From: Greg Kroah-Hartman gregkh@linuxfoundation.org
From: Peter Zijlstra peterz@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: stable@kernel.org Signed-off-by: Peter Zijlstra (Intel) peterz@infradead.org Signed-off-by: Ingo Molnar mingo@kernel.org Reviewed-by: Valentin Schneider valentin.schneider@arm.com Link: https://lkml.kernel.org/r/20210224131355.357743989@infradead.org Signed-off-by: Greg Kroah-Hartman gregkh@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 {