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Date: Mon, 20 Sep 2010 14:49:12 -0400 From: Mathieu Desnoyers <mathieu.desnoyers@...icios.com> To: Peter Zijlstra <peterz@...radead.org> Cc: Ingo Molnar <mingo@...e.hu>, LKML <linux-kernel@...r.kernel.org>, Mike Galbraith <efault@....de>, Linus Torvalds <torvalds@...ux-foundation.org>, Andrew Morton <akpm@...ux-foundation.org>, Steven Rostedt <rostedt@...dmis.org>, Thomas Gleixner <tglx@...utronix.de>, Tony Lindgren <tony@...mide.com> Subject: Re: [RFC PATCH] sched: START_NICE feature (temporarily niced forks) (v3) * Peter Zijlstra (peterz@...radead.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 -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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