tasklets: Replace spin wait in tasklet_kill()

tasklet_kill() spin waits for TASKLET_STATE_SCHED to be cleared invoking
yield() from inside the loop. yield() is an ill defined mechanism and the
result might still be wasting CPU cycles in a tight loop which is
especially painful in a guest when the CPU running the tasklet is scheduled
out.

tasklet_kill() is used in teardown paths and not performance critical at
all. Replace the spin wait with wait_var_event().

Signed-off-by: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lore.kernel.org/r/20210309084241.890532921@linutronix.de

diff --git a/kernel/softirq.c b/kernel/softirq.c
index ef6429a..ba89ca7 100644 (file)
--- a/kernel/softirq.c
+++ b/kernel/softirq.c
@@ -532,10 +532,12 @@ void __tasklet_hi_schedule(struct tasklet_struct *t)
 }
 EXPORT_SYMBOL(__tasklet_hi_schedule);
 
-static bool tasklet_should_run(struct tasklet_struct *t)
+static bool tasklet_clear_sched(struct tasklet_struct *t)
 {
-       if (test_and_clear_bit(TASKLET_STATE_SCHED, &t->state))
+       if (test_and_clear_bit(TASKLET_STATE_SCHED, &t->state)) {
+               wake_up_var(&t->state);
                return true;
+       }
 
        WARN_ONCE(1, "tasklet SCHED state not set: %s %pS\n",
                  t->use_callback ? "callback" : "func",
@@ -563,7 +565,7 @@ static void tasklet_action_common(struct softirq_action *a,
 
                if (tasklet_trylock(t)) {
                        if (!atomic_read(&t->count)) {
-                               if (tasklet_should_run(t)) {
+                               if (tasklet_clear_sched(t)) {
                                        if (t->use_callback)
                                                t->callback(t);
                                        else
@@ -623,13 +625,11 @@ void tasklet_kill(struct tasklet_struct *t)
        if (in_interrupt())
                pr_notice("Attempt to kill tasklet from interrupt\n");
 
-       while (test_and_set_bit(TASKLET_STATE_SCHED, &t->state)) {
-               do {
-                       yield();
-               } while (test_bit(TASKLET_STATE_SCHED, &t->state));
-       }
+       while (test_and_set_bit(TASKLET_STATE_SCHED, &t->state))
+               wait_var_event(&t->state, !test_bit(TASKLET_STATE_SCHED, &t->state));
+
        tasklet_unlock_wait(t);
-       clear_bit(TASKLET_STATE_SCHED, &t->state);
+       tasklet_clear_sched(t);
 }
 EXPORT_SYMBOL(tasklet_kill);