Re: [PATCH v2] sched/fair: Use all little CPUs for CPU-bound workload

[Pierre Gondois]

On 11/29/23 09:55, Dietmar Eggemann wrote:
> On 24/11/2023 16:33, Pierre Gondois wrote:
> > Running n CPU-bound tasks on an n CPUs platform:
> > - with asymmetric CPU capacity
> > - not having SD_SHARE_PKG_RESOURCES flag set at the DIE
>
> nit: DIE is now called PKG on tip sched/core.
>
> f577cd57bfaa - sched/topology: Rename 'DIE' domain to 'PKG' (2023-10-12
> Peter Zijlstra)
>
> >    sched domain level (i.e. not DynamIQ systems)
> > might result in a task placement where two tasks run on a big CPU
> > and none on a little CPU. This placement could be more optimal by
> > using all CPUs.
> >
> > Testing platform:
> > Juno-r2:
> > - 2 big CPUs (1-2), maximum capacity of 1024
> > - 4 little CPUs (0,3-5), maximum capacity of 383
> >
> > Testing workload ([1]):
> > Spawn 6 CPU-bound tasks. During the first 100ms (step 1), each tasks
> > is affine to a CPU, except for:
> > - one little CPU which is left idle.
> > - one big CPU which has 2 tasks affine.
> > After the 100ms (step 2), remove the cpumask affinity.
> >
> > Before patch:
> > During step 2, the load balancer running from the idle CPU tags sched
> > domains as:
> > - little CPUs: 'group_has_spare'. Indeed, 3 CPU-bound tasks run on a
> >    4 CPUs sched-domain, and the idle CPU provides enough spare
> >    capacity.
>
> What is meant by 'idle CPU provides enough spare capacity? I thought the
> task (util_avg ~ 512_ does not fit on the sched group [1,3-5] when we
> consider util_avg/capacity (383)

Right, I meant that when evaluating the 'group_type', there is enough spare
capacity when summing the utilization of CPUs in the the MC sched domain:

---
group_has_capacity()
{
	[...]
	if ((sgs->group_capacity * 100) >
			(sgs->group_util * imbalance_pct))
		return true;
	[...]
}
---

> The calculated imbalance of ~350 is too small for the task-size and
> that's why we need the 'shr_bound(util, env->sd->nr_balance_failed)' to
> let the task load-balance if nr_balance_failed = 2?

Yes exact, the tasks are too big and cannot fit this imbalance value
(representing the available spare capacity in the little CPUs in this case).

'shr_bound(...)' allows to progressively reduce the size of the tasks and
allow migrations after having tried to balance 'nr_balance_failed' times.


> [...]
>
> > Similar issue reported at:
> > https://lore.kernel.org/lkml/20230716014125.139577-1-qyousef@xxxxxxxxxxx/
> >
> > v1:
> > https://lore.kernel.org/all/20231110125902.2152380-1-pierre.gondois@xxxxxxx/
> >
> > Suggested-by: Vincent Guittot <vincent.guittot@xxxxxxxxxx>
> > Signed-off-by: Pierre Gondois <pierre.gondois@xxxxxxx>
>
> Even though this cures only classical big.LITTLE it might have a
> positive effect on today's Arm DynamIQ Android systems with Phantom SDs
> when running benchmarks like Geekbench.
>
> [...]
>
> Reviewed-by: Dietmar Eggemann <dietmar.eggemann@xxxxxxx>