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Subject: Re: [PATCH 5/6] Export ns irqtimes through /proc/stat -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>,
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Date: Tue, 26 Oct 2010 16:57:26 +0200
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On Tue, 2010-10-26 at 11:45 +0200, Peter Zijlstra wrote:
> On Mon, 2010-10-25 at 15:30 -0700, Venkatesh Pallipadi wrote:
> > +static void irqtime_account_process_tick(struct task_struct *p, int user_tick,
> > +                                               struct rq *rq)
> > +{
> > +       cputime_t one_jiffy_scaled = cputime_to_scaled(cputime_one_jiffy);
> > +       cputime64_t tmp = cputime_to_cputime64(cputime_one_jiffy);
> > +       struct cpu_usage_stat *cpustat = &kstat_this_cpu.cpustat;
> > +
> > +       if (irqtime_account_hi_update()) {
> > +               cpustat->irq = cputime64_add(cpustat->irq, tmp);
> > +       } else if (irqtime_account_si_update()) {
> > +               cpustat->softirq = cputime64_add(cpustat->softirq, tmp);
> > +       } else 
> 
> I'm still not sure about this else stmt, the above two conditions can
> basically 'eat' user/system ticks. What we need to show is that there is
> no bias towards either kind so the ratio is not affected -- can we make
> such an argument?

I think I can made a counter-argument: if either or both of these checks
are true we had system time in the last tick, hence there is a larger
chance this tick is a system tick.

Therefore it will not provide the same user/system ratio.

Hmm?

> >                if (user_tick) {
> > +               account_user_time(p, cputime_one_jiffy, one_jiffy_scaled);
> > +       } else if (p == rq->idle) {
> > +               account_idle_time(cputime_one_jiffy);
> > +       } else if (p->flags & PF_VCPU) { /* System time or guest time */
> > +               account_guest_time(p, cputime_one_jiffy, one_jiffy_scaled);
> > +       } else {
> > +               __account_system_time(p, cputime_one_jiffy, one_jiffy_scaled,
> > +                                       &cpustat->system);
> > +       }
> > +}