linux-kernel - Re: [PATCH 1/5] sched,time: Count actually elapsed irq & softirq time

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Date:	Tue, 09 Aug 2016 10:06:19 -0400
From:	Rik van Riel <riel@...hat.com>
To:	Wanpeng Li <kernellwp@...il.com>,
	Frederic Weisbecker <fweisbec@...il.com>
Cc:	Ingo Molnar <mingo@...nel.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Paolo Bonzini <pbonzini@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Wanpeng Li <wanpeng.li@...mail.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Radim Krcmar <rkrcmar@...hat.com>,
	Mike Galbraith <efault@....de>
Subject: Re: [PATCH 1/5] sched,time: Count actually elapsed irq & softirq
 time

On Tue, 2016-08-09 at 11:59 +0800, Wanpeng Li wrote:
> Hi Rik,
> 2016-07-13 22:50 GMT+08:00 Frederic Weisbecker <fweisbec@...il.com>:
> > From: Rik van Riel <riel@...hat.com>
> > 
> > Currently, if there was any irq or softirq time during 'ticks'
> > jiffies, the entire period will be accounted as irq or softirq
> > time.
> > 
> > This is inaccurate if only a subset of the time was actually spent
> > handling irqs, and could conceivably mis-count all of the ticks
> > during
> > a period as irq time, when there was some irq and some softirq
> > time.
> > 
> > This can actually happen when irqtime_account_process_tick is
> > called
> > from account_idle_ticks, which can pass a larger number of ticks
> > down
> > all at once.
> > 
> > Fix this by changing irqtime_account_hi_update,
> > irqtime_account_si_update,
> > and steal_account_process_ticks to work with cputime_t time units,
> > and
> > return the amount of time spent in each mode.
> 
> Do we need to minus st cputime from idle cputime in
> account_idle_ticks() when noirqtime is true? I try to add this logic
> w/ noirqtime and idle=poll boot parameter for a full dynticks guest,
> however, there is no difference, where I miss?

Yes, you are right. The code in account_idle_ticks()
could use the same treatment.

I am not sure why it would not work, though...

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