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SubjectRe: [PATCH] sched/numa: Acquire RCU lock for checking idle cores during NUMA balancing
On Thu, Feb 27, 2020 at 07:18:04PM +0000, Mel Gorman wrote:
> 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@lca.pw>
> Signed-off-by: Mel Gorman <mgorman@techsingularity.net>

From an RCU viewpoint:

Reviewed-by: Paul E. McKenney <paulmck@kernel.org>

> ---
> kernel/sched/fair.c | 2 ++
> 1 file changed, 2 insertions(+)
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 10f9e6729fcf..1592b6d26239 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));
>

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