Re: [RFC PATCH v3 21/21] sched/cputime: Cope with steal time going backwards or negative

[Sean Christopherson <seanjc@google.com>]

This should be posted To: something other than kvm@, in a separate series, else
it's bound to get lost/ignored.

On Wed, May 22, 2024, David Woodhouse wrote:
> From: David Woodhouse <dwmw@amazon.co.uk>
> 
> In steal_account_process_time(), a delta is calculated between the value
> returned by paravirt_steal_clock(), and this_rq()->prev_steal_time which
> is assumed to be the *previous* value returned by paravirt_steal_clock().
> 
> However, instead of just assigning the newly-read value directly into
> ->prev_steal_time for use in the next iteration, ->prev_steal_time is
> *incremented* by the calculated delta.
> 
> This used to be roughly the same, modulo conversion to jiffies and back,
> until commit 807e5b80687c0 ("sched/cputime: Add steal time support to
> full dynticks CPU time accounting") started clamping that delta to a
> maximum of the actual time elapsed. So now, if the value returned by
> paravirt_steal_clock() jumps by a large amount, instead of a *single*
> period of reporting 100% steal time, the system will report 100% steal
> time for as long as it takes to "catch up" with the reported value.
> Which is up to 584 years.
> 
> But there is a benefit to advancing ->prev_steal_time only by the time
> which was *accounted* as having been stolen. It means that any extra
> time truncated by the clamping will be accounted in the next sample
> period rather than lost. Given the stochastic nature of the sampling,
> that is more accurate overall.
> 
> So, continue to advance ->prev_steal_time by the accounted value as
> long as the delta isn't egregiously large (for which, use maxtime * 2).
> If the delta is more than that, just set ->prev_steal_time directly to
> the value returned by paravirt_steal_clock().
> 
> Fixes: 807e5b80687c0 ("sched/cputime: Add steal time support to full dynticks CPU time accounting")
> Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>
> ---
>  kernel/sched/cputime.c | 20 ++++++++++++++------
>  1 file changed, 14 insertions(+), 6 deletions(-)
> 
> diff --git a/kernel/sched/cputime.c b/kernel/sched/cputime.c
> index af7952f12e6c..3a8a8b38966d 100644
> --- a/kernel/sched/cputime.c
> +++ b/kernel/sched/cputime.c
> @@ -254,13 +254,21 @@ static __always_inline u64 steal_account_process_time(u64 maxtime)
>  {
>  #ifdef CONFIG_PARAVIRT
>  	if (static_key_false(&paravirt_steal_enabled)) {
> -		u64 steal;
> -
> -		steal = paravirt_steal_clock(smp_processor_id());
> -		steal -= this_rq()->prev_steal_time;
> -		steal = min(steal, maxtime);
> +		u64 steal, abs_steal;
> +
> +		abs_steal = paravirt_steal_clock(smp_processor_id());
> +		steal = abs_steal - this_rq()->prev_steal_time;
> +		if (unlikely(steal > maxtime)) {
> +			/*
> +			 * If the delta isn't egregious, it can be counted
> +			 * in the next time period. Only advance by maxtime.
> +			 */
> +			if (steal < maxtime * 2)
> +				abs_steal = this_rq()->prev_steal_time + maxtime;
> +			steal = maxtime;
> +		}
>  		account_steal_time(steal);
> -		this_rq()->prev_steal_time += steal;
> +		this_rq()->prev_steal_time = abs_steal;
>  
>  		return steal;
>  	}
> -- 
> 2.44.0
>