From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1756878Ab0ITQPf (ORCPT ); Mon, 20 Sep 2010 12:15:35 -0400 Received: from casper.infradead.org ([85.118.1.10]:60224 "EHLO casper.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1756839Ab0ITQPe convert rfc822-to-8bit (ORCPT ); Mon, 20 Sep 2010 12:15:34 -0400 Subject: Re: [RFC PATCH] sched: START_NICE feature (temporarily niced forks) (v3) From: Peter Zijlstra To: Mathieu Desnoyers Cc: Ingo Molnar , LKML , Mike Galbraith , Linus Torvalds , Andrew Morton , Steven Rostedt , Thomas Gleixner , Tony Lindgren In-Reply-To: <20100920160249.GB12624@Krystal> References: <20100914202503.GA1496@Krystal> <1284983026.2275.695.camel@laptop> <20100920160249.GB12624@Krystal> Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: 8BIT Date: Mon, 20 Sep 2010 18:15:19 +0200 Message-ID: <1284999319.2275.748.camel@laptop> Mime-Version: 1.0 X-Mailer: Evolution 2.28.3 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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. > 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. > > > @@ -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.