linux-kernel - Re: [PATCH v5 3/3] sched/cputime: Add steal time support to full dynticks CPU time accounting

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Message-ID: <20160608075210.GA8970@gmail.com>
Date:	Wed, 8 Jun 2016 09:52:10 +0200
From:	Ingo Molnar <mingo@...nel.org>
To:	Wanpeng Li <kernellwp@...il.com>
Cc:	Rik van Riel <riel@...hat.com>,
	Frédéric Weisbecker <fweisbec@...il.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	kvm <kvm@...r.kernel.org>, Wanpeng Li <wanpeng.li@...mail.com>,
	"Peter Zijlstra (Intel)" <peterz@...radead.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Paolo Bonzini <pbonzini@...hat.com>,
	Radim Krčmář <rkrcmar@...hat.com>
Subject: Re: [PATCH v5 3/3] sched/cputime: Add steal time support to full
 dynticks CPU time accounting

* Wanpeng Li <kernellwp@...il.com> wrote:

> 2016-06-08 15:22 GMT+08:00 Ingo Molnar <mingo@...nel.org>:
> >
> > * Wanpeng Li <kernellwp@...il.com> wrote:
> >
> >> From: Wanpeng Li <wanpeng.li@...mail.com>
> >>
> >> This patch adds guest steal-time support to full dynticks CPU
> >> time accounting. After the following commit:
> >>
> >> ff9a9b4c4334 ("sched, time: Switch VIRT_CPU_ACCOUNTING_GEN to jiffy granularity")
> >>
> >> ... time sampling became jiffy based, even if it's still listened
> >> to ring boundaries, so steal_account_process_tick() is reused
> >> to account how many 'ticks' are stolen-time, after the last accumulation.
> >
> > So the 'ring boundary' part still doesn't parse (neither grammatically nor
> > logically) - please rephrase it because I have no idea what you want to say here.
> 
> It is original from this slides.
> http://ertl.jp/~shinpei/conf/ospert13/slides/FredericWeisbecker.pdf,
> slide 28.

Yes, I now understand that this is meant as 'context tracking is active', but I 
don't understand the way you use it in this changelog's context.

Btw., the grammatically correct way to add that phrase would have been:

 ... time sampling became jiffy based, even if it's still listening to ring 
 boundaries, so steal_account_process_tick() is reused to account how many 
 'ticks' are stolen-time, after the last accumulation.

But I still don't understand it, nor did Paolo understand it.

Nor is there any 0/3 boilerplace description that gives some context about what 
these changes are about. Exactly what do you mean by 'add steal-time support' - we 
clearly had that before. So is your patch lifting some limitation? Or was 
steal-time accounting totally inactive with certain dynticks configurations? The 
changelog does not tell us anything about that...

I'd like to quote from a mail of Andrew Morton:

 "Please update the changelog to describe the current behavior.

  Please also describe why you think that behavior should be changed.
  ie: what's the reason for this patch."

Thanks,

	Ingo