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 <r...@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.
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