Re: [RFC PATCH] sched: START_NICE feature (temporarily niced forks) (v3)

[Mathieu Desnoyers <mathieu.desnoyers@efficios.com>]

* Peter Zijlstra (peterz@infradead.org) wrote:
> On Mon, 2010-09-20 at 12:02 -0400, Mathieu Desnoyers wrote:
> > > > Index: linux-2.6-lttng.git/kernel/sched_fair.c
> > > > ===================================================================
> > > > --- linux-2.6-lttng.git.orig/kernel/sched_fair.c
> > > > +++ linux-2.6-lttng.git/kernel/sched_fair.c
> > > > @@ -433,6 +433,14 @@ calc_delta_fair(unsigned long delta, str
> > > >     if (unlikely(se->load.weight != NICE_0_LOAD))
> > > >             delta = calc_delta_mine(delta, NICE_0_LOAD, &se->load);
> > > >  
> > > > +   if (se->fork_nice_penality) {
> > > > +           delta <<= se->fork_nice_penality;
> > > > +           if ((s64)(se->sum_exec_runtime - se->fork_nice_timeout) > 0) {
> > > > +                   se->fork_nice_penality = 0;
> > > > +                   se->fork_nice_timeout = 0;
> > > > +           }
> > > > +   }
> > > > +
> > > >     return delta;
> > > >  }
> > > 
> > > Something like this ought to live at every place where you use se->load,
> > > including sched_slice(), possibly wakeup_gran(), although that's more
> > > heuristic, so you could possibly leave it out there.
> > 
> > Agreed for wakeup_gran(). I'll just remove the duplicate "if
> > (unlikely(se->load.weight != NICE_0_LOAD))" check.
> > 
> > For sched_slice(), I don't know. sched_vslice() is used to take nice level into
> > account when placing new tasks. sched_slice() takes only the weight into
> > account, not the nice level. 
> 
> nice-level == weight
> 
> > So given that I want to mimic the nice level
> > impact, I'm not sure we have to take this into account at the sched_slice level.
> 
> If you renice, we change the weight, hence you need to propagate this
> penalty to every place we use the weight.

OK

> 
> > Also, I wonder if leaving it out of account_entity_enqueue/dequeue() calls to
> > add_cfs_task_weight() and inc/dec_cpu_load is OK ? Because it can be a pain to
> > reequilibrate the cpu and task weights when the timeout occurs.  The temporary
> > effect of this nice-on-fork is to make the tasks a little lighter, so the weight
> > is not accurate. But I wonder if we really care that much about it.
> 
> Yeah, propagating the accumulated weight effect is a bit of a bother
> like you noticed.
> 
> We can simply try, by lowering the effective weight and not propagating
> this to the accumulated weight, the effect is even stronger. Suppose you
> have 2 tasks of weight 1, then fork so that two tasks get half weight. 
> 
> Then if you propagate the accumulated weight it would look like:
> 1:.5:.5 with a total weight of 2, so that each of these light tasks get
> 1/4th the time. If, however you do not propagate, you get something
> like: 1:.5:.5 on 3, so that each of these light tasks gets 1/6th of the
> total time.
>  
> Its a bit of a trade-off, not propagating, simpler, less code, slightly
> wrong numbers, against propagating, more complex/expensive but slightly
> better numbers.
> 
> If you care you can implement both and measure it, but I'm not too
> bothered -- we can always fix it if it turns out to have definite
> down-sides.

Yeah, I think an approximation will be enough too. I'll keep my current approach
which does not update the accumulated weight.

> 
> > > > @@ -832,6 +840,11 @@ dequeue_entity(struct cfs_rq *cfs_rq, st
> > > >      */
> > > >     if (!(flags & DEQUEUE_SLEEP))
> > > >             se->vruntime -= cfs_rq->min_vruntime;
> > > > +
> > > > +   if (se->fork_nice_penality) {
> > > > +           se->fork_nice_penality = 0;
> > > > +           se->fork_nice_timeout = 0;
> > > > +   }
> > > >  }
> > > >  
> > > >  /*
> > > 
> > > So you want to reset this penalty on each de-schedule, not only sleep
> > > (but also preemptions)?
> > 
> > only sleeps. So I should put this within a 
> > 
> > if (flags & DEQUEUE_SLEEP) {
> >   ...
> > }
> > 
> > I suppose ? 
> 
> Yep.

OK, thanks !

I'll post v3 soon, incorporating the changes you recommended.

Mathieu


-- 
Mathieu Desnoyers
Operating System Efficiency R&D Consultant
EfficiOS Inc.
http://www.efficios.com