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

[Rik van Riel <riel@redhat.com>]

On 08/25/2010 05:43 PM, Glauber Costa wrote:
> 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);

I see one problem here.

What if get_steal_time_from_hypervisor() returns a smaller
amount of time than "cputime"?

Would it be better to account tmp as stealtime, and the
difference (cputime - tmp) as user/sys/... time?

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