linux-kernel - Re: [PATCH 1/2] workqueue: Move wq_update_unbound_numa() to the beginning of CPU

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Message-ID: <20160616193504.GB3262@mtj.duckdns.org>
Date:	Thu, 16 Jun 2016 15:35:04 -0400
From:	Tejun Heo <htejun@...il.com>
To:	Gautham R Shenoy <ego@...ux.vnet.ibm.com>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Michael Ellerman <mpe@...erman.id.au>,
	Abdul Haleem <abdhalee@...ux.vnet.ibm.com>,
	Aneesh Kumar <aneesh.kumar@...ux.vnet.ibm.com>,
	linuxppc-dev@...ts.ozlabs.org, linux-kernel@...r.kernel.org,
	kernel-team@...com
Subject: Re: [PATCH 1/2] workqueue: Move wq_update_unbound_numa() to the
 beginning of CPU_ONLINE

Hello,

So, the issue of the initial worker not having its affinity set
correctly wasn't caused by the order of the operations.  Reordering
just made set_cpus_allowed tried one more time late enough so that it
hides the race condition most of the time.  The problem is that
CPU_ONLINE callbacks are called while the cpu being onlined is online
but not active and select_fallback_rq() only considers active cpus, so
if a kthread gets scheduled in the meantime and it doesn't have any
cpu which is active in its allowed mask, it's allowed mask gets reset
to cpu_possible_mask.

Would something like the following make sense?

Thanks.
------ 8< ------
Subject: [PATCH] sched: allow kthreads to fallback 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.

Signed-off-by: Tejun Heo <tj@...nel.org>
Reported-by: Gautham R Shenoy <ego@...ux.vnet.ibm.com>
---
 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 017d539..a12e3db 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;
 		}