Re: [patch] sched: accurate user accounting

[malc]

On Mon, 26 Mar 2007, Con Kolivas wrote:

> 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.

I have no idea what type cputime64_t really is, so used this imaginary
LESS_THAN_ZERO thing.

Erm... i just looked at the code and suddenly it stopped making any sense
at all:

        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;

First `now' is assigned to `p->last_ran' and the very next line
compares those two values, and then the difference is taken.. I quite
frankly am either very tired or fail to see the point.. time_diff is
either always zero or there's always a race here.

--
vale
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