[tip: sched/urgent] sched: Fix migration_cpu_stop() requeueing

tip-bot2 for Peter Zijlstra
2021-03-01T10:21:49Z

The following commit has been merged into the sched/urgent branch of tip:

Commit-ID: b8e45e2a14bab684713f5dfc70c9e578c333dcdd
Gitweb: https://git.kernel.org/tip/b8e45e2a14bab684713f5dfc70c9e578c333dcdd
Author: Peter Zijlstra <peterz@infradead.org>
AuthorDate: Sat, 13 Feb 2021 13:10:35 +01:00
Committer: Peter Zijlstra <peterz@infradead.org>
CommitterDate: Mon, 01 Mar 2021 11:02:13 +01:00

sched: Fix migration_cpu_stop() requeueing

When affine_move_task(p) is called on a running task @p, which is not
otherwise already changing affinity, we'll first set
p->migration_pending and then do:

stop_one_cpu(cpu_of_rq(rq), migration_cpu_stop, &arg);

This then gets us to migration_cpu_stop() running on the CPU that was
previously running our victim task @p.

If we find that our task is no longer on that runqueue (this can
happen because of a concurrent migration due to load-balance etc.),
then we'll end up at the:

} else if (dest_cpu < 1 || pending) {

branch. Which we'll take because we set pending earlier. Here we first
check if the task @p has already satisfied the affinity constraints,
if so we bail early [A]. Otherwise we'll reissue migration_cpu_stop()
onto the CPU that is now hosting our task @p:

stop_one_cpu_nowait(cpu_of(rq), migration_cpu_stop,
&pending->arg, &pending->stop_work);

Except, we've never initialized pending->arg, which will be all 0s.

This then results in running migration_cpu_stop() on the next CPU with
arg->p == NULL, which gives the by now obvious result of fireworks.

The cure is to change affine_move_task() to always use pending->arg,
furthermore we can use the exact same pattern as the
SCA_MIGRATE_ENABLE case, since we'll block on the pending->done
completion anyway, no point in adding yet another completion in
stop_one_cpu().

This then gives a clear distinction between the two
migration_cpu_stop() use cases:

- sched_exec() / migrate_task_to() : arg->pending == NULL
- affine_move_task() : arg->pending != NULL;

And we can have it ignore p->migration_pending when !arg->pending. Any
stop work from sched_exec() / migrate_task_to() is in addition to stop
works from affine_move_task(), which will be sufficient to issue the
completion.

Fixes: 6d337eab041d ("sched: Fix migrate_disable() vs set_cpus_allowed_ptr()")
Cc: stable@kernel.org
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Valentin Schneider <valentin.schneider@arm.com>
Link: https://lkml.kernel.org/r/20210224131355.357743989@infradead.org
---
kernel/sched/core.c | 39 ++++++++++++++++++++++++++++-----------
1 file changed, 28 insertions(+), 11 deletions(-)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index ca2bb62..79ddba5 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -1922,6 +1922,24 @@ static int migration_cpu_stop(void *data)
rq_lock(rq, &rf);

pending = p->migration_pending;
+ if (pending && !arg->pending) {
+ /*
+ * This happens from sched_exec() and migrate_task_to(),
+ * neither of them care about pending and just want a task to
+ * maybe move about.
+ *
+ * Even if there is a pending, we can ignore it, since
+ * affine_move_task() will have it's own stop_work's in flight
+ * which will manage the completion.
+ *
+ * Notably, pending doesn't need to match arg->pending. This can
+ * happen when tripple concurrent affine_move_task() first sets
+ * pending, then clears pending and eventually sets another
+ * pending.
+ */
+ pending = NULL;
+ }
+
/*
* If task_rq(p) != rq, it cannot be migrated here, because we're
* holding rq->lock, if p->on_rq == 0 it cannot get enqueued because
@@ -2194,10 +2212,6 @@ static int affine_move_task(struct rq *rq, struct task_struct *p, struct rq_flag
int dest_cpu, unsigned int flags)
{
struct set_affinity_pending my_pending = { }, *pending = NULL;
- struct migration_arg arg = {
- .task = p,
- .dest_cpu = dest_cpu,
- };
bool complete = false;

/* Can the task run on the task's current CPU? If so, we're done */
@@ -2235,6 +2249,12 @@ static int affine_move_task(struct rq *rq, struct task_struct *p, struct rq_flag
/* Install the request */
refcount_set(&my_pending.refs, 1);
init_completion(&my_pending.done);
+ my_pending.arg = (struct migration_arg) {
+ .task = p,
+ .dest_cpu = -1, /* any */
+ .pending = &my_pending,
+ };
+
p->migration_pending = &my_pending;
} else {
pending = p->migration_pending;
@@ -2265,12 +2285,6 @@ static int affine_move_task(struct rq *rq, struct task_struct *p, struct rq_flag
p->migration_flags &= ~MDF_PUSH;
task_rq_unlock(rq, p, rf);

- pending->arg = (struct migration_arg) {
- .task = p,
- .dest_cpu = -1,
- .pending = pending,
- };
-
stop_one_cpu_nowait(cpu_of(rq), migration_cpu_stop,
&pending->arg, &pending->stop_work);

@@ -2283,8 +2297,11 @@ static int affine_move_task(struct rq *rq, struct task_struct *p, struct rq_flag
* is_migration_disabled(p) checks to the stopper, which will
* run on the same CPU as said p.
*/
+ refcount_inc(&pending->refs); /* pending->{arg,stop_work} */
task_rq_unlock(rq, p, rf);
- stop_one_cpu(cpu_of(rq), migration_cpu_stop, &arg);
+
+ stop_one_cpu_nowait(cpu_of(rq), migration_cpu_stop,
+ &pending->arg, &pending->stop_work);

} else {