Re: [patch] sched: accurate user accounting

[Con Kolivas <kernel@kolivas.org>]

On Monday 26 March 2007 03:14, malc wrote:
> 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.

Bah major thinko error on my part! That will teach me to post patches untested 
at 1:30 am. I'll try again shortly sorry.

-- 
-ck