linux-kernel - [PATCH] nohz: don't kick non-idle CPUs in tick_nohz_full_kick

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Message-Id: <20180712181922.23073-1-ynorov@caviumnetworks.com>
Date:   Thu, 12 Jul 2018 21:19:22 +0300
From:   Yury Norov <ynorov@...iumnetworks.com>
To:     Frederic Weisbecker <fweisbec@...il.com>,
        Ingo Molnar <mingo@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>
Cc:     Yury Norov <ynorov@...iumnetworks.com>,
        "Goutham, Sunil" <Sunil.Goutham@...ium.com>,
        Chris Metcalf <cmetcalf@...lanox.com>,
        linux-kernel@...r.kernel.org
Subject: [PATCH] nohz: don't kick non-idle CPUs in tick_nohz_full_kick_cpu()

IIUC, tick_nohz_full_kick_cpu() is intended to wakeup idle CPUs
that will not be poked by scheduler because they are actually
nohz_full.

But in fact this function kicks all CPUs listed in tick_nohz_full_mask,
namely:
 - idle CPUs;
 - CPUs runnung normal tasks;
 - CPUs running isolated tasks [1];

For normal tasks it introduces unneeded latency, and for isolated tasks
it's fatal because isolation gets broken and task receives SIGKILL.

The patch below makes tick_nohz_full_kick_cpu() kicking only idle CPUs.
Non-idle nohz_full CPUs will observe changed system settings just like
non-idle normal (i.e. not nohz_full) CPUs, at next reschedule.

[1] https://lkml.org/lkml/2017/11/3/589

Signed-off-by: Yury Norov <ynorov@...iumnetworks.com>
---
 kernel/time/tick-sched.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c
index c026145eba2f..1c24c700e75a 100644
--- a/kernel/time/tick-sched.c
+++ b/kernel/time/tick-sched.c
@@ -247,7 +247,7 @@ static void tick_nohz_full_kick(void)
  */
 void tick_nohz_full_kick_cpu(int cpu)
 {
-	if (!tick_nohz_full_cpu(cpu))
+	if (!(tick_nohz_full_cpu(cpu) && idle_cpu(cpu)))
 		return;
 
 	irq_work_queue_on(&per_cpu(nohz_full_kick_work, cpu), cpu);
-- 
2.17.1