[PATCH v3] sched/cputime: Resync steal time when guest & host lose sync

[Wanpeng Li <kernellwp@gmail.com>]

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

Commit:

  	57430218317e ("sched/cputime: Count actually elapsed irq & softirq time")

... triggered a regression:

| An i5 laptop, 4 pCPUs, 4vCPUs for one full dynticks guest, there are four
| cpu hog processes(for loop) running in the guest, I hot-unplug the pCPUs 
| on host one by one until there is only one left, then observe the top in 
| guest, there are 100% st for cpu0(housekeeping), and 75% st for other cpus
| (nohz full mode). However, w/o this commit, 75% for all the four cpus.

When a guest is interrupted for a longer amount of time, missed clock ticks 
are not redelivered later. Because of that, we should not limit the amount 
of steal time accounted to the amount of time that the calling functions 
think have passed.

However, the interval returned by account_other_time() is NOT rounded down 
to the nearest jiffy, while the base interval in get_vtime_delta() it is 
subtracted from is, so the max cputime limit is required to avoid underflow.

This patch fix the regression by limiting the account_other_time() from 
get_vtime_delta() to avoid underflow, and let other three call sites
(account_other_time() and steal_account_process_time()) account however 
much steal time the host told us elapsed. 

Suggested-by: Rik van Riel <riel@redhat.com> 
Suggested-by: Paolo Bonzini <pbonzini@redhat.com>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Rik van Riel <riel@redhat.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Radim Krcmar <rkrcmar@redhat.com>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Wanpeng Li <wanpeng.li@hotmail.com>
---
v2 -> v3:
 * update code comments
v1 -> v2:
 * add code comments and update the changelog

 kernel/sched/cputime.c | 18 +++++++++++++++---
 1 file changed, 15 insertions(+), 3 deletions(-)

diff --git a/kernel/sched/cputime.c b/kernel/sched/cputime.c
index 9858266..2b9e5e5 100644
--- a/kernel/sched/cputime.c
+++ b/kernel/sched/cputime.c
@@ -263,6 +263,11 @@ void account_idle_time(cputime_t cputime)
 		cpustat[CPUTIME_IDLE] += (__force u64) cputime;
 }
 
+/*
+ * When a guest is interrupted for a longer amount of time, missed clock
+ * ticks are not redelivered later. Due to that, this function may on
+ * occasion account more time than the calling functions think elapsed.
+ */
 static __always_inline cputime_t steal_account_process_time(cputime_t maxtime)
 {
 #ifdef CONFIG_PARAVIRT
@@ -371,7 +376,7 @@ static void irqtime_account_process_tick(struct task_struct *p, int user_tick,
 	 * idle, or potentially user or system time. Due to rounding,
 	 * other time can exceed ticks occasionally.
 	 */
-	other = account_other_time(cputime);
+	other = account_other_time(ULONG_MAX);
 	if (other >= cputime)
 		return;
 	cputime -= other;
@@ -486,7 +491,7 @@ void account_process_tick(struct task_struct *p, int user_tick)
 	}
 
 	cputime = cputime_one_jiffy;
-	steal = steal_account_process_time(cputime);
+	steal = steal_account_process_time(ULONG_MAX);
 
 	if (steal >= cputime)
 		return;
@@ -516,7 +521,7 @@ void account_idle_ticks(unsigned long ticks)
 	}
 
 	cputime = jiffies_to_cputime(ticks);
-	steal = steal_account_process_time(cputime);
+	steal = steal_account_process_time(ULONG_MAX);
 
 	if (steal >= cputime)
 		return;
@@ -694,6 +699,13 @@ static cputime_t get_vtime_delta(struct task_struct *tsk)
 	unsigned long now = READ_ONCE(jiffies);
 	cputime_t delta, other;
 
+	/*
+	 * Unlike tick based timing, vtime based timing never has lost
+	 * ticks, and no need for steal time accounting to make up for
+	 * lost ticks. Vtime accounts a rounded version of actual
+	 * elapsed time. Limit account_other_time to prevent rounding
+	 * errors from causing elapsed vtime to go negative.
+	 */
 	delta = jiffies_to_cputime(now - tsk->vtime_snap);
 	other = account_other_time(delta);
 	WARN_ON_ONCE(tsk->vtime_snap_whence == VTIME_INACTIVE);
-- 
1.9.1