This patch has helped me and some others having migration_init() troubles.
The migration_mask's semantics are altered for use as a lock word, and
some of its other functionality is deferred to a new counter and struct
completion to provide protection against pathological cases encountered
in practice.
Cheers,
Bill
diff -Nru a/kernel/sched.c b/kernel/sched.c
--- a/kernel/sched.c Fri Apr 12 04:16:07 2002
+++ b/kernel/sched.c Fri Apr 12 04:16:07 2002
@@ -1669,7 +1669,16 @@
down(&req.sem);
}
+/*
+ * Treat the bits of migration_mask as lock bits.
+ * If the bit corresponding to the cpu a migration_thread is
+ * running on then we have failed to claim our cpu and must
+ * yield in order to find another.
+ */
static volatile unsigned long migration_mask;
+static atomic_t migration_threads_seeking_cpu;
+static struct completion migration_complete
+ = COMPLETION_INITIALIZER(migration_complete);
static int migration_thread(void * unused)
{
@@ -1693,26 +1702,54 @@
* task binds itself to the current CPU.
*/
- /* wait for all migration threads to start up. */
- while (!migration_mask)
- yield();
+ preempt_disable();
- for (;;) {
- preempt_disable();
- if (test_and_clear_bit(smp_processor_id(), &migration_mask))
- current->cpus_allowed = 1 << smp_processor_id();
- if (test_thread_flag(TIF_NEED_RESCHED))
- schedule();
- if (!migration_mask)
- break;
+ /*
+ * Enter the loop with preemption disabled so that
+ * smp_processor_id() remains valid through the check. The
+ * interior of the wait loop re-enables preemption in an
+ * attempt to get scheduled off the current cpu. When the
+ * loop is exited the lock bit in migration_mask is acquired
+ * and preemption is disabled on the way out. This way the
+ * cpu acquired remains valid when ->cpus_allowed is set.
+ */
+ while (test_and_set_bit(smp_processor_id(), &migration_mask)) {
preempt_enable();
+ yield();
+ preempt_disable();
}
+
+ current->cpus_allowed = 1 << smp_processor_id();
rq = this_rq();
rq->migration_thread = current;
+
+ /*
+ * Now that we've bound ourselves to a cpu, post to
+ * migration_threads_seeking_cpu and wait for everyone else.
+ * Preemption should remain disabled and the cpu should remain
+ * in busywait. Yielding the cpu will allow the livelock
+ * where where a timing pattern causes an idle task seeking a
+ * migration_thread to always find the unbound migration_thread
+ * running on the cpu's it tries to steal tasks from.
+ */
+ atomic_dec(&migration_threads_seeking_cpu);
+ while (atomic_read(&migration_threads_seeking_cpu))
+ cpu_relax();
+
preempt_enable();
sprintf(current->comm, "migration_CPU%d", smp_processor_id());
+ /*
+ * Everyone's found their cpu, so now wake migration_init().
+ * Multiple wakeups are harmless; removal from the waitqueue
+ * has locking built-in, and waking an empty queue is valid.
+ */
+ complete(&migration_complete);
+
+ /*
+ * Initiate the event loop.
+ */
for (;;) {
runqueue_t *rq_src, *rq_dest;
struct list_head *head;
@@ -1760,33 +1797,31 @@
void __init migration_init(void)
{
- unsigned long tmp, orig_cache_decay_ticks;
+ unsigned long orig_cache_decay_ticks;
int cpu;
- tmp = 0;
- for (cpu = 0; cpu < smp_num_cpus; cpu++) {
- if (kernel_thread(migration_thread, NULL,
- CLONE_FS | CLONE_FILES | CLONE_SIGNAL) < 0)
- BUG();
- tmp |= (1UL << cpu_logical_map(cpu));
- }
+ atomic_set(&migration_threads_seeking_cpu, smp_num_cpus);
- migration_mask = tmp;
+ preempt_disable();
orig_cache_decay_ticks = cache_decay_ticks;
cache_decay_ticks = 0;
- for (cpu = 0; cpu < smp_num_cpus; cpu++) {
- int logical = cpu_logical_map(cpu);
+ for (cpu = 0; cpu < smp_num_cpus; cpu++)
+ if (kernel_thread(migration_thread, NULL,
+ CLONE_FS | CLONE_FILES | CLONE_SIGNAL) < 0)
+ BUG();
- while (!cpu_rq(logical)->migration_thread) {
- set_current_state(TASK_INTERRUPTIBLE);
- schedule_timeout(2);
- }
- }
- if (migration_mask)
- BUG();
+ /*
+ * We cannot have missed the wakeup for the migration_thread
+ * bound for the cpu migration_init() is running on cannot
+ * acquire this cpu until migration_init() has yielded it by
+ * means of wait_for_completion().
+ */
+ wait_for_completion(&migration_complete);
cache_decay_ticks = orig_cache_decay_ticks;
+
+ preempt_enable();
}
#endif
-
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This archive was generated by hypermail 2b29 : Mon Apr 15 2002 - 22:00:25 EST