[tip:sched/core] sched/core: Remove get_cpu() from sched_fork()

[tip-bot for Sebastian Andrzej Siewior]

Commit-ID:  af0fffd9300b97d8875aa745bc78e2f6fdb3c1f0
Gitweb:     https://git.kernel.org/tip/af0fffd9300b97d8875aa745bc78e2f6fdb3c1f0
Author:     Sebastian Andrzej Siewior <bige...@linutronix.de>
AuthorDate: Fri, 6 Jul 2018 15:06:15 +0200
Committer:  Ingo Molnar <mi...@kernel.org>
CommitDate: Mon, 16 Jul 2018 00:16:29 +0200

sched/core: Remove get_cpu() from sched_fork()

get_cpu() disables preemption for the entire sched_fork() function.
This get_cpu() was introduced in commit:

  dd41f596cda0 ("sched: cfs core code")

... which also invoked sched_balance_self() and this function
required preemption do be off.

Today, sched_balance_self() seems to be moved to ->task_fork callback
which is invoked while the ->pi_lock is held.

set_load_weight() could invoke reweight_task() which then via $callchain
might end up in smp_processor_id() but since `update_load' is false
this won't happen.

I didn't find any this_cpu*() or similar usage during the initialisation
of the task_struct.

The `cpu' value (from get_cpu()) is only used later in __set_task_cpu()
while the ->pi_lock lock is held.

Based on this it is possible to remove get_cpu() and use
smp_processor_id() for the `cpu' variable without breaking anything.

Signed-off-by: Sebastian Andrzej Siewior <bige...@linutronix.de>
Signed-off-by: Peter Zijlstra (Intel) <pet...@infradead.org>
Cc: Linus Torvalds <torva...@linux-foundation.org>
Cc: Peter Zijlstra <pet...@infradead.org>
Cc: Thomas Gleixner <t...@linutronix.de>
Link: http://lkml.kernel.org/r/20180706130615.g2ex2kmfu5kcv...@linutronix.de
Signed-off-by: Ingo Molnar <mi...@kernel.org>
---
 kernel/sched/core.c | 13 ++++---------
 1 file changed, 4 insertions(+), 9 deletions(-)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index ba6bb805693a..c3cf7d992159 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -2294,7 +2294,6 @@ static inline void init_schedstats(void) {}
 int sched_fork(unsigned long clone_flags, struct task_struct *p)
 {
        unsigned long flags;
-       int cpu = get_cpu();
 
        __sched_fork(clone_flags, p);
        /*
@@ -2330,14 +2329,12 @@ int sched_fork(unsigned long clone_flags, struct 
task_struct *p)
                p->sched_reset_on_fork = 0;
        }
 
-       if (dl_prio(p->prio)) {
-               put_cpu();
+       if (dl_prio(p->prio))
                return -EAGAIN;
-       } else if (rt_prio(p->prio)) {
+       else if (rt_prio(p->prio))
                p->sched_class = &rt_sched_class;
-       } else {
+       else
                p->sched_class = &fair_sched_class;
-       }
 
        init_entity_runnable_average(&p->se);
 
@@ -2353,7 +2350,7 @@ int sched_fork(unsigned long clone_flags, struct 
task_struct *p)
         * We're setting the CPU for the first time, we don't migrate,
         * so use __set_task_cpu().
         */
-       __set_task_cpu(p, cpu);
+       __set_task_cpu(p, smp_processor_id());
        if (p->sched_class->task_fork)
                p->sched_class->task_fork(p);
        raw_spin_unlock_irqrestore(&p->pi_lock, flags);
@@ -2370,8 +2367,6 @@ int sched_fork(unsigned long clone_flags, struct 
task_struct *p)
        plist_node_init(&p->pushable_tasks, MAX_PRIO);
        RB_CLEAR_NODE(&p->pushable_dl_tasks);
 #endif
-
-       put_cpu();
        return 0;
 }