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Subject: Re: [PATCH 6/6] Account ksoftirqd time as cpustat softirq -v1
From: Peter Zijlstra <peterz@infradead.org>
To: Venkatesh Pallipadi <venki@google.com>
Cc: Ingo Molnar <mingo@elte.hu>, "H. Peter Anvin" <hpa@zytor.com>,
        Thomas Gleixner <tglx@linutronix.de>,
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        linux-kernel@vger.kernel.org, Paul Turner <pjt@google.com>,
        Eric Dumazet <eric.dumazet@gmail.com>,
        Shaun Ruffell <sruffell@digium.com>,
        Yong Zhang <yong.zhang0@gmail.com>
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Date: Tue, 26 Oct 2010 11:50:18 +0200
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On Tue, 2010-10-26 at 11:33 +0200, Peter Zijlstra wrote:
> On Mon, 2010-10-25 at 15:30 -0700, Venkatesh Pallipadi wrote:
> > softirq time in ksoftirqd context is not accounted in ns granularity
> > per cpu softirq stats, as we want that to be a part of ksoftirqd
> > exec_runtime.
> > 
> > Accounting them as softirq on /proc/stat separately.
> > 
> > Tested-by: Shaun Ruffell <sruffell@digium.com>
> > 
> > Signed-off-by: Venkatesh Pallipadi <venki@google.com>
> > ---
> >  kernel/sched.c |    8 ++++++++
> >  1 files changed, 8 insertions(+), 0 deletions(-)
> > 
> > diff --git a/kernel/sched.c b/kernel/sched.c
> > index 49f6f61..0955050 100644
> > --- a/kernel/sched.c
> > +++ b/kernel/sched.c
> > @@ -3617,6 +3617,14 @@ static void irqtime_account_process_tick(struct task_struct *p, int user_tick,
> >  		cpustat->irq = cputime64_add(cpustat->irq, tmp);
> >  	} else if (irqtime_account_si_update()) {
> >  		cpustat->softirq = cputime64_add(cpustat->softirq, tmp);
> > +	} else if (this_cpu_ksoftirqd() == p) {
> > +		/*
> > +		 * ksoftirqd time do not get accounted in cpu_softirq_time.
> > +		 * So, we have to handle it separately here.
> > +		 * Also, p->stime needs to be updated for ksoftirqd.
> > +		 */
> > +		__account_system_time(p, cputime_one_jiffy, one_jiffy_scaled,
> > +					&cpustat->softirq);
> >  	} else if (user_tick) {
> >  		account_user_time(p, cputime_one_jiffy, one_jiffy_scaled);
> >  	} else if (p == rq->idle) {
> 
> 
> I'm somewhat confused by this patch.. This is significantly different
> from the thing proposed last time around, which was to use:
> 
>   cpustat->softirq + this_cpu_ksoftirqd()->se.sum_exec_runtime
> 
> The above looses the fine grained aspect of the accounting and simply
> charges a whole jiffy if the current process happens to be ksoftirqd.

Btw, both these solutions can cause si + us + ni + sy > 100%, are we ok
with that?