Hi Vincent,
On Fri, 2015-07-31 at 11:49 +0200, Vincent Guittot wrote:
> Since commit d4573c3e1c99 ("sched: Improve load balancing in the presence
> of idle CPUs"), the ILB CPU starts with the idle load balancing of other
> idle CPUs and finishes with itself in order to speed up the spread of tasks
> in all idle CPUs.
>
> The this_rq->next_balance is still used in nohz_idle_balance as an
> intermediate step to gather the shortest next balance before updating
> nohz.next_balance. But the former has not been updated yet and is likely to
> be set with the current jiffies. As a result, the nohz.next_balance will be
> set with current jiffies instead of the real next balance date. This
> generates spurious kicks of nohz ilde balance.
>
> nohz_idle_balance must set the nohz.next_balance without taking into
> account this_rq->next_balance which is not updated yet. Then, this_rq will
> update nohz.next_update with its next_balance once updated and if necessary.
>
> Signed-off-by: Vincent Guittot <[email protected]>
> ---
> kernel/sched/fair.c | 33 +++++++++++++++++++++++++++++----
> 1 file changed, 29 insertions(+), 4 deletions(-)
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 587a2f6..2b02089 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -7779,8 +7779,21 @@ static void rebalance_domains(struct rq *rq, enum
> cpu_idle_type idle)
> * When the cpu is attached to null domain for ex, it will not be
> * updated.
> */
> - if (likely(update_next_balance))
> + if (likely(update_next_balance)) {
> rq->next_balance = next_balance;
> +
> + /*
> + * If this cpu has been elected to perform the nohz idle
> + * balance. Other idle cpus have already rebalance with
> + * nohz_idle_balance and the nohz.next_balaance has been
> + * updated accordingly. This cpu has now run the idle load
> + * balance for itself and we need to update the
> + * nohz.next_balance accordingly.
> + */
> + if ((idle == CPU_IDLE) &&
> + time_after(nohz.next_balance, rq->next_balance))
> + nohz.next_balance = rq->next_balance;
The code accessing the "nohz" structure may also need to be put in an:
#ifdef CONFIG_NO_HZ_COMMON
in order for it to compile when !CONFIG_NO_HZ_COMMON.
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