sched: Prepare for per-cpu preempt_count (a233f112) · Commits · 戴 / test

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>

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

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