linux-kernel - [PATCH v4 2/3] sched/cputime: Fix prev steal time accouting during cpu hotplug

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Date:	Tue,  7 Jun 2016 16:33:08 +0800
From:	Wanpeng Li <kernellwp@...il.com>
To:	linux-kernel@...r.kernel.org, kvm@...r.kernel.org
Cc:	Wanpeng Li <wanpeng.li@...mail.com>,
	Ingo Molnar <mingo@...nel.org>,
	"Peter Zijlstra (Intel)" <peterz@...radead.org>,
	Rik van Riel <riel@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Paolo Bonzini <pbonzini@...hat.com>,
	Radim Krčmář <rkrcmar@...hat.com>
Subject: [PATCH v4 2/3] sched/cputime: Fix prev steal time accouting during cpu hotplug

From: Wanpeng Li <wanpeng.li@...mail.com>

Commit e9532e69b8d1 ("sched/cputime: Fix steal time accounting vs. CPU hotplug") 
set rq->prev_* to 0 after a cpu hotplug comes back in order to fix the scenario:

| 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.

However, it is still buggy.

rq->prev_steal_time = 0:

As Rik pointed out:

| setting rq->prev_irq_time to 0 in the guest, and then getting a giant value from 
| the host, could result in a very large of steal_jiffies.

rq->prev_steal_time_rq = 0:

| steal = paravirt_steal_clock(cpu_of(rq));
| steal -= rq->prev_steal_time_rq;
|
| if (unlikely(steal > delta))
|	 steal = delta;
|
| rq->prev_steal_time_rq += steal;
| delta -= steal;
|
| rq->clock_task += delta;

steal is a giant value and rq->prev_steal_time_rq is 0, rq->prev_steal_time_rq 
grows in delta granularity, rq->clock_task can't ramp up until rq->prev_steal_time_rq 
catches up steal clock since delta value will be 0 after reducing steal time from 
normal execution time. That's why I obersved that cpuhg/1-12 continue running 
until rq->prev_steal_time_rq catches up steal clock timestamp.

I believe rq->prev_irq_time has similar issue. So this patch fix it by setting 
rq->prev_* to current irq time and steal clock timestamp after a cpu hotplug 
comes back.

Fixes: 'commit e9532e69b8d1 ("sched/cputime: Fix steal time accounting vs. CPU hotplug")'
Acked-by: Rik van Riel <riel@...hat.com>
Cc: Ingo Molnar <mingo@...nel.org>
Cc: Peter Zijlstra (Intel) <peterz@...radead.org>
Cc: Rik van Riel <riel@...hat.com>
Cc: Thomas Gleixner <tglx@...utronix.de>
Cc: Frederic Weisbecker <fweisbec@...il.com>
Cc: Paolo Bonzini <pbonzini@...hat.com>
Cc: Radim Krčmář <rkrcmar@...hat.com>
Signed-off-by: Wanpeng Li <wanpeng.li@...mail.com>
---
 kernel/sched/sched.h | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
index 72f1f30..e6758af 100644
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -1813,12 +1813,12 @@ static inline void cpufreq_trigger_update(u64 time) {}
 static inline void account_reset_rq(struct rq *rq)
 {
 #ifdef CONFIG_IRQ_TIME_ACCOUNTING
-	rq->prev_irq_time = 0;
+	rq->prev_irq_time = irq_time_read(cpu_of(rq));
 #endif
 #ifdef CONFIG_PARAVIRT
-	rq->prev_steal_time = 0;
+	rq->prev_steal_time = paravirt_steal_clock(cpu_of(rq));
 #endif
 #ifdef CONFIG_PARAVIRT_TIME_ACCOUNTING
-	rq->prev_steal_time_rq = 0;
+	rq->prev_steal_time_rq = paravirt_steal_clock(cpu_of(rq));
 #endif
 }
-- 
1.9.1