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From: Erich Focht <efocht@ess.nec.de>
To: Ingo Molnar <mingo@elte.hu>, linux-kernel@vger.kernel.org
Subject: Re: [patch] HT scheduler, sched-2.5.59-E2
Date: Tue, 4 Feb 2003 10:31:20 +0100
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Hi Ingo,

On Monday 03 February 2003 19:23, Ingo Molnar wrote:
> -#define
> CAN_MIGRATE_TASK(p,rq,this_cpu)                                        \
> -       ((jiffies - (p)->sleep_timestamp > cache_decay_ticks) &&        \
> -               !task_running(rq, p) &&                                 \
> -                       ((p)->cpus_allowed & (1UL << (this_cpu)))) +#define
> CAN_MIGRATE_TASK(p,rq,cpu)                                     \
> +       ((idle || (jiffies - (p)->last_run > cache_decay_ticks)) && \
> +               !task_running(p) && task_allowed(p, cpu))

at least for NUMA systems this is too aggressive (though I believe
normal SMP systems could be hurt, too).

The problem: freshly forked tasks are stolen by idle cpus on the same
node before they exec. This actually disables the sched_balance_exec()
mechanism as the tasks to be balanced already run alone on other
CPUs. Which means: the whole benefit of having balanced nodes
(maximize the memory bandwidth) is gone.

The change below is less aggressive but in the same philosophy. Could
you please take it instead?

> CAN_MIGRATE_TASK(p,rq,cpu)                                     \
> +       ((jiffies - (p)->last_run > (cache_decay_ticks >> idle)) && \
> +               !task_running(p) && task_allowed(p, cpu))

Regards,
Erich

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