rcu_read_lock();
    - if (test_idle_cores(core, true))
    + if (test_idle_cores(core))
    goto unlock;

    for_each_cpu(cpu, cpu_smt_mask(core)) {
    @@ -6367,9 +6367,9 @@ static inline void set_idle_cores(int cpu, int val)
    {
    }

    -static inline bool test_idle_cores(int cpu, bool def)
    +static inline bool test_idle_cores(int cpu)
    {
    - return def;
    + return false;
    }

    static inline int select_idle_core(struct task_struct *p, int core, struct cpumask *cpus, int *idle_cpu)
    @@ -6608,7 +6608,7 @@ static int select_idle_sibling(struct task_struct *p, int prev, int target)
    return target;

    if (sched_smt_active()) {
    - has_idle_core = test_idle_cores(target, false);
    + has_idle_core = test_idle_cores(target);

    if (!has_idle_core && cpus_share_cache(prev, target)) {
    i = select_idle_smt(p, prev);
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