From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S934785AbcIUOvF (ORCPT ); Wed, 21 Sep 2016 10:51:05 -0400 Received: from mail-qt0-f195.google.com ([209.85.216.195]:36200 "EHLO mail-qt0-f195.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S933376AbcIUOvD (ORCPT ); Wed, 21 Sep 2016 10:51:03 -0400 MIME-Version: 1.0 From: Yuriy Romanenko Date: Wed, 21 Sep 2016 07:51:02 -0700 X-Google-Sender-Auth: j4NhxvocwL7niBm48BTuknT7AsQ Message-ID: Subject: [PATCH 1/1] Fix tasks being forgotten for a long time on SMP To: Ingo Molnar , Peter Zijlstra , linux-kernel@vger.kernel.org Content-Type: text/plain; charset=UTF-8 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Now without HTML >>From e9a304ae91fa2a4427bde7d3aea18296d0ebb27f Mon Sep 17 00:00:00 2001 From: Yuriy Romanenko Date: Tue, 20 Sep 2016 17:47:28 -0700 Subject: [PATCH] Fix tasks being forgotten for a long time on SMP Observed occasional very high latency on an embedded SMP system between a task becoming ready to run and actually running with low system load, impacting interactive usage. A sched_wake() from CPUx on CPUy puts the task into the run queue and marks it runnable, but does not trigger an IPI to have the scheduler re-run on CPUy and see if the current task needs to get pre-empted and does not wake up CPUy if it is asleep. This is especially evident when a CPU is in SWFI and simply does not wake up even though it now has a runnable task. This is probably not the most elegant fix and definitely generates some unnecessary scheduler runs, but it's better for overall latency. Signed-off-by: Yuriy Romanenko --- kernel/sched/core.c | 8 ++++++++ 1 file changed, 8 insertions(+) diff --git a/kernel/sched/core.c b/kernel/sched/core.c index 860070f..7c334b7 100644 --- a/kernel/sched/core.c +++ b/kernel/sched/core.c @@ -1686,6 +1686,14 @@ static void ttwu_do_wakeup(struct rq *rq, struct task_struct *p, int wake_flags, trace_sched_wakeup(p); #ifdef CONFIG_SMP + /* + * If the task is not on the current cpu, there is a chance + * the other cpu might be asleep and will not get to our task + * for a really long time. Send an IPI to avoid that + */ + if (task_cpu(p) != smp_processor_id()) + smp_send_reschedule(task_cpu(p)); + if (p->sched_class->task_woken) { /* * Our task @p is fully woken up and running; so its safe to -- 2.7.4