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Commit 2cec55a2 authored by Joonwoo Park's avatar Joonwoo Park
Browse files

sched: Prevent race conditions where upmigrate_min_nice changes 



When upmigrate_min_nice is changed dec_nr_big_small_task() can trigger
BUG_ON(rq->nr_big_tasks < 0).  This happens when there is a task which was
considered as non-big task due to its nice > upmigrate_min_nice and later
upmigrate_min_nice is changed to higher value so the task becomes big task.
In this case runqueue still has nr_big_tasks = 0 incorrectly with current
implementation.  Consequently next scheduler tick sees a big task to
schedule and try to decrease nr_big_tasks which is already 0.

Introduce sched_upmigrate_min_nice which is updated atomically and re-count
the number of big and small tasks to fix BUG_ON() triggering.

Change-Id: I6f5fc62ed22bbe5c52ec71613082a6e64f406e58
Signed-off-by: default avatarJoonwoo Park <joonwoop@codeaurora.org>
parent f6e9e18e

kernel/sched/fair.c

+37 −15
Original line number Diff line number Diff line
@@ -1324,6 +1324,7 @@ unsigned int __read_mostly sysctl_sched_downmigrate_pct = 60; 
 * Tasks whose nice value is > sysctl_sched_upmigrate_min_nice are never

 * considered as "big" tasks.

 */

static int __read_mostly sched_upmigrate_min_nice = 15;

int __read_mostly sysctl_sched_upmigrate_min_nice = 15;



/*
@@ -1374,6 +1375,8 @@ void set_hmp_defaults(void) 
	sched_init_task_load_windows =

		div64_u64((u64)sysctl_sched_init_task_load_pct *

			  (u64)sched_ravg_window, 100);



	sched_upmigrate_min_nice = sysctl_sched_upmigrate_min_nice;

}



u32 sched_get_init_task_load(struct task_struct *p)
@@ -1470,7 +1473,7 @@ static inline int is_big_task(struct task_struct *p) 
	int nice = TASK_NICE(p);



	/* Todo: Provide cgroup-based control as well? */

	if (nice > sysctl_sched_upmigrate_min_nice)

	if (nice > sched_upmigrate_min_nice)

		return 0;



	load = scale_load_to_cpu(load, task_cpu(p));
@@ -1658,7 +1661,7 @@ static int task_will_fit(struct task_struct *p, int cpu) 
			return 1;

	} else {

		/* Todo: Provide cgroup-based control as well? */

		if (nice > sysctl_sched_upmigrate_min_nice)

		if (nice > sched_upmigrate_min_nice)

			return 1;



		load = scale_load_to_cpu(task_load(p), cpu);
@@ -2197,11 +2200,16 @@ int sched_hmp_proc_update_handler(struct ctl_table *table, int write, 
	int ret;

	unsigned int *data = (unsigned int *)table->data;

	unsigned int old_val = *data;

	int update_min_nice = 0;



	ret = proc_dointvec_minmax(table, write, buffer, lenp, ppos);



	if (ret || !write || !sched_enable_hmp)

		return ret;



	if (write && (old_val == *data))

		return 0;



	if (data == &sysctl_sched_min_runtime) {

		sched_min_runtime = ((u64) sysctl_sched_min_runtime) * 1000;

		return 0;
@@ -2212,6 +2220,23 @@ int sched_hmp_proc_update_handler(struct ctl_table *table, int write, 
		return -EINVAL;

	}



	if (data == (unsigned int *)&sysctl_sched_upmigrate_min_nice)

		update_min_nice = 1;



	if (update_min_nice) {

		if ((*(int *)data) < -20 || (*(int *)data) > 19) {

			*data = old_val;

			return -EINVAL;

		}

	} else {

		/* all tunables other than min_nice are in percentage */

		if (sysctl_sched_downmigrate_pct >

		    sysctl_sched_upmigrate_pct || *data > 100) {

			*data = old_val;

			return -EINVAL;

		}

	}



	/*

	 * Big/Small task tunable change will need to re-classify tasks on

	 * runqueue as big and small and set their counters appropriately.
@@ -2220,18 +2245,16 @@ int sched_hmp_proc_update_handler(struct ctl_table *table, int write, 
	 * includes taking runqueue lock of all online cpus and re-initiatizing

	 * their big/small counter values based on changed criteria.

	 */

	if ((*data != old_val) &&

		(data == &sysctl_sched_upmigrate_pct ||

		data == &sysctl_sched_small_task_pct)) {

	if ((data == &sysctl_sched_upmigrate_pct ||

	     data == &sysctl_sched_small_task_pct || update_min_nice)) {

		get_online_cpus();

		pre_big_small_task_count_change(cpu_online_mask);

	}



	set_hmp_defaults();



	if ((*data != old_val) &&

		(data == &sysctl_sched_upmigrate_pct ||

		data == &sysctl_sched_small_task_pct)) {

	if ((data == &sysctl_sched_upmigrate_pct ||

	     data == &sysctl_sched_small_task_pct || update_min_nice)) {

		post_big_small_task_count_change(cpu_online_mask);

		put_online_cpus();

	}
@@ -2346,8 +2369,7 @@ static inline int migration_needed(struct rq *rq, struct task_struct *p) 
		return 0;



	/* Todo: cgroup-based control? */

	if (nice > sysctl_sched_upmigrate_min_nice &&

		rq->capacity > min_capacity)

	if (nice > sched_upmigrate_min_nice && rq->capacity > min_capacity)

		return MOVE_TO_LITTLE_CPU;



	if (!task_will_fit(p, cpu_of(rq)))

kernel/sysctl.c

+1 −1
Original line number Diff line number Diff line
@@ -414,7 +414,7 @@ static struct ctl_table kern_table[] = { 
		.data		= &sysctl_sched_upmigrate_min_nice,

		.maxlen		= sizeof(unsigned int),

		.mode		= 0644,

		.proc_handler	= proc_dointvec,

		.proc_handler	= sched_hmp_proc_update_handler,

	},

	{

		.procname	= "sched_prefer_idle",