sched: Prepare to use balance_push in ttwu()

In preparation of using the balance_push state in ttwu() we need it to
provide a reliable and consistent state.

The immediate problem is that rq->balance_callback gets cleared every
schedule() and then re-set in the balance_push_callback() itself. This
is not a reliable signal, so add a variable that stays set during the
entire time.

Also move setting it before the synchronize_rcu() in
sched_cpu_deactivate(), such that we get guaranteed visibility to
ttwu(), which is a preempt-disable region.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Valentin Schneider <valentin.schneider@arm.com>
Tested-by: Valentin Schneider <valentin.schneider@arm.com>
Link: https://lkml.kernel.org/r/20210121103506.966069627@infradead.org

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 8da0fd7..16946b5 100644 (file)
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -7320,6 +7320,7 @@ static void balance_push_set(int cpu, bool on)
        struct rq_flags rf;
 
        rq_lock_irqsave(rq, &rf);
+       rq->balance_push = on;
        if (on) {
                WARN_ON_ONCE(rq->balance_callback);
                rq->balance_callback = &balance_push_callback;
@@ -7489,17 +7490,17 @@ int sched_cpu_deactivate(unsigned int cpu)
        int ret;
 
        set_cpu_active(cpu, false);
+       balance_push_set(cpu, true);
+
        /*
-        * We've cleared cpu_active_mask, wait for all preempt-disabled and RCU
-        * users of this state to go away such that all new such users will
-        * observe it.
+        * We've cleared cpu_active_mask / set balance_push, wait for all
+        * preempt-disabled and RCU users of this state to go away such that
+        * all new such users will observe it.
         *
         * Do sync before park smpboot threads to take care the rcu boost case.
         */
        synchronize_rcu();
 
-       balance_push_set(cpu, true);
-
        rq_lock_irqsave(rq, &rf);
        if (rq->rd) {
                update_rq_clock(rq);

diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
index 12ada79..bb09988 100644 (file)
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -975,6 +975,7 @@ struct rq {
        unsigned long           cpu_capacity_orig;
 
        struct callback_head    *balance_callback;
+       unsigned char           balance_push;
 
        unsigned char           nohz_idle_balance;
        unsigned char           idle_balance;