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[PATCH v4 1/3] KVM: fix steal clock warp during guest cpu hotplug

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  [PATCH v4 1/3] KVM: fix steal clock warp during guest cpu hotplug Wanpeng Li <kernellwp@gmail.com> - 2016-06-07 10:40 +0200
    [PATCH v4 3/3] sched/cputime: Add steal time support to full dynticks CPU time accounting Wanpeng Li <kernellwp@gmail.com> - 2016-06-07 10:40 +0200

#1415871 — [PATCH v4 1/3] KVM: fix steal clock warp during guest cpu hotplug

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

I observed that sometimes st is 100% instantaneous, then idle is 100% 
even if there is a cpu hog on the guest cpu after the cpu hotplug comes 
back(N.B. this can not always be readily reproduced). I add trace to 
capture it as below:

cpuhp/1-12    [001] d.h1   167.461657: account_process_tick: steal = 1291385514, prev_steal_time = 0         
cpuhp/1-12    [001] d.h1   167.461659: account_process_tick: steal_jiffies = 1291          
<idle>-0     [001] d.h1   167.462663: account_process_tick: steal = 18732255, prev_steal_time = 1291000000          
<idle>-0     [001] d.h1   167.462664: account_process_tick: steal_jiffies = 18446744072437

The steal clock warp and then steal_jiffies underflow.

Rik also pointed out to me:
 
| I have seen stuff like that with live migration too, in the past 

The root cause of steal clock warp during hotplug is kvm_steal_time reset 
to 0 after cpu hotplug comes back which should be preexiting guest value. 
This patch fix it by don't reset kvm_steal_time during guest cpu hotplug.

Suggested-by: Paolo Bonzini <pbonzini@redhat.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Radim Krčmář <rkrcmar@redhat.com>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Rik van Riel <riel@redhat.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Signed-off-by: Wanpeng Li <wanpeng.li@hotmail.com>
---
v2 -> v3:
 * fix the root cause
v1 -> v2:
 * update patch subject, description and comments
 * deal with the case where steal time suddenly increases by a ludicrous amount

 arch/x86/kernel/kvm.c | 2 --
 1 file changed, 2 deletions(-)

diff --git a/arch/x86/kernel/kvm.c b/arch/x86/kernel/kvm.c
index eea2a6f..1ef5e48 100644
--- a/arch/x86/kernel/kvm.c
+++ b/arch/x86/kernel/kvm.c
@@ -301,8 +301,6 @@ static void kvm_register_steal_time(void)
 	if (!has_steal_clock)
 		return;
 
-	memset(st, 0, sizeof(*st));
-
 	wrmsrl(MSR_KVM_STEAL_TIME, (slow_virt_to_phys(st) | KVM_MSR_ENABLED));
 	pr_info("kvm-stealtime: cpu %d, msr %llx\n",
 		cpu, (unsigned long long) slow_virt_to_phys(st));
-- 
1.9.1

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#1415874 — [PATCH v4 3/3] sched/cputime: Add steal time support to full dynticks CPU time accounting

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

This patch adds guest steal-time support to full dynticks CPU
time accounting. After the following commit:

ff9a9b4c4334 ("sched, time: Switch VIRT_CPU_ACCOUNTING_GEN to jiffy granularity")

... time sampling became jiffy based, even if it's still listened
to ring boundaries, so steal_account_process_tick() is reused
to account how many 'ticks' are stolen-time, after the last accumulation.

Suggested-and-Reviewed-by: Rik van Riel <riel@redhat.com>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Rik van Riel <riel@redhat.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Radim Krčmář <rkrcmar@redhat.com>
Signed-off-by: Wanpeng Li <wanpeng.li@hotmail.com>
---
v3 -> v4:
 * fix grammar errors, thanks Ingo
 * cleanup fragile codes, thanks Ingo
v2 -> v3:
 * convert steal time jiffies to cputime
v1 -> v2:
 * fix divide zero bug, thanks Rik

 kernel/sched/cputime.c | 23 ++++++++++++++++-------
 1 file changed, 16 insertions(+), 7 deletions(-)

diff --git a/kernel/sched/cputime.c b/kernel/sched/cputime.c
index 75f98c5..9ff036b 100644
--- a/kernel/sched/cputime.c
+++ b/kernel/sched/cputime.c
@@ -257,7 +257,7 @@ void account_idle_time(cputime_t cputime)
 		cpustat[CPUTIME_IDLE] += (__force u64) cputime;
 }
 
-static __always_inline bool steal_account_process_tick(void)
+static __always_inline unsigned long steal_account_process_tick(void)
 {
 #ifdef CONFIG_PARAVIRT
 	if (static_key_false(&paravirt_steal_enabled)) {
@@ -279,7 +279,7 @@ static __always_inline bool steal_account_process_tick(void)
 		return steal_jiffies;
 	}
 #endif
-	return false;
+	return 0;
 }
 
 /*
@@ -691,9 +691,13 @@ static cputime_t get_vtime_delta(struct task_struct *tsk)
 
 static void __vtime_account_system(struct task_struct *tsk)
 {
-	cputime_t delta_cpu = get_vtime_delta(tsk);
+	cputime_t delta_time = get_vtime_delta(tsk);
+	cputime_t steal_time = jiffies_to_cputime(steal_account_process_tick());
 
-	account_system_time(tsk, irq_count(), delta_cpu, cputime_to_scaled(delta_cpu));
+	if (steal_time < delta_time) {
+		delta_time -= steal_time;
+		account_system_time(tsk, irq_count(), delta_time, cputime_to_scaled(delta_time));
+	}
 }
 
 void vtime_account_system(struct task_struct *tsk)
@@ -718,13 +722,18 @@ void vtime_gen_account_irq_exit(struct task_struct *tsk)
 
 void vtime_account_user(struct task_struct *tsk)
 {
-	cputime_t delta_cpu;
+	cputime_t delta_time, steal_time;
 
 	write_seqcount_begin(&tsk->vtime_seqcount);
 	tsk->vtime_snap_whence = VTIME_SYS;
 	if (vtime_delta(tsk)) {
-		delta_cpu = get_vtime_delta(tsk);
-		account_user_time(tsk, delta_cpu, cputime_to_scaled(delta_cpu));
+		delta_time = get_vtime_delta(tsk);
+		steal_time = jiffies_to_cputime(steal_account_process_tick());
+
+		if (steal_time < delta_time) {
+			delta_time -= steal_time;
+			account_user_time(tsk, delta_time, cputime_to_scaled(delta_time));
+		}
 	}
 	write_seqcount_end(&tsk->vtime_seqcount);
 }
-- 
1.9.1

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