Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755245AbbGPBMy (ORCPT ); Wed, 15 Jul 2015 21:12:54 -0400 Received: from youngberry.canonical.com ([91.189.89.112]:56450 "EHLO youngberry.canonical.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1755154AbbGPBMJ (ORCPT ); Wed, 15 Jul 2015 21:12:09 -0400 From: Kamal Mostafa To: linux-kernel@vger.kernel.org, stable@vger.kernel.org, kernel-team@lists.ubuntu.com Cc: Ben Segall , "Peter Zijlstra (Intel)" , Andrew Morton , "H. Peter Anvin" , Linus Torvalds , Roman Gushchin , Thomas Gleixner , pjt@google.com, Ingo Molnar , Kamal Mostafa Subject: [PATCH 3.19.y-ckt 210/251] sched/fair: Prevent throttling in early pick_next_task_fair() Date: Wed, 15 Jul 2015 18:08:51 -0700 Message-Id: <1437008972-9140-211-git-send-email-kamal@canonical.com> X-Mailer: git-send-email 1.9.1 In-Reply-To: <1437008972-9140-1-git-send-email-kamal@canonical.com> References: <1437008972-9140-1-git-send-email-kamal@canonical.com> X-Extended-Stable: 3.19 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 3118 Lines: 95 3.19.8-ckt4 -stable review patch. If anyone has any objections, please let me know. ------------------ From: Ben Segall commit 54d27365cae88fbcc853b391dcd561e71acb81fa upstream. The optimized task selection logic optimistically selects a new task to run without first doing a full put_prev_task(). This is so that we can avoid a put/set on the common ancestors of the old and new task. Similarly, we should only call check_cfs_rq_runtime() to throttle eligible groups if they're part of the common ancestry, otherwise it is possible to end up with no eligible task in the simple task selection. Imagine: /root /prev /next /A /B If our optimistic selection ends up throttling /next, we goto simple and our put_prev_task() ends up throttling /prev, after which we're going to bug out in set_next_entity() because there aren't any tasks left. Avoid this scenario by only throttling common ancestors. Reported-by: Mohammed Naser Reported-by: Konstantin Khlebnikov Signed-off-by: Ben Segall [ munged Changelog ] Signed-off-by: Peter Zijlstra (Intel) Cc: Andrew Morton Cc: H. Peter Anvin Cc: Linus Torvalds Cc: Peter Zijlstra Cc: Roman Gushchin Cc: Thomas Gleixner Cc: pjt@google.com Fixes: 678d5718d8d0 ("sched/fair: Optimize cgroup pick_next_task_fair()") Link: http://lkml.kernel.org/r/xm26wq1oswoq.fsf@sword-of-the-dawn.mtv.corp.google.com Signed-off-by: Ingo Molnar Signed-off-by: Kamal Mostafa --- kernel/sched/fair.c | 25 ++++++++++++++----------- 1 file changed, 14 insertions(+), 11 deletions(-) diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c index 44cfeb3..9e78f9f 100644 --- a/kernel/sched/fair.c +++ b/kernel/sched/fair.c @@ -5028,18 +5028,21 @@ again: * entity, update_curr() will update its vruntime, otherwise * forget we've ever seen it. */ - if (curr && curr->on_rq) - update_curr(cfs_rq); - else - curr = NULL; + if (curr) { + if (curr->on_rq) + update_curr(cfs_rq); + else + curr = NULL; - /* - * This call to check_cfs_rq_runtime() will do the throttle and - * dequeue its entity in the parent(s). Therefore the 'simple' - * nr_running test will indeed be correct. - */ - if (unlikely(check_cfs_rq_runtime(cfs_rq))) - goto simple; + /* + * This call to check_cfs_rq_runtime() will do the + * throttle and dequeue its entity in the parent(s). + * Therefore the 'simple' nr_running test will indeed + * be correct. + */ + if (unlikely(check_cfs_rq_runtime(cfs_rq))) + goto simple; + } se = pick_next_entity(cfs_rq, curr); cfs_rq = group_cfs_rq(se); -- 1.9.1 -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/