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    Subject[PATCH 1/5 v0.6] sched/umcg: add WF_CURRENT_CPU and externise ttwu
    Date
    Add WF_CURRENT_CPU wake flag that advices the scheduler to
    move the wakee to the current CPU. This is useful for fast on-CPU
    context switching use cases such as UMCG.

    In addition, make ttwu external rather than static so that
    the flag could be passed to it from outside of sched/core.c.

    Signed-off-by: Peter Oskolkov <posk@google.com>
    ---
    kernel/sched/core.c | 3 +--
    kernel/sched/fair.c | 4 ++++
    kernel/sched/sched.h | 15 +++++++++------
    3 files changed, 14 insertions(+), 8 deletions(-)

    diff --git a/kernel/sched/core.c b/kernel/sched/core.c
    index b36b5d7f2617..12a9d053e724 100644
    --- a/kernel/sched/core.c
    +++ b/kernel/sched/core.c
    @@ -3878,8 +3878,7 @@ static void ttwu_queue(struct task_struct *p, int cpu, int wake_flags)
    * Return: %true if @p->state changes (an actual wakeup was done),
    * %false otherwise.
    */
    -static int
    -try_to_wake_up(struct task_struct *p, unsigned int state, int wake_flags)
    +int try_to_wake_up(struct task_struct *p, unsigned int state, int wake_flags)
    {
    unsigned long flags;
    int cpu, success = 0;
    diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
    index e26d622762a9..7face28b1830 100644
    --- a/kernel/sched/fair.c
    +++ b/kernel/sched/fair.c
    @@ -6863,6 +6863,10 @@ select_task_rq_fair(struct task_struct *p, int prev_cpu, int wake_flags)
    if (wake_flags & WF_TTWU) {
    record_wakee(p);

    + if ((wake_flags & WF_CURRENT_CPU) &&
    + cpumask_test_cpu(cpu, p->cpus_ptr))
    + return cpu;
    +
    if (sched_energy_enabled()) {
    new_cpu = find_energy_efficient_cpu(p, prev_cpu);
    if (new_cpu >= 0)
    diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
    index 148d5d3255a5..2a79c302ee81 100644
    --- a/kernel/sched/sched.h
    +++ b/kernel/sched/sched.h
    @@ -2040,13 +2040,14 @@ static inline int task_on_rq_migrating(struct task_struct *p)
    }

    /* Wake flags. The first three directly map to some SD flag value */
    -#define WF_EXEC 0x02 /* Wakeup after exec; maps to SD_BALANCE_EXEC */
    -#define WF_FORK 0x04 /* Wakeup after fork; maps to SD_BALANCE_FORK */
    -#define WF_TTWU 0x08 /* Wakeup; maps to SD_BALANCE_WAKE */
    +#define WF_EXEC 0x02 /* Wakeup after exec; maps to SD_BALANCE_EXEC */
    +#define WF_FORK 0x04 /* Wakeup after fork; maps to SD_BALANCE_FORK */
    +#define WF_TTWU 0x08 /* Wakeup; maps to SD_BALANCE_WAKE */

    -#define WF_SYNC 0x10 /* Waker goes to sleep after wakeup */
    -#define WF_MIGRATED 0x20 /* Internal use, task got migrated */
    -#define WF_ON_CPU 0x40 /* Wakee is on_cpu */
    +#define WF_SYNC 0x10 /* Waker goes to sleep after wakeup */
    +#define WF_MIGRATED 0x20 /* Internal use, task got migrated */
    +#define WF_ON_CPU 0x40 /* Wakee is on_cpu */
    +#define WF_CURRENT_CPU 0x80 /* Prefer to move the wakee to the current CPU. */

    #ifdef CONFIG_SMP
    static_assert(WF_EXEC == SD_BALANCE_EXEC);
    @@ -3054,6 +3055,8 @@ static inline bool is_per_cpu_kthread(struct task_struct *p)
    extern void swake_up_all_locked(struct swait_queue_head *q);
    extern void __prepare_to_swait(struct swait_queue_head *q, struct swait_queue *wait);

    +extern int try_to_wake_up(struct task_struct *tsk, unsigned int state, int wake_flags);
    +
    #ifdef CONFIG_PREEMPT_DYNAMIC
    extern int preempt_dynamic_mode;
    extern int sched_dynamic_mode(const char *str);
    --
    2.25.1
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