linux-kernel - [PATCH 1/2] tick-sched: Do not clear the iowait and idle times

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Message-Id: <20200610210549.61193-2-tom.hromatka@oracle.com>
Date:   Wed, 10 Jun 2020 15:05:48 -0600
From:   Tom Hromatka <tom.hromatka@...cle.com>
To:     tom.hromatka@...cle.com, linux-kernel@...r.kernel.org,
        linux-fsdevel@...r.kernel.org
Cc:     fweisbec@...il.com, tglx@...utronix.de, mingo@...nel.org,
        adobriyan@...il.com
Subject: [PATCH 1/2] tick-sched: Do not clear the iowait and idle times

A customer reported that when a cpu goes offline and then comes back
online, the overall cpu idle and iowait data in /proc/stat decreases.
This is wreaking havoc with their cpu usage calculations.

Prior to this patch:

	        user nice system    idle iowait
	cpu  1390748  636 209444 9802206  19598
	cpu1  178384   75  24545 1392450   3025

take cpu1 offline and bring it back online

	        user nice system    idle iowait
	cpu  1391209  636 209682 8453440  16595
	cpu1  178440   75  24572     627      0

To prevent this, do not clear the idle and iowait times for the
cpu that has come back online.

With this patch:

	        user nice system    idle iowait
	cpu   129913   17  17590  166512    704
	cpu1   15916    3   2395   20989     47

take cpu1 offline and bring it back online

	        user nice system    idle iowait
	cpu   130089   17  17686  184625    711
        cpu1   15942    3   2401   23088     47

Signed-off-by: Tom Hromatka <tom.hromatka@...cle.com>
---
 kernel/time/tick-sched.c | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c
index 3e2dc9b8858c..8103bad7bbd6 100644
--- a/kernel/time/tick-sched.c
+++ b/kernel/time/tick-sched.c
@@ -1375,13 +1375,22 @@ void tick_setup_sched_timer(void)
 void tick_cancel_sched_timer(int cpu)
 {
 	struct tick_sched *ts = &per_cpu(tick_cpu_sched, cpu);
+	ktime_t idle_sleeptime, iowait_sleeptime;
 
 # ifdef CONFIG_HIGH_RES_TIMERS
 	if (ts->sched_timer.base)
 		hrtimer_cancel(&ts->sched_timer);
 # endif
 
+	/* save off and restore the idle_sleeptime and the iowait_sleeptime
+	 * to avoid discontinuities and ensure that they are monotonically
+	 * increasing
+	 */
+	idle_sleeptime = ts->idle_sleeptime;
+	iowait_sleeptime = ts->iowait_sleeptime;
 	memset(ts, 0, sizeof(*ts));
+	ts->idle_sleeptime = idle_sleeptime;
+	ts->iowait_sleeptime = iowait_sleeptime;
 }
 #endif
 
-- 
2.25.3