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

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Date:	Wed, 10 Aug 2016 07:35:29 +0800
From:	Wanpeng Li <kernellwp@...il.com>
To:	Rik van Riel <riel@...hat.com>
Cc:	Frederic Weisbecker <fweisbec@...il.com>,
	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

2016-08-10 7:25 GMT+08:00 Wanpeng Li <kernellwp@...il.com>:
> 2016-08-09 22:06 GMT+08:00 Rik van Riel <riel@...hat.com>:
>> 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...
>
> Actually I observed a regression caused by this patch. I use a i5

The regression is caused by your commit "sched,time: Count actually
elapsed irq & softirq time".

> laptop, 4 pCPUs, 4vCPUs for one full dynticks guest, there are four
> cpu hog processes(for loop) running in the guest, I hot-unplug the
> pCPUs on host one by one until there is only one left, then observe
> the top in guest, there are 100% st for cpu0(housekeeping), and 75% st
> for other cpus(nohz full). However, w/o this patch, 75% for all the
> four cpus.
>
> I try to figure out this recently, any tip is a great appreciated. :)
>
> Regards,
> Wapeng Li