Re: [PATCH 2/3] sched: drop group_capacity to 1 only if remote group has no running tasks

[Suresh Siddha <suresh.b.siddha@intel.com>]

On Mon, 2010-09-27 at 17:29 -0700, Nikhil Rao wrote:
> When SD_PREFER_SIBLING is set on a sched domain, drop group_capacity to 1
> only if the remote sched group has no running tasks. This addresses the case
> where you have two tasks on one socket and the other socket is idle, in which
> case you drop the capacity to 1. If the remote group has >=1 running task, then
> there is no difference from a cache-sharing perspective.
> 
> Signed-off-by: Nikhil Rao <ncrao@google.com>
> ---
>  kernel/sched_fair.c |    2 +-
>  1 files changed, 1 insertions(+), 1 deletions(-)
> 
> diff --git a/kernel/sched_fair.c b/kernel/sched_fair.c
> index de8a6a0..33a7985 100644
> --- a/kernel/sched_fair.c
> +++ b/kernel/sched_fair.c
> @@ -2548,7 +2548,7 @@ static inline void update_sd_lb_stats(struct sched_domain *sd, int this_cpu,
>  		 * first, lower the sg capacity to one so that we'll try
>  		 * and move all the excess tasks away.
>  		 */
> -		if (prefer_sibling)
> +		if (prefer_sibling && !sgs.sum_nr_running)
>  			sgs.group_capacity = min(sgs.group_capacity, 1UL);
>  
>  		if (local_group) {

Nikhil, Doesn't this break the case of:

two sockets with dual-core and HT. Four tasks currently scheduled as:
three on socket-0 (two threads on core-0 running two tasks and 1 thread
on core-1 running one task). One on socket-1 (one thread on core-0
running a task, with other core-1 idle)

We would like to move the task from core-0 socket-0 to core-1 socket-1,
while we are load balancing at the socket level (it might be smp or numa
level depending on system).

thanks,
suresh