Messages in this thread | | | Date | Wed, 29 Nov 2023 09:55:33 +0100 | Subject | Re: [PATCH v2] sched/fair: Use all little CPUs for CPU-bound workload | From | Dietmar Eggemann <> |
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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?
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> Similar issue reported at: > https://lore.kernel.org/lkml/20230716014125.139577-1-qyousef@layalina.io/ > > v1: > https://lore.kernel.org/all/20231110125902.2152380-1-pierre.gondois@arm.com/ > > Suggested-by: Vincent Guittot <vincent.guittot@linaro.org> > Signed-off-by: Pierre Gondois <pierre.gondois@arm.com>
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.
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Reviewed-by: Dietmar Eggemann <dietmar.eggemann@arm.com>
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