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

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Date: Fri, 24 May 2024 15:25:31 +0100
From: Paul Durrant <xadimgnik@...il.com>
To: David Woodhouse <dwmw2@...radead.org>, kvm@...r.kernel.org
Cc: Paolo Bonzini <pbonzini@...hat.com>, Jonathan Corbet <corbet@....net>,
 Sean Christopherson <seanjc@...gle.com>, Thomas Gleixner
 <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>,
 Borislav Petkov <bp@...en8.de>, Dave Hansen <dave.hansen@...ux.intel.com>,
 x86@...nel.org, "H. Peter Anvin" <hpa@...or.com>,
 Peter Zijlstra <peterz@...radead.org>, Juri Lelli <juri.lelli@...hat.com>,
 Vincent Guittot <vincent.guittot@...aro.org>,
 Dietmar Eggemann <dietmar.eggemann@....com>,
 Steven Rostedt <rostedt@...dmis.org>, Ben Segall <bsegall@...gle.com>,
 Mel Gorman <mgorman@...e.de>, Daniel Bristot de Oliveira
 <bristot@...hat.com>, Valentin Schneider <vschneid@...hat.com>,
 Shuah Khan <shuah@...nel.org>, linux-doc@...r.kernel.org,
 linux-kernel@...r.kernel.org, jalliste@...zon.co.uk, sveith@...zon.de,
 zide.chen@...el.com, Dongli Zhang <dongli.zhang@...cle.com>,
 Chenyi Qiang <chenyi.qiang@...el.com>
Subject: Re: [RFC PATCH v3 21/21] sched/cputime: Cope with steal time going
 backwards or negative

On 22/05/2024 01:17, David Woodhouse wrote:
> From: David Woodhouse <dwmw@...zon.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@...zon.co.uk>
> ---
>   kernel/sched/cputime.c | 20 ++++++++++++++------
>   1 file changed, 14 insertions(+), 6 deletions(-)
> 

Reviewed-by: Paul Durrant <paul@....org>