path: root/kernel
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authorWanpeng Li <wanpeng.li@hotmail.com>2016-08-17 10:05:46 +0800
committerIngo Molnar <mingo@kernel.org>2016-08-18 11:19:48 +0200
commit03cbc732639ddcad15218c4b2046d255851ff1e3 (patch)
tree6120b4b2a074f3e212e08ea661baa43b7936ed2c /kernel
parent173be9a14f7b2e901cf77c18b1aafd4d672e9d9e (diff)
sched/cputime: Resync steal time when guest & host lose sync
Commit: 57430218317e ("sched/cputime: Count actually elapsed irq & softirq time") ... fixed a bug but also triggered a regression: On 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 CPU utilization via 'top' in the guest, it shows: 100% st for cpu0(housekeeping) 75% st for other CPUs (nohz full mode) However, w/o this commit it shows the correct 75% for all 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 fixes the regression by limiting the account_other_time() from get_vtime_delta() to avoid underflow, and lets the other three call sites (in 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> Signed-off-by: Wanpeng Li <wanpeng.li@hotmail.com> Reviewed-by: Rik van Riel <riel@redhat.com> Cc: Frederic Weisbecker <fweisbec@gmail.com> Cc: Linus Torvalds <torvalds@linux-foundation.org> Cc: Mike Galbraith <efault@gmx.de> Cc: Peter Zijlstra <peterz@infradead.org> Cc: Radim Krcmar <rkrcmar@redhat.com> Cc: Thomas Gleixner <tglx@linutronix.de> Cc: kvm@vger.kernel.org Link: http://lkml.kernel.org/r/1471399546-4069-1-git-send-email-wanpeng.li@hotmail.com [ Improved the changelog. ] Signed-off-by: Ingo Molnar <mingo@kernel.org>
Diffstat (limited to 'kernel')
1 files changed, 15 insertions, 3 deletions
diff --git a/kernel/sched/cputime.c b/kernel/sched/cputime.c
index 2ee83b200504..a846cf89eb96 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)
@@ -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)
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)
@@ -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)
@@ -699,6 +704,13 @@ 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);