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

[Dietmar Eggemann]

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)

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?

[...]

> 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>