Re: [PATCH 2/6] sched: make sched_slice() group scheduling savvy

[Srivatsa Vaddagiri <vatsa@linux.vnet.ibm.com>]

On Wed, Oct 31, 2007 at 10:10:32PM +0100, Peter Zijlstra wrote:
> Currently the ideal slice length does not take group scheduling into account.
> Change it so that it properly takes all the runnable tasks on this cpu into
> account and caluclate the weight according to the grouping hierarchy.
> 
> Also fixes a bug in vslice which missed a factor NICE_0_LOAD.
> 
> Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
> CC: Srivatsa Vaddagiri <vatsa@linux.vnet.ibm.com>
> ---
>  kernel/sched_fair.c |   42 +++++++++++++++++++++++++++++++-----------
>  1 file changed, 31 insertions(+), 11 deletions(-)
> 
> Index: linux-2.6/kernel/sched_fair.c
> ===================================================================
> --- linux-2.6.orig/kernel/sched_fair.c
> +++ linux-2.6/kernel/sched_fair.c
> @@ -331,10 +331,15 @@ static u64 __sched_period(unsigned long 
>   */
>  static u64 sched_slice(struct cfs_rq *cfs_rq, struct sched_entity *se)
>  {
> -	u64 slice = __sched_period(cfs_rq->nr_running);
> +	unsigned long nr_running = rq_of(cfs_rq)->nr_running;
> +	u64 slice = __sched_period(nr_running);
> 
> -	slice *= se->load.weight;
> -	do_div(slice, cfs_rq->load.weight);
> +	for_each_sched_entity(se) {
> +		cfs_rq = cfs_rq_of(se);
> +
> +		slice *= se->load.weight;
> +		do_div(slice, cfs_rq->load.weight);
> +	}
> 
>  	return slice;


Lets say we have two groups A and B on CPU0, of equal weight (1024).

Further,

A has 1 task (A0)
B has 1000 tasks (B0 .. B999) 

Agreed its a extreme case, but illustrates the problem I have in mind
for this patch.

All tasks of same weight=1024.

Before this patch
=================

	sched_slice(grp A) = 20ms * 1/2 = 10ms
	sched_slice(A0) = 20ms

	sched_slice(grp B) = 20ms * 1/2 = 10ms
	sched_slice(B0) = (20ms * 1000/20) * 1 / 1000 = 1ms
	sched_slice(B1) = ... = sched_slice(B99) = 1 ms

Fairness between groups and tasks would be obtained as below:

    A0       B0-B9     A0    B10-B19     A0     B20-B29
 |--------|--------|--------|--------|--------|--------|-----//--| 
 0       10ms	   20ms	   30ms     40ms     50ms     60ms

After this patch
================

	sched_slice(grp A) = (20ms * 1001/20) * 1/2 ~= 500ms
	sched_slice(A0) = 500ms

	sched_slice(grp B) = 500ms
	sched_slice(B0) = 0.5ms 

Fairness between groups and tasks would be obtained as below:

	    A0		          B0 - B99  	            A0
 |-----------------------|-----------------------|-----------------------|
 0		        500ms			1000ms 			1500ms

Did I get it right? If so, I don't like the fact that group A is allowed to run 
for a long time (500ms) before giving chance to group B.

Can I know what real problem is being addressed by this change to
sched_slice()?

-- 
Regards,
vatsa