Patch "sched/cputime: Fix steal time accounting vs. CPU hotplug" has been added to the 3.14-stable tree

This is a note to let you know that I've just added the patch titled

sched/cputime: Fix steal time accounting vs. CPU hotplug

to the 3.14-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
sched-cputime-fix-steal-time-accounting-vs.-cpu-hotplug.patch
and it can be found in the queue-3.14 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: Thomas Gleixner <[email protected]>
Date: Fri, 4 Mar 2016 15:59:42 +0100
Subject: sched/cputime: Fix steal time accounting vs. CPU hotplug

From: Thomas Gleixner <[email protected]>

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.

---
 kernel/sched/core.c  |    1 +
 kernel/sched/sched.h |   13 +++++++++++++
 2 files changed, 14 insertions(+)

@@ -5064,6 +5064,7 @@ 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:

@@ -1563,3 +1563,16 @@ 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
+}

Patches currently in stable-queue which might be from [email protected] are

queue-3.14/bitops-do-not-default-to-__clear_bit-for-__clear_bit_unlock.patch queue-3.14/sched-cputime-fix-steal-time-accounting-vs.-cpu-hotplug.patch queue-3.14/x86-iopl-fix-iopl-capability-check-on-xen-pv.patch queue-3.14/x86-apic-fix-suspicious-rcu-usage-in-smp_trace_call_function_interrupt.patch