linux-kernel - [patch 2/2] nohz: change signal tick dependency to wakeup CPUs of member tasks

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Date:   Wed, 07 Oct 2020 15:01:53 -0300
From:   Marcelo Tosatti <mtosatti@...hat.com>
To:     linux-kernel@...r.kernel.org
Cc:     Frederic Weisbecker <frederic@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Nitesh Narayan Lal <nitesh@...hat.com>,
        Peter Xu <peterx@...hat.com>,
        Marcelo Tosatti <mtosatti@...hat.com>
Subject: [patch 2/2] nohz: change signal tick dependency to wakeup CPUs of member tasks

Rather than waking up all nohz_full CPUs on the system, only wakeup 
the target CPUs of member threads of the signal.

Reduces interruptions to nohz_full CPUs.

Signed-off-by: Marcelo Tosatti <mtosatti@...hat.com>

Index: linux-2.6/kernel/time/tick-sched.c
===================================================================
--- linux-2.6.orig/kernel/time/tick-sched.c
+++ linux-2.6/kernel/time/tick-sched.c
@@ -398,7 +398,15 @@ EXPORT_SYMBOL_GPL(tick_nohz_dep_clear_ta
  */
 void tick_nohz_dep_set_signal(struct signal_struct *sig, enum tick_dep_bits bit)
 {
-	tick_nohz_dep_set_all(&sig->tick_dep_mask, bit);
+	int prev;
+
+	prev = atomic_fetch_or(BIT(bit), &sig->tick_dep_mask);
+	if (!prev) {
+		rcu_read_lock();
+		for_each_thread(sig, t)
+			tick_nohz_kick_task(t);
+		rcu_read_unlock();
+	}
 }
 
 void tick_nohz_dep_clear_signal(struct signal_struct *sig, enum tick_dep_bits bit)