Re: Bug in SCHED_IDLE interaction with group scheduling?

From: Ingo Molnar
Date: Sat Jul 11 2009 - 04:02:50 EST



* Paul Turner <pjt@xxxxxxxxxx> wrote:

> Hi Peter,
>
> It seems like there's an assumption in the sched_idle policy check in
> place_entity that the sched_entity we are looking at belongs to a task?
>
> - Paul
>
> --
>
> sched: bug in SCHED_IDLE interaction with group scheduling
>
> One of the isolation modifications for SCHED_IDLE is the
> unitization of sleeper credit. However the check for this assumes
> that the sched_entity we're placing always belongs to a task.
>
> This is potentially not true with group scheduling and leaves us
> rummaging randomly when we try to pull the policy.
>
> Signed-off-by: Paul Turner <pjt@xxxxxxxxxx>
> ---
> kernel/sched_fair.c | 3 ++-
> 1 files changed, 2 insertions(+), 1 deletions(-)
>
> diff --git a/kernel/sched_fair.c b/kernel/sched_fair.c
> index ba7fd6e..7c248dc 100644
> --- a/kernel/sched_fair.c
> +++ b/kernel/sched_fair.c
> @@ -687,7 +687,8 @@ place_entity(struct cfs_rq *cfs_rq, struct sched_entity *se, int initial)
> * all of which have the same weight.
> */
> if (sched_feat(NORMALIZED_SLEEPER) &&
> - task_of(se)->policy != SCHED_IDLE)
> + (!entity_is_task(se) ||
> + task_of(se)->policy != SCHED_IDLE))
> thresh = calc_delta_fair(thresh, se);

Ah, nice catch!

( Sidenote: we keep having these subtle bugs where we call
task_of(se) on a non-task entity - it's been the fifth such
incident or so. Perhaps we should add an active debug check to
task_of(), in the case of CONFIG_SCHED_DEBUG=y. )

Ingo
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