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sched: Prepare for per-cpu preempt_count

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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: Peter Zijlstra <peterz@infradead.org>
Link: http://lkml.kernel.org/n/tip-4k0b7oy1rcdyzochwiixuwi9@git.kernel.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>
This commit is contained in:
Peter Zijlstra 2013-09-23 19:04:26 +02:00 committed by Ingo Molnar
parent bdb4380658
commit a233f1120c
2 changed files with 9 additions and 5 deletions
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12
include/linux/sched.h
View File

@ -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

2
kernel/sched/core.c
View File

@ -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