linux-kernel - [PATCH 3/3] sched/numa: Acquire RCU lock when checking idle cores during NUMA balancing

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Date:   Tue,  3 Mar 2020 11:02:58 +0000
From:   Mel Gorman <mgorman@...hsingularity.net>
To:     Ingo Molnar <mingo@...nel.org>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Juri Lelli <juri.lelli@...hat.com>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ben Segall <bsegall@...gle.com>,
        Valentin Schneider <valentin.schneider@....com>,
        Lukasz Luba <lukasz.luba@....com>,
        Phil Auld <pauld@...hat.com>, Hillf Danton <hdanton@...a.com>,
        Anshuman Khandual <anshuman.khandual@....com>,
        Naresh Kamboju <naresh.kamboju@...aro.org>,
        Paul McKenney <paulmck@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Mel Gorman <mgorman@...hsingularity.net>
Subject: [PATCH 3/3] sched/numa: Acquire RCU lock when checking idle cores during NUMA balancing

Qian Cai reported the following

  The linux-next commit ff7db0bf24db ("sched/numa: Prefer using an idle CPU as a
migration target instead of comparing tasks") introduced a boot warning,

  [   86.520534][    T1] WARNING: suspicious RCU usage
  [   86.520540][    T1] 5.6.0-rc3-next-20200227 #7 Not tainted
  [   86.520545][    T1] -----------------------------
  [   86.520551][    T1] kernel/sched/fair.c:5914 suspicious rcu_dereference_check() usage!
  [   86.520555][    T1]
  [   86.520555][    T1] other info that might help us debug this:
  [   86.520555][    T1]
  [   86.520561][    T1]
  [   86.520561][    T1] rcu_scheduler_active = 2, debug_locks = 1
  [   86.520567][    T1] 1 lock held by systemd/1:
  [   86.520571][    T1]  #0: ffff8887f4b14848 (&mm->mmap_sem#2){++++}, at: do_page_fault+0x1d2/0x998
  [   86.520594][    T1]
  [   86.520594][    T1] stack backtrace:
  [   86.520602][    T1] CPU: 1 PID: 1 Comm: systemd Not tainted 5.6.0-rc3-next-20200227 #7

task_numa_migrate() checks for idle cores when updating NUMA-related statistics.
This relies on reading a RCU-protected structure in test_idle_cores() via this
call chain

task_numa_migrate
  -> update_numa_stats
    -> numa_idle_core
      -> test_idle_cores

While the locking could be fine-grained, it is more appropriate to acquire
the RCU lock for the entire scan of the domain. This patch removes the
warning triggered at boot time.

Fixes: ff7db0bf24db ("sched/numa: Prefer using an idle CPU as a migration target instead of comparing tasks")
Reported-by: Qian Cai <cai@....pw>
Signed-off-by: Mel Gorman <mgorman@...hsingularity.net>
Reviewed-by: Lukasz Luba <lukasz.luba@....com>
Reviewed-by: Paul E. McKenney <paulmck@...nel.org>
---
 kernel/sched/fair.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 33622bbfcea7..23a6fe298720 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -1595,6 +1595,7 @@ static void update_numa_stats(struct task_numa_env *env,
 	memset(ns, 0, sizeof(*ns));
 	ns->idle_cpu = -1;
 
+	rcu_read_lock();
 	for_each_cpu(cpu, cpumask_of_node(nid)) {
 		struct rq *rq = cpu_rq(cpu);
 
@@ -1614,6 +1615,7 @@ static void update_numa_stats(struct task_numa_env *env,
 			idle_core = numa_idle_core(idle_core, cpu);
 		}
 	}
+	rcu_read_unlock();
 
 	ns->weight = cpumask_weight(cpumask_of_node(nid));
 
-- 
2.16.4