linux-kernel - [tip:sched/core] sched/fair: Fix SCHED_HRTICK bug leading to late preemption of tasks

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Date:   Thu, 22 Sep 2016 07:01:55 -0700
From:   tip-bot for Srivatsa Vaddagiri <tipbot@...or.com>
To:     linux-tip-commits@...r.kernel.org
Cc:     torvalds@...ux-foundation.org, hpa@...or.com,
        linux-kernel@...r.kernel.org, tglx@...utronix.de, mingo@...nel.org,
        peterz@...radead.org, joonwoop@...eaurora.org, vatsa@...eaurora.org
Subject: [tip:sched/core] sched/fair: Fix SCHED_HRTICK bug leading to late
 preemption of tasks

Commit-ID:  8bf46a39be910937d4c9e8d999a7438a7ae1a75b
Gitweb:     http://git.kernel.org/tip/8bf46a39be910937d4c9e8d999a7438a7ae1a75b
Author:     Srivatsa Vaddagiri <vatsa@...eaurora.org>
AuthorDate: Fri, 16 Sep 2016 18:28:51 -0700
Committer:  Ingo Molnar <mingo@...nel.org>
CommitDate: Thu, 22 Sep 2016 15:20:18 +0200

sched/fair: Fix SCHED_HRTICK bug leading to late preemption of tasks

SCHED_HRTICK feature is useful to preempt SCHED_FAIR tasks on-the-dot
(just when they would have exceeded their ideal_runtime).

It makes use of a per-CPU hrtimer resource and hence arming that
hrtimer should be based on total SCHED_FAIR tasks a CPU has across its
various cfs_rqs, rather than being based on number of tasks in a
particular cfs_rq (as implemented currently).

As a result, with current code, its possible for a running task (which
is the sole task in its cfs_rq) to be preempted much after its
ideal_runtime has elapsed, resulting in increased latency for tasks in
other cfs_rq on same CPU.

Fix this by arming sched hrtimer based on total number of SCHED_FAIR
tasks a CPU has across its various cfs_rqs.

Signed-off-by: Srivatsa Vaddagiri <vatsa@...eaurora.org>
Signed-off-by: Joonwoo Park <joonwoop@...eaurora.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
Cc: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: Peter Zijlstra <peterz@...radead.org>
Cc: Thomas Gleixner <tglx@...utronix.de>
Link: http://lkml.kernel.org/r/1474075731-11550-1-git-send-email-joonwoop@codeaurora.org
Signed-off-by: Ingo Molnar <mingo@...nel.org>
---
 kernel/sched/fair.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 986c10c..8fb4d19 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -4469,7 +4469,7 @@ static void hrtick_start_fair(struct rq *rq, struct task_struct *p)
 
 	WARN_ON(task_rq(p) != rq);
 
-	if (cfs_rq->nr_running > 1) {
+	if (rq->cfs.h_nr_running > 1) {
 		u64 slice = sched_slice(cfs_rq, se);
 		u64 ran = se->sum_exec_runtime - se->prev_sum_exec_runtime;
 		s64 delta = slice - ran;