Re: [PATCH V4 1/2] sched/fair: Fix load_balance() affinity redo path

[Jeffrey Hugo]

On 6/2/2017 4:27 PM, Jeffrey Hugo wrote:
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index d711093..84255ab 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -6737,10 +6737,10 @@ int can_migrate_task(struct task_struct *p, struct lb_env *env)
>   		 * our sched_group. We may want to revisit it if we couldn't
>   		 * meet load balance goals by pulling other tasks on src_cpu.
>   		 *
> -		 * Also avoid computing new_dst_cpu if we have already computed
> -		 * one in current iteration.
> +		 * Avoid computing new_dst_cpu for NEWLY_IDLE or if we have
> +		 * already computed one in current iteration.
>   		 */
> -		if (!env->dst_grpmask || (env->flags & LBF_DST_PINNED))
> +		if (env->idle == CPU_NEWLY_IDLE || (env->flags & LBF_DST_PINNED))
>   			return 0;

Self NACK.  This breaks active_load_balance_cpu_stop().  Looks like 
env->idle == CPU_IDLE, but env->dst_grpmask is uninitialized, so it can 
be NULL, which causes a null pointer dereference a few lines later.

I'm still having a look to see what makes sense to address the issue.


--
Jeffrey Hugo
Qualcomm Datacenter Technologies as an affiliate of Qualcomm 
Technologies, Inc.
Qualcomm Technologies, Inc. is a member of the
Code Aurora Forum, a Linux Foundation Collaborative Project.