Commit af0fffd9 authored Jul 06, 2018 by Sebastian Andrzej Siewior Committed by Ingo Molnar Jul 16, 2018

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:

  dd41f596 ("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 <bigeasy@linutronix.de>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/20180706130615.g2ex2kmfu5kcvlq6@linutronix.de


Signed-off-by: Ingo Molnar <mingo@kernel.org>

parent 45f5519e

kernel/sched/core.c

+4 −9

Original line number	Diff line number	Diff line
		@@ -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;
		}