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From: Wanpeng Li <kernellwp@gmail.com>
Date: Wed, 10 Aug 2016 07:25:58 +0800
Message-ID: <CANRm+CzxWo9i_=Ygn4CAF0L5=1avtnX=ahwMK96fj3hnQW-_zA@mail.gmail.com>
Subject: Re: [PATCH 1/5] sched,time: Count actually elapsed irq & softirq time
To: Rik van Riel <riel@redhat.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>, Ingo Molnar <mingo@kernel.org>,
        LKML <linux-kernel@vger.kernel.org>,
        Paolo Bonzini <pbonzini@redhat.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Wanpeng Li <wanpeng.li@hotmail.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Radim Krcmar <rkrcmar@redhat.com>, Mike Galbraith <efault@gmx.de>
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2016-08-09 22:06 GMT+08:00 Rik van Riel <riel@redhat.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@gmail.com>:
>> > From: Rik van Riel <riel@redhat.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
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