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Commit a233f112 authored by Peter Zijlstra's avatar Peter Zijlstra Committed by Ingo Molnar
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sched: Prepare for per-cpu preempt_count 



When using per-cpu preempt_count variables we need to save/restore the
preempt_count on context switch (into per task storage; for instance
the old thread_info::preempt_count variable) because of
PREEMPT_ACTIVE.

However, this means that on fork() the preempt_count value of the last
context switch gets copied and if we had a PREEMPT_ACTIVE switch right
before cloning a child task the child task will now too have
PREEMPT_ACTIVE set and start its life with an extra PREEMPT_ACTIVE
count.

Therefore we need to make init_task_preempt_count() unconditional;
this resets whatever preempt_count we inherited from our parent
process.

Doing so for !per-cpu implementations is harmless.

For !PREEMPT_COUNT kernels we need to be careful not to start life
with an increased preempt_count.

Signed-off-by: default avatarPeter Zijlstra <peterz@infradead.org>
Link: http://lkml.kernel.org/n/tip-4k0b7oy1rcdyzochwiixuwi9@git.kernel.org


Signed-off-by: default avatarIngo Molnar <mingo@kernel.org>
parent bdb43806

include/linux/sched.h

+9 −3
Original line number Diff line number Diff line
@@ -428,6 +428,14 @@ struct task_cputime { 
		.sum_exec_runtime = 0,				\

	}



#define PREEMPT_ENABLED		(PREEMPT_NEED_RESCHED)



#ifdef CONFIG_PREEMPT_COUNT

#define PREEMPT_DISABLED	(1 + PREEMPT_ENABLED)

#else

#define PREEMPT_DISABLED	PREEMPT_ENABLED

#endif



/*

 * Disable preemption until the scheduler is running.

 * Reset by start_kernel()->sched_init()->init_idle().
@@ -435,9 +443,7 @@ struct task_cputime { 
 * We include PREEMPT_ACTIVE to avoid cond_resched() from working

 * before the scheduler is active -- see should_resched().

 */

#define INIT_PREEMPT_COUNT	(1 + PREEMPT_ACTIVE + PREEMPT_NEED_RESCHED)

#define PREEMPT_ENABLED		(PREEMPT_NEED_RESCHED)

#define PREEMPT_DISABLED	(1 + PREEMPT_NEED_RESCHED)

#define INIT_PREEMPT_COUNT	(PREEMPT_DISABLED + PREEMPT_ACTIVE)



/**

 * struct thread_group_cputimer - thread group interval timer counts

kernel/sched/core.c

+0 −2
Original line number Diff line number Diff line
@@ -1722,9 +1722,7 @@ void sched_fork(struct task_struct *p) 
#if defined(CONFIG_SMP)

	p->on_cpu = 0;

#endif

#ifdef CONFIG_PREEMPT_COUNT

	init_task_preempt_count(p);

#endif

#ifdef CONFIG_SMP

	plist_node_init(&p->pushable_tasks, MAX_PRIO);

#endif