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From:   Frederic Weisbecker <frederic@kernel.org>
To:     Peter Zijlstra <peterz@infradead.org>,
        Thomas Gleixner <tglx@linutronix.de>
Cc:     LKML <linux-kernel@vger.kernel.org>,
        "Zhou Ti (x2019cwm)" <x2019cwm@stfx.ca>,
        "Rafael J . Wysocki" <rafael.j.wysocki@intel.com>,
        Yunfeng Ye <yeyunfeng@huawei.com>,
        Frederic Weisbecker <frederic@kernel.org>,
        "Paul E . McKenney" <paulmck@kernel.org>,
        Marcelo Tosatti <mtosatti@redhat.com>,
        Ingo Molnar <mingo@kernel.org>
Subject: [PATCH 01/10] tick/nohz: Prevent tick_nohz_get_sleep_length() from returning negative value
Date:   Thu, 11 Mar 2021 13:36:59 +0100
Message-Id: <20210311123708.23501-2-frederic@kernel.org>
In-Reply-To: <20210311123708.23501-1-frederic@kernel.org>
References: <20210311123708.23501-1-frederic@kernel.org>
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From: "Zhou Ti (x2019cwm)" <x2019cwm@stfx.ca>

If the hardware clock happens to fire its interrupts late, two possible
issues can happen while calling tick_nohz_get_sleep_length(). Either:

1) The next clockevent device event is due past the last idle entry time.

or:

2) The last timekeeping update happened before the last idle entry time
   and the next timer callback expires before the last idle entry time.

Make sure that both cases are handled to avoid returning a negative
duration to the cpuidle governors.

Signed-off-by: Ti Zhou <x2019cwm@stfx.ca>
Cc: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
---
 kernel/time/tick-sched.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c
index e10a4af88737..22b6a46818cf 100644
--- a/kernel/time/tick-sched.c
+++ b/kernel/time/tick-sched.c
@@ -1142,6 +1142,9 @@ ktime_t tick_nohz_get_sleep_length(ktime_t *delta_next)
 
 	*delta_next = ktime_sub(dev->next_event, now);
 
+	if (unlikely(*delta_next < 0))
+		*delta_next = 0;
+
 	if (!can_stop_idle_tick(cpu, ts))
 		return *delta_next;
 
@@ -1156,6 +1159,9 @@ ktime_t tick_nohz_get_sleep_length(ktime_t *delta_next)
 	next_event = min_t(u64, next_event,
 			   hrtimer_next_event_without(&ts->sched_timer));
 
+	if (unlikely(next_event < now))
+		next_event = now;
+
 	return ktime_sub(next_event, now);
 }
 
-- 
2.25.1