From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753398Ab0DSSwV (ORCPT ); Mon, 19 Apr 2010 14:52:21 -0400 Received: from bombadil.infradead.org ([18.85.46.34]:48062 "EHLO bombadil.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753096Ab0DSSwU (ORCPT ); Mon, 19 Apr 2010 14:52:20 -0400 Subject: Re: [REGRESSION 2.6.30][PATCH v3] sched: update load count only once per cpu in 10 tick update window From: Peter Zijlstra To: Chase Douglas Cc: linux-kernel@vger.kernel.org, Thomas Gleixner , Andrew Morton , Ingo Molnar , "Rafael J. Wysocki" , kernel-team In-Reply-To: <1271200751-18697-1-git-send-email-chase.douglas@canonical.com> References: <1271200751-18697-1-git-send-email-chase.douglas@canonical.com> Content-Type: text/plain; charset="UTF-8" Date: Mon, 19 Apr 2010 20:52:10 +0200 Message-ID: <1271703130.1676.214.camel@laptop> Mime-Version: 1.0 X-Mailer: Evolution 2.28.3 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, 2010-04-13 at 16:19 -0700, Chase Douglas wrote: > There's a period of 10 ticks where calc_load_tasks is updated by all the > cpus for the load avg. Usually all the cpus do this during the first > tick. If any cpus go idle, calc_load_tasks is decremented accordingly. > However, if they wake up calc_load_tasks is not incremented. Thus, if > cpus go idle during the 10 tick period, calc_load_tasks may be > decremented to a non-representative value. This issue can lead to > systems having a load avg of exactly 0, even though the real load avg > could theoretically be up to NR_CPUS. > > This change defers calc_load_tasks accounting after each cpu updates the > count until after the 10 tick update window. > > A few points: > > * A global atomic deferral counter, and not per-cpu vars, is needed > because a cpu may go NOHZ idle and not be able to update the global > calc_load_tasks variable for subsequent load calculations. > * It is not enough to add calls to account for the load when a cpu is > awakened: > - Load avg calculation must be independent of cpu load. > - If a cpu is awakend by one tasks, but then has more scheduled before > the end of the update window, only the first task will be accounted. OK, so what you're saying is that because we update calc_load_tasks from entering idle, we decrease earlier than a regular 10 tick sample interval would? Hence you batch these early updates into _deferred and let the next 10 tick sample roll them over? So the only early updates can come from pick_next_task_idle()->calc_load_account_active(), so why not specialize that callchain instead of the below? Also, since its all NO_HZ, why not stick this in with the ILB? Once people get around to making that scale better, this can hitch a ride. Something like the below perhaps? It does run partially from softirq context, but since there's a distinct lack of synchronization here that didn't seem like an immediate problem. --- kernel/sched.c | 10 ++++++---- kernel/sched_fair.c | 4 +++- kernel/sched_idletask.c | 2 -- 3 files changed, 9 insertions(+), 7 deletions(-) diff --git a/kernel/sched.c b/kernel/sched.c index 95eaecc..cdd4d8c 100644 --- a/kernel/sched.c +++ b/kernel/sched.c @@ -2959,6 +2959,11 @@ static void calc_load_account_active(struct rq *this_rq) { long nr_active, delta; + if (!time_after_eq(jiffies, this_rq->calc_load_update)) + return; + + this_rq->calc_load_update += LOAD_FREQ; + nr_active = this_rq->nr_running; nr_active += (long) this_rq->nr_uninterruptible; @@ -2998,10 +3003,7 @@ static void update_cpu_load(struct rq *this_rq) this_rq->cpu_load[i] = (old_load*(scale-1) + new_load) >> i; } - if (time_after_eq(jiffies, this_rq->calc_load_update)) { - this_rq->calc_load_update += LOAD_FREQ; - calc_load_account_active(this_rq); - } + calc_load_account_active(this_rq); } #ifdef CONFIG_SMP diff --git a/kernel/sched_fair.c b/kernel/sched_fair.c index 88d3053..2c267ef 100644 --- a/kernel/sched_fair.c +++ b/kernel/sched_fair.c @@ -3394,9 +3394,11 @@ static void run_rebalance_domains(struct softirq_action *h) if (need_resched()) break; + rq = cpu_rq(balance_cpu); + calc_load_account_active(rq); + rebalance_domains(balance_cpu, CPU_IDLE); - rq = cpu_rq(balance_cpu); if (time_after(this_rq->next_balance, rq->next_balance)) this_rq->next_balance = rq->next_balance; } diff --git a/kernel/sched_idletask.c b/kernel/sched_idletask.c index bea2b8f..6ca191f 100644 --- a/kernel/sched_idletask.c +++ b/kernel/sched_idletask.c @@ -23,8 +23,6 @@ static void check_preempt_curr_idle(struct rq *rq, struct task_struct *p, int fl static struct task_struct *pick_next_task_idle(struct rq *rq) { schedstat_inc(rq, sched_goidle); - /* adjust the active tasks as we might go into a long sleep */ - calc_load_account_active(rq); return rq->idle; }