From: Greg Kroah-Hartman <gre...@linuxfoundation.org>
From: Peter Zijlstra <pet...@infradead.org>
commit 8a6edb5257e2a84720fe78cb179eca58ba76126f upstream.
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: sta...@kernel.org
Signed-off-by: Peter Zijlstra (Intel) <pet...@infradead.org>
Signed-off-by: Ingo Molnar <mi...@kernel.org>
Reviewed-by: Valentin Schneider <valentin.schnei...@arm.com>
Link: https://lkml.kernel.org/r/20210224131355.357743...@infradead.org
Signed-off-by: Greg Kroah-Hartman <gre...@linuxfoundation.org>
---
kernel/sched/core.c | 39 ++++++++++++++++++++++++++++-----------
1 file changed, 28 insertions(+), 11 deletions(-)
--- 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 *r
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 *r
/* 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 *r
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 *r
* 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 {
