linux-kernel - Re: [patch] sched: accurate user accounting

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Date:	Mon, 26 Mar 2007 01:28:09 +1000
From:	Con Kolivas <kernel@...ivas.org>
To:	malc <av1474@...tv.ru>
Cc:	Ingo Molnar <mingo@...e.hu>,
	linux list <linux-kernel@...r.kernel.org>, zwane@...radead.org,
	ck list <ck@....kolivas.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [patch] sched: accurate user accounting

On Monday 26 March 2007 01:19, malc wrote:
> On Mon, 26 Mar 2007, Con Kolivas wrote:
> > So before we go any further with this patch, can you try the following
> > one and see if this simple sanity check is enough?
>
> Sure (compiling the kernel now), too bad old axiom that testing can not
> confirm absence of bugs holds.
>
> I have one nit and one request from clarification. Question first (i
> admit i haven't looked at the surroundings of the patch maybe things
> would have been are self evident if i did):
>
> What this patch amounts to is that the accounting logic is moved from
> timer interrupt to the place where scheduler switches task (or something
> to that effect)?

Both the scheduler tick and context switch now. So yes it adds overhead as I 
said, although we already do update_cpu_clock on context switch, but it's not 
this complex.

> [..snip..]
>
> >  * These are the 'tuning knobs' of the scheduler:
> > @@ -3017,8 +3018,53 @@ EXPORT_PER_CPU_SYMBOL(kstat);
> > static inline void
> > update_cpu_clock(struct task_struct *p, struct rq *rq, unsigned long long
> > now) {
> > -	p->sched_time += now - p->last_ran;
> > +	struct cpu_usage_stat *cpustat = &kstat_this_cpu.cpustat;
> > +	cputime64_t time_diff;
> > +
> > 	p->last_ran = rq->most_recent_timestamp = now;
> > +	/* Sanity check. It should never go backwards or ruin accounting */
> > +	if (unlikely(now < p->last_ran))
> > +		return;
> > +	time_diff = now - p->last_ran;
>
> A nit. Anything wrong with:
>
> time_diff = now - p->last_ran;
> if (unlikeley (LESS_THAN_ZERO (time_diff))
>          return;

Does LESS_THAN_ZERO work on a cputime64_t on all arches? I can't figure that 
out just by looking myself which is why I did it the other way.

-- 
-ck
-
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