Commit 475ea6c6 authored May 26, 2021 by Valentin Schneider Committed by Peter Zijlstra Jun 01, 2021

sched: Don't defer CPU pick to migration_cpu_stop()



Will reported that the 'XXX __migrate_task() can fail' in migration_cpu_stop()
can happen, and it *is* sort of a big deal. Looking at it some more, one
will note there is a glaring hole in the deferred CPU selection:

  (w/ CONFIG_CPUSET=n, so that the affinity mask passed via taskset doesn't
  get AND'd with cpu_online_mask)

  $ taskset -pc 0-2 $PID
  # offline CPUs 3-4
  $ taskset -pc 3-5 $PID
    `\
      $PID may stay on 0-2 due to the cpumask_any_distribute() picking an
      offline CPU and __migrate_task() refusing to do anything due to
      cpu_is_allowed().

set_cpus_allowed_ptr() goes to some length to pick a dest_cpu that matches
the right constraints vs affinity and the online/active state of the
CPUs. Reuse that instead of discarding it in the affine_move_task() case.

Fixes: 6d337eab ("sched: Fix migrate_disable() vs set_cpus_allowed_ptr()")
Reported-by: Will Deacon <will@kernel.org>
Signed-off-by: Valentin Schneider <valentin.schneider@arm.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/20210526205751.842360-2-valentin.schneider@arm.com

parent 08f7c2f4

kernel/sched/core.c

+12 −8

Original line number	Diff line number	Diff line
		@@ -2273,7 +2273,6 @@ static int migration_cpu_stop(void *data)
		struct migration_arg *arg = data;
		struct set_affinity_pending *pending = arg->pending;
		struct task_struct *p = arg->task;
		int dest_cpu = arg->dest_cpu;
		struct rq *rq = this_rq();
		bool complete = false;
		struct rq_flags rf;
		@@ -2311,19 +2310,15 @@ static int migration_cpu_stop(void *data)
		if (pending) {
		p->migration_pending = NULL;
		complete = true;
		}

		if (dest_cpu < 0) {
		if (cpumask_test_cpu(task_cpu(p), &p->cpus_mask))
		goto out;

		dest_cpu = cpumask_any_distribute(&p->cpus_mask);
		}

		if (task_on_rq_queued(p))
		rq = __migrate_task(rq, &rf, p, dest_cpu);
		rq = __migrate_task(rq, &rf, p, arg->dest_cpu);
		else
		p->wake_cpu = dest_cpu;
		p->wake_cpu = arg->dest_cpu;

		/*
		* XXX __migrate_task() can fail, at which point we might end
		@@ -2606,7 +2601,7 @@ static int affine_move_task(struct rq rq, struct task_struct p, struct rq_flag
		init_completion(&my_pending.done);
		my_pending.arg = (struct migration_arg) {
		.task = p,
		.dest_cpu = -1, /* any */
		.dest_cpu = dest_cpu,
		.pending = &my_pending,
		};

		@@ -2614,6 +2609,15 @@ static int affine_move_task(struct rq rq, struct task_struct p, struct rq_flag
		} else {
		pending = p->migration_pending;
		refcount_inc(&pending->refs);
		/*
		* Affinity has changed, but we've already installed a
		* pending. migration_cpu_stop() must see this, else
		* we risk a completion of the pending despite having a
		* task on a disallowed CPU.
		*
		* Serialized by p->pi_lock, so this is safe.
		*/
		pending->arg.dest_cpu = dest_cpu;
		}
		}
		pending = p->migration_pending;