Re: [PATCH] sched: Don't try to catch up excess steal time.

[Srikar Dronamraju <srikar@linux.ibm.com>]

* Suleiman Souhlal <suleiman@google.com> [2024-08-06 20:11:57]:

> When steal time exceeds the measured delta when updating clock_task, we
> currently try to catch up the excess in future updates.
> However, this results in inaccurate run times for the future clock_task
> measurements, as they end up getting additional steal time that did not
> actually happen, from the previous excess steal time being paid back.
> 
> For example, suppose a task in a VM runs for 10ms and had 15ms of steal
> time reported while it ran. clock_task rightly doesn't advance. Then, a
> different task runs on the same rq for 10ms without any time stolen.
> Because of the current catch up mechanism, clock_sched inaccurately ends
> up advancing by only 5ms instead of 10ms even though there wasn't any
> actual time stolen. The second task is getting charged for less time
> than it ran, even though it didn't deserve it.
> In other words, tasks can end up getting more run time than they should
> actually get.
> 
> So, we instead don't make future updates pay back past excess stolen time.
> 
> Signed-off-by: Suleiman Souhlal <suleiman@google.com>
> ---
>  kernel/sched/core.c | 6 ++++--
>  1 file changed, 4 insertions(+), 2 deletions(-)
> 
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index bcf2c4cc0522..42b37da2bda6 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -728,13 +728,15 @@ static void update_rq_clock_task(struct rq *rq, s64 delta)
>  #endif
>  #ifdef CONFIG_PARAVIRT_TIME_ACCOUNTING
>  	if (static_key_false((&paravirt_steal_rq_enabled))) {
> -		steal = paravirt_steal_clock(cpu_of(rq));
> +		u64 prev_steal;
> +
> +		steal = prev_steal = paravirt_steal_clock(cpu_of(rq));
>  		steal -= rq->prev_steal_time_rq;
>  
>  		if (unlikely(steal > delta))
>  			steal = delta;
>  
> -		rq->prev_steal_time_rq += steal;
> +		rq->prev_steal_time_rq = prev_steal;
>  		delta -= steal;
>  	}
>  #endif


Agree with the change.

Probably, we could have achieved by just moving a line above
Something like this?

#ifdef CONFIG_PARAVIRT_TIME_ACCOUNTING
	if (static_key_false((&paravirt_steal_rq_enabled))) {
		steal = paravirt_steal_clock(cpu_of(rq));
		steal -= rq->prev_steal_time_rq;
		rq->prev_steal_time_rq += steal;

		if (unlikely(steal > delta))
			steal = delta;

		delta -= steal;
	}
#endif


-- 
Thanks and Regards
Srikar Dronamraju