Linux - Difference between migrations and switches?

Migration is when a thread, usually after a context switch, get scheduled on a different CPU than it was scheduled before.

EDIT 1:

Here is more info on Wikipedia about the migration: https://en.wikipedia.org/wiki/Process_migration

Here is the kernel code increasing the counter:https://github.com/torvalds/linux/blob/master/kernel/sched/core.c#L1175

if (task_cpu(p) != new_cpu) {    ...    p->se.nr_migrations++;

EDIT 2:

A thread can migrate to another CPU in the following cases:

1. During exec()
2. During fork()
3. During thread wake-up.
4. If thread affinity mask has changed.
5. When the current CPU is getting offline.

For more info please have a look at functions set_task_cpu(), move_queued_task(), migrate_tasks() in the same source file: https://github.com/torvalds/linux/blob/master/kernel/sched/core.c

The policies scheduler follows are described in select_task_rq(), which depend on the class of scheduler you are using. The basic version of the policier:

if (p->nr_cpus_allowed > 1)    cpu = p->sched_class->select_task_rq(p, cpu, sd_flags, wake_flags);else    cpu = cpumask_any(&p->cpus_allowed);

Source: https://github.com/torvalds/linux/blob/master/kernel/sched/core.c#L1534

So in order to avoid the migration, set the CPU affinity mask for your threads using sched_setaffinity(2) system call or corresponding POSIX API pthread_setaffinity_np(3).

Here is the definition of select_task_rq() for the Completely Fair Scheduler:https://github.com/torvalds/linux/blob/master/kernel/sched/fair.c#L5860

The logic is quite complicated, but basically, we either select sibling idle CPU or find a least busy new one.

Hope this answers your question.