Re: [PATCH] sched/numa: Acquire RCU lock for checking idle cores during NUMA balancing

["Paul E. McKenney" <paulmck@kernel.org>]

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));
>