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SubjectRe: [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@google.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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