[RFC v2 4/7] change kernel accounting to include steal time

Glauber Costa
2010-08-30T16:23:53Z

This patch proposes a common steal time implementation. When no
steal time is accounted, we just add a branch to the current
accounting code, that shouldn't add much overhead.

When we do want to register steal time, we proceed as following:
- if we would account user or system time in this tick, and there is
out-of-cpu time registered, we skip it altogether, and account steal
time only.
- if we would account user or system time in this tick, and we got the
cpu for the whole slice, we proceed normaly.
- if we are idle in this tick, we flush out-of-cpu time to give it the
chance to update whatever last-measure internal variable it may have.

This approach is simple, but proved to work well for my test scenarios.
in a UP guest on UP host, with a cpu-hog in both guest and host shows
~ 50 % steal time. steal time is also accounted proportionally, if
nice values are given to the host cpu-hog.

A cpu-hog in the host with no load in the guest, produces 0 % steal time,
with 100 % idle, as one would expect.

Signed-off-by: Glauber Costa <glommer@redhat.com>
---
include/linux/sched.h | 1 +
kernel/sched.c | 29 +++++++++++++++++++++++++++++
2 files changed, 30 insertions(+), 0 deletions(-)

diff --git a/include/linux/sched.h b/include/linux/sched.h
index 0478888..e571ddd 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -312,6 +312,7 @@ long io_schedule_timeout(long timeout);
extern void cpu_init (void);
extern void trap_init(void);
extern void update_process_times(int user);
+extern cputime_t (*hypervisor_steal_time)(void);
extern void scheduler_tick(void);

extern void sched_show_task(struct task_struct *p);
diff --git a/kernel/sched.c b/kernel/sched.c
index f52a880..9695c92 100644
--- a/kernel/sched.c
+++ b/kernel/sched.c
@@ -3157,6 +3157,16 @@ unsigned long long thread_group_sched_runtime(struct task_struct *p)
return ns;
}

+cputime_t (*hypervisor_steal_time)(void) = NULL;
+
+static inline cputime_t get_steal_time_from_hypervisor(void)
+{
+ if (!hypervisor_steal_time)
+ return 0;
+ return hypervisor_steal_time();
+}
+
+
/*
* Account user cpu time to a process.
* @p: the process that the cpu time gets accounted to
@@ -3169,6 +3179,12 @@ void account_user_time(struct task_struct *p, cputime_t cputime,
struct cpu_usage_stat *cpustat = &kstat_this_cpu.cpustat;
cputime64_t tmp;

+ tmp = get_steal_time_from_hypervisor();
+ if (tmp) {
+ account_steal_time(tmp);
+ return;
+ }
+
/* Add user time to process. */
p->utime = cputime_add(p->utime, cputime);
p->utimescaled = cputime_add(p->utimescaled, cputime_scaled);
@@ -3234,6 +3250,12 @@ void account_system_time(struct task_struct *p, int hardirq_offset,
return;
}

+ tmp = get_steal_time_from_hypervisor();
+ if (tmp) {
+ account_steal_time(tmp);
+ return;
+ }
+
/* Add system time to process. */
p->stime = cputime_add(p->stime, cputime);
p->stimescaled = cputime_add(p->stimescaled, cputime_scaled);
@@ -3276,6 +3298,13 @@ void account_idle_time(cputime_t cputime)
cputime64_t cputime64 = cputime_to_cputime64(cputime);
struct rq *rq = this_rq();

+ /*
+ * if we're idle, we don't account it as steal time, since we did
+ * not want to run anyway. We do call the steal function, however, to
+ * give the guest the chance to flush its internal buffers
+ */
+ get_steal_time_from_hypervisor();
+
if (atomic_read(&rq->nr_iowait) > 0)
cpustat->iowait = cputime64_add(cpustat->iowait, cputime64);
else
--
1.6.2.2