tracing: Fix hwlat kthread migration

The hwlat tracer creates a kernel thread at start of the tracer. It is
pinned to a single CPU and will move to the next CPU after each period of
running. If the user modifies the migration thread's affinity, it will not
change after that happens.

The original code created the thread at the first instance it was called,
but later was changed to destroy the thread after the tracer was finished,
and would not be created until the next instance of the tracer was
established. The code that initialized the affinity was only called on the
initial instantiation of the tracer. After that, it was not initialized, and
the previous affinity did not match the current newly created one, making
it appear that the user modified the thread's affinity when it did not, and
the thread failed to migrate again.

Cc: stable@vger.kernel.org
Fixes: 0330f7aa8ee6 ("tracing: Have hwlat trace migrate across tracing_cpumask CPUs")
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>

path: root/kernel/sched/cpupri.h

#ifndef _LINUX_CPUPRI_H
#define _LINUX_CPUPRI_H

#include <linux/sched.h>

#define CPUPRI_NR_PRIORITIES	(MAX_RT_PRIO + 2)

#define CPUPRI_INVALID -1
#define CPUPRI_IDLE     0
#define CPUPRI_NORMAL   1
/* values 2-101 are RT priorities 0-99 */

struct cpupri_vec {
	atomic_t	count;
	cpumask_var_t	mask;
};

struct cpupri {
	struct cpupri_vec pri_to_cpu[CPUPRI_NR_PRIORITIES];
	int *cpu_to_pri;
};

#ifdef CONFIG_SMP
int  cpupri_find(struct cpupri *cp,
		 struct task_struct *p, struct cpumask *lowest_mask);
void cpupri_set(struct cpupri *cp, int cpu, int pri);
int cpupri_init(struct cpupri *cp);
void cpupri_cleanup(struct cpupri *cp);
#endif

#endif /* _LINUX_CPUPRI_H */