From: Rusty Russell , Nick Piggin The current sched_balance_exec() sets the task's cpus_allowed mask temporarily to move it to a different CPU. This has several issues, including the fact that a task will see its affinity at a bogus value. So we change the migration_req_t to explicitly specify a destination CPU, rather than the migration thread deriving it from cpus_allowed. If the requested CPU is no longer valid (racing with another set_cpus_allowed, say), it can be ignored: if the task is not allowed on this CPU, there will be another migration request pending. This change allows sched_balance_exec() to tell the migration thread what to do without changing the cpus_allowed mask. So we rename __set_cpus_allowed() to move_task(), as the cpus_allowed mask is now set by the caller. And move_task_away(), which the migration thread uses to actually perform the move, is renamed __move_task(). I also ignore offline CPUs in sched_best_cpu(), so sched_migrate_task() doesn't need to check for offline CPUs. Ulterior motive: this approach also plays well with CPU Hotplug. Previously that patch might have seen a task with cpus_allowed only containing the dying CPU (temporarily due to sched_balance_exec) and forcibly reset it to all cpus, which might be wrong. The other approach is to hold the cpucontrol sem around sched_balance_exec(), which is too much of a bottleneck. --- 25-akpm/kernel/sched.c | 67 ++++++++++++++++++++++++------------------------- 1 files changed, 33 insertions(+), 34 deletions(-) diff -puN kernel/sched.c~sched-directed-migration kernel/sched.c --- 25/kernel/sched.c~sched-directed-migration 2004-04-27 01:30:48.814749160 -0700 +++ 25-akpm/kernel/sched.c 2004-04-27 01:30:48.820748248 -0700 @@ -530,37 +530,30 @@ inline int task_curr(task_t *p) typedef struct { struct list_head list; task_t *task; + int dest_cpu; struct completion done; } migration_req_t; /* - * The task's runqueue lock must be held, and the new mask must be valid. + * The task's runqueue lock must be held. * Returns true if you have to wait for migration thread. */ -static int __set_cpus_allowed(task_t *p, cpumask_t new_mask, - migration_req_t *req) +static int migrate_task(task_t *p, int dest_cpu, migration_req_t *req) { runqueue_t *rq = task_rq(p); - p->cpus_allowed = new_mask; - /* - * Can the task run on the task's current CPU? If not then - * migrate the thread off to a proper CPU. - */ - if (cpu_isset(task_cpu(p), new_mask)) - return 0; - /* * If the task is not on a runqueue (and not running), then * it is sufficient to simply update the task's cpu field. */ if (!p->array && !task_running(rq, p)) { - set_task_cpu(p, any_online_cpu(p->cpus_allowed)); + set_task_cpu(p, dest_cpu); return 0; } init_completion(&req->done); req->task = p; + req->dest_cpu = dest_cpu; list_add(&req->list, &rq->migration_queue); return 1; } @@ -1044,7 +1037,7 @@ unsigned long nr_running(void) { unsigned long i, sum = 0; - for (i = 0; i < NR_CPUS; i++) + for_each_cpu(i) sum += cpu_rq(i)->nr_running; return sum; @@ -1134,29 +1127,19 @@ static void sched_migrate_task(task_t *p runqueue_t *rq; migration_req_t req; unsigned long flags; - cpumask_t old_mask, new_mask = cpumask_of_cpu(dest_cpu); lock_cpu_hotplug(); rq = task_rq_lock(p, &flags); - old_mask = p->cpus_allowed; - if (!cpu_isset(dest_cpu, old_mask) || !cpu_online(dest_cpu)) + if (!cpu_isset(dest_cpu, p->cpus_allowed)) goto out; /* force the process onto the specified CPU */ - if (__set_cpus_allowed(p, new_mask, &req)) { + if (migrate_task(p, dest_cpu, &req)) { /* Need to wait for migration thread. */ task_rq_unlock(rq, &flags); wake_up_process(rq->migration_thread); wait_for_completion(&req.done); - - /* If we raced with sys_sched_setaffinity, don't - * restore mask. */ - rq = task_rq_lock(p, &flags); - if (likely(cpus_equal(p->cpus_allowed, new_mask))) { - /* Restore old mask: won't need migration - * thread, since current cpu is allowed. */ - BUG_ON(__set_cpus_allowed(p, old_mask, NULL)); - } + return; } out: task_rq_unlock(rq, &flags); @@ -1174,7 +1157,7 @@ static int sched_best_cpu(struct task_st best_cpu = this_cpu = task_cpu(p); min_load = INT_MAX; - for (i = 0; i < NR_CPUS; i++) { + for_each_online_cpu(i) { unsigned long load; if (!cpu_isset(i, domain->span)) continue; @@ -3078,7 +3061,12 @@ int set_cpus_allowed(task_t *p, cpumask_ goto out; } - if (__set_cpus_allowed(p, new_mask, &req)) { + p->cpus_allowed = new_mask; + /* Can the task run on the task's current CPU? If so, we're done */ + if (cpu_isset(task_cpu(p), new_mask)) + goto out; + + if (migrate_task(p, any_online_cpu(new_mask), &req)) { /* Need help from migration thread: drop lock and wait. */ task_rq_unlock(rq, &flags); wake_up_process(rq->migration_thread); @@ -3092,16 +3080,28 @@ out: EXPORT_SYMBOL_GPL(set_cpus_allowed); -/* Move (not current) task off this cpu, onto dest cpu. */ -static void move_task_away(struct task_struct *p, int dest_cpu) +/* + * Move (not current) task off this cpu, onto dest cpu. We're doing + * this because either it can't run here any more (set_cpus_allowed() + * away from this CPU, or CPU going down), or because we're + * attempting to rebalance this task on exec (sched_balance_exec). + * + * So we race with normal scheduler movements, but that's OK, as long + * as the task is no longer on this CPU. + */ +static void __migrate_task(struct task_struct *p, int dest_cpu) { runqueue_t *rq_dest; rq_dest = cpu_rq(dest_cpu); double_rq_lock(this_rq(), rq_dest); + /* Already moved. */ if (task_cpu(p) != smp_processor_id()) - goto out; /* Already moved */ + goto out; + /* Affinity changed (again). */ + if (!cpu_isset(dest_cpu, p->cpus_allowed)) + goto out; set_task_cpu(p, dest_cpu); if (p->array) { @@ -3154,8 +3154,7 @@ static int migration_thread(void * data) list_del_init(head->next); spin_unlock(&rq->lock); - move_task_away(req->task, - any_online_cpu(req->task->cpus_allowed)); + __migrate_task(req->task, req->dest_cpu); local_irq_enable(); complete(&req->done); } @@ -3212,7 +3211,7 @@ void migrate_all_tasks(void) tsk->pid, tsk->comm, src_cpu); } - move_task_away(tsk, dest_cpu); + __migrate_task(tsk, dest_cpu); } while_each_thread(t, tsk); write_unlock(&tasklist_lock); _