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From: Valentin Schneider <valentin.schneider@arm.com>
Date: Fri, 23 Oct 2020 12:12:17 +0200
Subject: [PATCH 19/19] sched: Comment affine_move_task()
Origin: https://www.kernel.org/pub/linux/kernel/projects/rt/5.10/older/patches-5.10.8-rt24.tar.xz
Signed-off-by: Valentin Schneider <valentin.schneider@arm.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/20201013140116.26651-2-valentin.schneider@arm.com
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
---
kernel/sched/core.c | 81 ++++++++++++++++++++++++++++++++++++++++++++++++++--
1 file changed, 79 insertions(+), 2 deletions(-)
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -2080,7 +2080,75 @@ void do_set_cpus_allowed(struct task_str
}
/*
- * This function is wildly self concurrent, consider at least 3 times.
+ * This function is wildly self concurrent; here be dragons.
+ *
+ *
+ * When given a valid mask, __set_cpus_allowed_ptr() must block until the
+ * designated task is enqueued on an allowed CPU. If that task is currently
+ * running, we have to kick it out using the CPU stopper.
+ *
+ * Migrate-Disable comes along and tramples all over our nice sandcastle.
+ * Consider:
+ *
+ * Initial conditions: P0->cpus_mask = [0, 1]
+ *
+ * P0@CPU0 P1
+ *
+ * migrate_disable();
+ * <preempted>
+ * set_cpus_allowed_ptr(P0, [1]);
+ *
+ * P1 *cannot* return from this set_cpus_allowed_ptr() call until P0 executes
+ * its outermost migrate_enable() (i.e. it exits its Migrate-Disable region).
+ * This means we need the following scheme:
+ *
+ * P0@CPU0 P1
+ *
+ * migrate_disable();
+ * <preempted>
+ * set_cpus_allowed_ptr(P0, [1]);
+ * <blocks>
+ * <resumes>
+ * migrate_enable();
+ * __set_cpus_allowed_ptr();
+ * <wakes local stopper>
+ * `--> <woken on migration completion>
+ *
+ * Now the fun stuff: there may be several P1-like tasks, i.e. multiple
+ * concurrent set_cpus_allowed_ptr(P0, [*]) calls. CPU affinity changes of any
+ * task p are serialized by p->pi_lock, which we can leverage: the one that
+ * should come into effect at the end of the Migrate-Disable region is the last
+ * one. This means we only need to track a single cpumask (i.e. p->cpus_mask),
+ * but we still need to properly signal those waiting tasks at the appropriate
+ * moment.
+ *
+ * This is implemented using struct set_affinity_pending. The first
+ * __set_cpus_allowed_ptr() caller within a given Migrate-Disable region will
+ * setup an instance of that struct and install it on the targeted task_struct.
+ * Any and all further callers will reuse that instance. Those then wait for
+ * a completion signaled at the tail of the CPU stopper callback (1), triggered
+ * on the end of the Migrate-Disable region (i.e. outermost migrate_enable()).
+ *
+ *
+ * (1) In the cases covered above. There is one more where the completion is
+ * signaled within affine_move_task() itself: when a subsequent affinity request
+ * cancels the need for an active migration. Consider:
+ *
+ * Initial conditions: P0->cpus_mask = [0, 1]
+ *
+ * P0@CPU0 P1 P2
+ *
+ * migrate_disable();
+ * <preempted>
+ * set_cpus_allowed_ptr(P0, [1]);
+ * <blocks>
+ * set_cpus_allowed_ptr(P0, [0, 1]);
+ * <signal completion>
+ * <awakes>
+ *
+ * Note that the above is safe vs a concurrent migrate_enable(), as any
+ * pending affinity completion is preceded an uninstallion of
+ * p->migration_pending done with p->pi_lock held.
*/
static int affine_move_task(struct rq *rq, struct task_struct *p, struct rq_flags *rf,
int dest_cpu, unsigned int flags)
@@ -2124,6 +2192,7 @@ static int affine_move_task(struct rq *r
if (!(flags & SCA_MIGRATE_ENABLE)) {
/* serialized by p->pi_lock */
if (!p->migration_pending) {
+ /* Install the request */
refcount_set(&my_pending.refs, 1);
init_completion(&my_pending.done);
p->migration_pending = &my_pending;
@@ -2167,7 +2236,11 @@ static int affine_move_task(struct rq *r
}
if (task_running(rq, p) || p->state == TASK_WAKING) {
-
+ /*
+ * Lessen races (and headaches) by delegating
+ * is_migration_disabled(p) checks to the stopper, which will
+ * run on the same CPU as said p.
+ */
task_rq_unlock(rq, p, rf);
stop_one_cpu(cpu_of(rq), migration_cpu_stop, &arg);
@@ -2192,6 +2265,10 @@ static int affine_move_task(struct rq *r
if (refcount_dec_and_test(&pending->refs))
wake_up_var(&pending->refs);
+ /*
+ * Block the original owner of &pending until all subsequent callers
+ * have seen the completion and decremented the refcount
+ */
wait_var_event(&my_pending.refs, !refcount_read(&my_pending.refs));
return 0;
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