linux-kernel - Re: [PATCH] sched: fix exec_start/task

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Message-ID: <20140516080424.GM11096@twins.programming.kicks-ass.net>
Date:	Fri, 16 May 2014 10:04:24 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Ben Segall <bsegall@...gle.com>
Cc:	Ingo Molnar <mingo@...hat.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] sched: fix exec_start/task_hot on migrated tasks

On Thu, May 15, 2014 at 03:59:20PM -0700, Ben Segall wrote:
> task_hot checks exec_start on any runnable task, but if it has been
> migrated since the it last ran, then exec_start is a clock_task from
> another cpu. If the old cpu's clock_task was sufficiently far ahead of
> this cpu's then the task will not be considered for another migration
> until it has run. Instead reset exec_start whenever a task is migrated,
> since it is presumably no longer hot anyway.
> 
> Signed-off-by: Ben Segall <bsegall@...gle.com>
> ---
>  kernel/sched/fair.c |    3 +++
>  1 file changed, 3 insertions(+)
> 
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 28ccf50..9f8dfeb 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -4544,6 +4544,9 @@ migrate_task_rq_fair(struct task_struct *p, int next_cpu)
>  		atomic_long_add(se->avg.load_avg_contrib,
>  						&cfs_rq->removed_load);
>  	}
> +
> +	/* We have migrated, no longer consider this task hot */
> +	se.exec_start = 0;
>  }

0 isn't strictly the right thing to do here, since the clock can wrap,
but being wrong every ~585 years isn't too big an issue for this.

Thanks!

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