Re: [PATCH,RFC] sched: fix "impossible" load balancing oops

[Peter Zijlstra <peterz@infradead.org>]

On Fri, Aug 21, 2015 at 02:21:22PM -0400, Rik van Riel wrote:
> The load balancing code can run into the situation where
> the source and destination runqueues are the same, in
> the active balancing code.
> 
>         /*
>          * This condition is "impossible", if it occurs
>          * we need to fix it. Originally reported by
>          * Bjorn Helgaas on a 128-cpu setup.
>          */
>         BUG_ON(busiest_rq == target_rq);
> 
> This happens despite not triggering the BUG_ON(busiest == env.dst_rq)
> line after find_busiest_queue.
> 
> From code inspection, it appears there is a condition where this can happen.
> 
> Specifically, if we encounter only pinned tasks on a CPU, can_migrate_task
> will set env->new_dst_cpu to a CPU in the env->dst_grpmask.  If the group
> includes the source cpu, we may end up setting env.dst_cpu to the same
> as dst.src_cpu.
> 
> The fix would be to clear the source cpu from env.dst_grpmask, to ensure
> we never select the source cpu as the destination.
> 
> Signed-off-by: Rik van Riel <riel@redhat.com>
> ---
>  kernel/sched/fair.c | 2 ++
>  1 file changed, 2 insertions(+)
> 
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index d113c3b..514a369 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -7172,6 +7172,8 @@ redo:
>  	env.src_cpu = busiest->cpu;
>  	env.src_rq = busiest;
>  
> +	cpumask_clear_cpu(busiest->cpu, env.dst_grpmask);
> +
>  	ld_moved = 0;
>  	if (busiest->nr_running > 1) {
>  		/*

Right, so real problem 'recently' introduced by:

  88b8dac0a14c ("sched: Improve balance_cpu() to consider other cpus in its group as target of (pinned) task")

But I think there's a wee problem with the solution, if I'm not still
entirely asleep, it appears dst_grpmask might be NULL in case of
CPU_NEWLY_IDLE, which would make that cpumask_clear_cpu() do something
naughty.