linux-kernel - [PATCH 3.16 066/217] sched/cputime: Fix steal time accounting vs. CPU hotplug

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Message-ID: <lsq.1461711741.414091614@decadent.org.uk>
Date:	Wed, 27 Apr 2016 01:02:21 +0200
From:	Ben Hutchings <ben@...adent.org.uk>
To:	linux-kernel@...r.kernel.org, stable@...r.kernel.org
CC:	akpm@...ux-foundation.org,
	"Frederic Weisbecker" <fweisbec@...il.com>,
	"Glauber Costa" <glommer@...allels.com>,
	"Ingo Molnar" <mingo@...nel.org>,
	"Linus Torvalds" <torvalds@...ux-foundation.org>,
	"Peter Zijlstra" <peterz@...radead.org>,
	"Thomas Gleixner" <tglx@...utronix.de>,
	"Rik van Riel" <riel@...hat.com>
Subject: [PATCH 3.16 066/217] sched/cputime: Fix steal time accounting vs.
 CPU hotplug

3.16.35-rc1 review patch.  If anyone has any objections, please let me know.

------------------

From: Thomas Gleixner <tglx@...utronix.de>

commit e9532e69b8d1d1284e8ecf8d2586de34aec61244 upstream.

On CPU hotplug the steal time accounting can keep a stale rq->prev_steal_time
value over CPU down and up. So after the CPU comes up again the delta
calculation in steal_account_process_tick() wreckages itself due to the
unsigned math:

	 u64 steal = paravirt_steal_clock(smp_processor_id());

	 steal -= this_rq()->prev_steal_time;

So if steal is smaller than rq->prev_steal_time we end up with an insane large
value which then gets added to rq->prev_steal_time, resulting in a permanent
wreckage of the accounting. As a consequence the per CPU stats in /proc/stat
become stale.

Nice trick to tell the world how idle the system is (100%) while the CPU is
100% busy running tasks. Though we prefer realistic numbers.

None of the accounting values which use a previous value to account for
fractions is reset at CPU hotplug time. update_rq_clock_task() has a sanity
check for prev_irq_time and prev_steal_time_rq, but that sanity check solely
deals with clock warps and limits the /proc/stat visible wreckage. The
prev_time values are still wrong.

Solution is simple: Reset rq->prev_*_time when the CPU is plugged in again.

Signed-off-by: Thomas Gleixner <tglx@...utronix.de>
Acked-by: Rik van Riel <riel@...hat.com>
Cc: Frederic Weisbecker <fweisbec@...il.com>
Cc: Glauber Costa <glommer@...allels.com>
Cc: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: Peter Zijlstra <peterz@...radead.org>
Fixes: commit 095c0aa83e52 "sched: adjust scheduler cpu power for stolen time"
Fixes: commit aa483808516c "sched: Remove irq time from available CPU power"
Fixes: commit e6e6685accfa "KVM guest: Steal time accounting"
Link: http://lkml.kernel.org/r/alpine.DEB.2.11.1603041539490.3686@nanos
Signed-off-by: Ingo Molnar <mingo@...nel.org>
Signed-off-by: Ben Hutchings <ben@...adent.org.uk>
---
 kernel/sched/core.c  |  1 +
 kernel/sched/sched.h | 13 +++++++++++++
 2 files changed, 14 insertions(+)

--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -5111,6 +5111,7 @@ migration_call(struct notifier_block *nf

 	case CPU_UP_PREPARE:
 		rq->calc_load_update = calc_load_update;
+		account_reset_rq(rq);
 		break;

 	case CPU_ONLINE:
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -1548,3 +1548,16 @@ static inline u64 irq_time_read(int cpu)
 }
 #endif /* CONFIG_64BIT */
 #endif /* CONFIG_IRQ_TIME_ACCOUNTING */
+
+static inline void account_reset_rq(struct rq *rq)
+{
+#ifdef CONFIG_IRQ_TIME_ACCOUNTING
+	rq->prev_irq_time = 0;
+#endif
+#ifdef CONFIG_PARAVIRT
+	rq->prev_steal_time = 0;
+#endif
+#ifdef CONFIG_PARAVIRT_TIME_ACCOUNTING
+	rq->prev_steal_time_rq = 0;
+#endif
+}