Re: [PATCH] sched: fix env->src_cpu for active migration

[Damien Wyart <damien.wyart@gmail.com>]

Hi,

I tested this on top of 3.8-rc7 and this made the machine (x86_64, Core
i7 920) unable to boot (very early as nothing at all is displayed on
screen). Nothing in the kernel log (after booting with a working
kernel).

Double-checked by just backing out only this patch and this made the
machine boot again.

Damien

> need_active_balance uses env->src_cpu which is set only if there is more
> than 1 task on the run queue. We must set the src_cpu field unconditionnally
> otherwise the test "env->src_cpu > env->dst_cpu" will always fail if there is
> only 1 task on the run queue
> 
> Signed-off-by: Vincent Guittot <vincent.guittot@linaro.org>
> ---
>  kernel/sched/fair.c |    6 ++++--
>  1 file changed, 4 insertions(+), 2 deletions(-)
> 
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 81fa536..32938ea 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -5044,6 +5044,10 @@ redo:
>  
>  	ld_moved = 0;
>  	lb_iterations = 1;
> +
> +	env.src_cpu   = busiest->cpu;
> +	env.src_rq    = busiest;
> +
>  	if (busiest->nr_running > 1) {
>  		/*
>  		 * Attempt to move tasks. If find_busiest_group has found
> @@ -5052,8 +5056,6 @@ redo:
>  		 * correctly treated as an imbalance.
>  		 */
>  		env.flags |= LBF_ALL_PINNED;
> -		env.src_cpu   = busiest->cpu;
> -		env.src_rq    = busiest;
>  		env.loop_max  = min(sysctl_sched_nr_migrate, busiest->nr_running);
>  
>  		update_h_load(env.src_cpu);