From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754690AbbDTIZN (ORCPT ); Mon, 20 Apr 2015 04:25:13 -0400 Received: from m50-110.126.com ([123.125.50.110]:40364 "EHLO m50-110.126.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754655AbbDTIZG (ORCPT ); Mon, 20 Apr 2015 04:25:06 -0400 From: Xunlei Pang To: linux-kernel@vger.kernel.org Cc: Peter Zijlstra , Steven Rostedt , Juri Lelli , Xunlei Pang Subject: [PATCH v6 2/3] sched/rt: Fix wrong SMP scheduler behavior for equal prio cases Date: Mon, 20 Apr 2015 16:22:47 +0800 Message-Id: <1429518168-7965-2-git-send-email-xlpang@126.com> X-Mailer: git-send-email 1.9.1 In-Reply-To: <1429518168-7965-1-git-send-email-xlpang@126.com> References: <1429518168-7965-1-git-send-email-xlpang@126.com> X-CM-TRANSID: jdKowAD3__uLtzRVzV2uAQ--.4394S3 X-Coremail-Antispam: 1Uf129KBjvJXoWxGFWrZr4kur48Gw4xXF13Jwb_yoWrJw48pa 48Jw1jyw4kJa9Fgrn3Xr48Zr43Gw1vq3y5JF93K3y5KanYqFy0qFnYvry3tFWrKr1vga17 tF4ktrsxGr1UZF7anT9S1TB71UUUUUUqnTZGkaVYY2UrUUUUjbIjqfuFe4nvWSU5nxnvy2 9KBjDUYxBIdaVFxhVjvjDU0xZFpf9x07jlPfPUUUUU= X-Originating-IP: [210.21.223.3] X-CM-SenderInfo: p0ost0bj6rjloofrz/1tbiJxbhv01sBUvfIAAAs- Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org From: Xunlei Pang Currently, SMP RT scheduler has some trouble in dealing with equal prio cases. For example, in check_preempt_equal_prio(): When RT1(current task) gets preempted by RT2, if there is a migratable RT3 with same prio, RT3 will be pushed away instead of RT1 afterwards, because RT1 will be enqueued to the tail of the pushable list when going through succeeding put_prev_task_rt() triggered by resched. This broke FIFO. Furthermore, this is also problematic for normal preempted cases if there're some rt tasks queued with the same prio as current. Because current will be put behind these tasks in the pushable queue. So, if a task is running and gets preempted by a higher priority task (or even with same priority for migrating), this patch ensures that it is put ahead of any existing task with the same priority in the pushable queue. Suggested-by: Steven Rostedt Signed-off-by: Xunlei Pang --- v5->v6: Add wrapped enqueue_pushable_task_preempted() and enqueue_pushable_task() for __enqueue_pushable_task() to avoid the "bool head" parameter everywhere. kernel/sched/rt.c | 41 +++++++++++++++++++++++++++++++++++++---- 1 file changed, 37 insertions(+), 4 deletions(-) diff --git a/kernel/sched/rt.c b/kernel/sched/rt.c index 575da76..8679eff 100644 --- a/kernel/sched/rt.c +++ b/kernel/sched/rt.c @@ -359,17 +359,32 @@ static inline void set_post_schedule(struct rq *rq) rq->post_schedule = has_pushable_tasks(rq); } -static void enqueue_pushable_task(struct rq *rq, struct task_struct *p) +static void +__enqueue_pushable_task(struct rq *rq, struct task_struct *p, bool head) { plist_del(&p->pushable_tasks, &rq->rt.pushable_tasks); plist_node_init(&p->pushable_tasks, p->prio); - plist_add(&p->pushable_tasks, &rq->rt.pushable_tasks); + if (head) + plist_add_head(&p->pushable_tasks, &rq->rt.pushable_tasks); + else + plist_add_tail(&p->pushable_tasks, &rq->rt.pushable_tasks); /* Update the highest prio pushable task */ if (p->prio < rq->rt.highest_prio.next) rq->rt.highest_prio.next = p->prio; } +static inline +void enqueue_pushable_task_preempted(struct rq *rq, struct task_struct *p) +{ + __enqueue_pushable_task(rq, p, true); +} + +static inline void enqueue_pushable_task(struct rq *rq, struct task_struct *p) +{ + __enqueue_pushable_task(rq, p, false); +} + static void dequeue_pushable_task(struct rq *rq, struct task_struct *p) { plist_del(&p->pushable_tasks, &rq->rt.pushable_tasks); @@ -385,6 +400,11 @@ static void dequeue_pushable_task(struct rq *rq, struct task_struct *p) #else +static inline +void enqueue_pushable_task_preempted(struct rq *rq, struct task_struct *p) +{ +} + static inline void enqueue_pushable_task(struct rq *rq, struct task_struct *p) { } @@ -1506,8 +1526,21 @@ static void put_prev_task_rt(struct rq *rq, struct task_struct *p) * The previous task needs to be made eligible for pushing * if it is still active */ - if (on_rt_rq(&p->rt) && p->nr_cpus_allowed > 1) - enqueue_pushable_task(rq, p); + if (on_rt_rq(&p->rt) && p->nr_cpus_allowed > 1) { + /* + * put_prev_task_rt() is called by many functions, + * pick_next_task_rt() is the only one may have + * PREEMPT_ACTIVE set. So if detecting p(current + * task) is preempted in such case, we should + * enqueue it to the front of the pushable plist, + * as there may be multiple tasks with the same + * priority as p. + */ + if (preempt_count() & PREEMPT_ACTIVE) + enqueue_pushable_task_preempted(rq, p); + else + enqueue_pushable_task(rq, p); + } } #ifdef CONFIG_SMP -- 1.9.1