linux-kernel - Re: [RFC PATCH 24/30] cputime: Increment kcpustat directly on irqtime account

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Date:	Mon, 1 Dec 2014 17:15:45 +0100
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Martin Schwidefsky <schwidefsky@...ibm.com>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Tony Luck <tony.luck@...el.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Heiko Carstens <heiko.carstens@...ibm.com>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Oleg Nesterov <oleg@...hat.com>,
	Paul Mackerras <paulus@...ba.org>,
	Wu Fengguang <fengguang.wu@...el.com>,
	Ingo Molnar <mingo@...nel.org>, Rik van Riel <riel@...hat.com>
Subject: Re: [RFC PATCH 24/30] cputime: Increment kcpustat directly on
 irqtime account

On Mon, Dec 01, 2014 at 03:41:28PM +0100, Martin Schwidefsky wrote:
> On Fri, 28 Nov 2014 19:23:54 +0100
> Frederic Weisbecker <fweisbec@...il.com> wrote:
> 
> > The irqtime is accounted is nsecs and stored in
> > cpu_irq_time.hardirq_time and cpu_irq_time.softirq_time. Once the
> > accumulated amount reaches a new jiffy, this one gets accounted to the
> > kcpustat.
> > 
> > This was necessary when kcpustat was stored in cputime_t, which could at
> > worst have a jiffies granularity. But now kcpustat is stored in nsecs
> > so this whole discretization game with temporary irqtime storage has
> > become unnecessary.
> > 
> > We can now directly account the irqtime to the kcpustat.
> 
> Isn't the issue here that two different approaches to cputime accounting
> get mixed here? On the one hand a cputime_t based on jiffies and on the
> other CONFIG_IRQ_TIME_ACCOUNTING which uses sched_clock_cpu() to create
> the accounting deltas.

There is no other way really because cputime_t can wrap very low granularity
time unit such as jiffies. And there is no way to account irqtime with jiffies
since IRQ duration is supposed to be below 1 ms.

So irqtime is accounted with a high precision clock, nsecs based and periodically
accounted as cputime_t once we accumulate enough for a cputime_t unit.

And turning cputime_t to nsecs simplifies that.
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