linux-kernel - [tip:sched/urgent] sched/core: Allow kthreads to fall back to online && !active cpus

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Message-ID: <tip-feb245e304f343cf5e4f9123db36354144dce8a4@git.kernel.org>
Date:	Fri, 24 Jun 2016 02:00:32 -0700
From:	tip-bot for Tejun Heo <tipbot@...or.com>
To:	linux-tip-commits@...r.kernel.org
Cc:	htejun@...il.com, ego@...ux.vnet.ibm.com, peterz@...radead.org,
	tglx@...utronix.de, tj@...nel.org, torvalds@...ux-foundation.org,
	mpe@...erman.id.au, linux-kernel@...r.kernel.org,
	aneesh.kumar@...ux.vnet.ibm.com, abdhalee@...ux.vnet.ibm.com,
	mingo@...nel.org, hpa@...or.com
Subject: [tip:sched/urgent] sched/core: Allow kthreads to fall back to
 online && !active cpus

Commit-ID:  feb245e304f343cf5e4f9123db36354144dce8a4
Gitweb:     http://git.kernel.org/tip/feb245e304f343cf5e4f9123db36354144dce8a4
Author:     Tejun Heo <htejun@...il.com>
AuthorDate: Thu, 16 Jun 2016 15:35:04 -0400
Committer:  Ingo Molnar <mingo@...nel.org>
CommitDate: Fri, 24 Jun 2016 08:26:53 +0200

sched/core: Allow kthreads to fall back to online && !active cpus

During CPU hotplug, CPU_ONLINE callbacks are run while the CPU is
online but not active.  A CPU_ONLINE callback may create or bind a
kthread so that its cpus_allowed mask only allows the CPU which is
being brought online.  The kthread may start executing before the CPU
is made active and can end up in select_fallback_rq().

In such cases, the expected behavior is selecting the CPU which is
coming online; however, because select_fallback_rq() only chooses from
active CPUs, it determines that the task doesn't have any viable CPU
in its allowed mask and ends up overriding it to cpu_possible_mask.

CPU_ONLINE callbacks should be able to put kthreads on the CPU which
is coming online.  Update select_fallback_rq() so that it follows
cpu_online() rather than cpu_active() for kthreads.

Reported-by: Gautham R Shenoy <ego@...ux.vnet.ibm.com>
Tested-by: Gautham R. Shenoy <ego@...ux.vnet.ibm.com>
Signed-off-by: Tejun Heo <tj@...nel.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
Cc: Abdul Haleem <abdhalee@...ux.vnet.ibm.com>
Cc: Aneesh Kumar <aneesh.kumar@...ux.vnet.ibm.com>
Cc: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: Michael Ellerman <mpe@...erman.id.au>
Cc: Peter Zijlstra <peterz@...radead.org>
Cc: Thomas Gleixner <tglx@...utronix.de>
Cc: kernel-team@...com
Cc: linuxppc-dev@...ts.ozlabs.org
Link: http://lkml.kernel.org/r/20160616193504.GB3262@mtj.duckdns.org
Signed-off-by: Ingo Molnar <mingo@...nel.org>
---
 kernel/sched/core.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 4135ac8..51d7105 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -1536,7 +1536,9 @@ static int select_fallback_rq(int cpu, struct task_struct *p)
 	for (;;) {
 		/* Any allowed, online CPU? */
 		for_each_cpu(dest_cpu, tsk_cpus_allowed(p)) {
-			if (!cpu_active(dest_cpu))
+			if (!(p->flags & PF_KTHREAD) && !cpu_active(dest_cpu))
+				continue;
+			if (!cpu_online(dest_cpu))
 				continue;
 			goto out;
 		}