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Subject[tip: sched/core] sched: Don't defer CPU pick to migration_cpu_stop()
The following commit has been merged into the sched/core branch of tip:

Commit-ID: 475ea6c60279e9f2ddf7e4cf2648cd8ae0608361
Gitweb: https://git.kernel.org/tip/475ea6c60279e9f2ddf7e4cf2648cd8ae0608361
Author: Valentin Schneider <valentin.schneider@arm.com>
AuthorDate: Wed, 26 May 2021 21:57:50 +01:00
Committer: Peter Zijlstra <peterz@infradead.org>
CommitterDate: Tue, 01 Jun 2021 16:00:11 +02:00

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: 6d337eab041d ("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
---
kernel/sched/core.c | 20 ++++++++++++--------
1 file changed, 12 insertions(+), 8 deletions(-)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index e205c19..7e59466 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -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;
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