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sched/numa: Acquire RCU lock for checking idle cores during NUMA balancing 

Qian Cai reported the following bug:

  The linux-next commit ff7db0bf24 ("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.

Reported-by: Qian Cai <cai@lca.pw>
Reviewed-by: Paul E. McKenney <paulmck@kernel.org>
Signed-off-by: Mel Gorman <mgorman@techsingularity.net>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Fixes: ff7db0bf24 ("sched/numa: Prefer using an idle CPU as a migration target instead of comparing tasks")
Link: https://lkml.kernel.org/r/20200227191804.GJ3818@techsingularity.net
This commit is contained in:
Mel Gorman 2020-02-27 19:18:04 +00:00 committed by Ingo Molnar
parent 76c389ab2b
commit 0621df3154
1 changed files with 2 additions and 0 deletions
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2
kernel/sched/fair.c
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@ -1608,6 +1608,7 @@ 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);
@ -1627,6 +1628,7 @@ 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));