Re: [PATCH v2] sched: fix nohz.next_balance update

Vincent Guittot
2015-08-27T11:21:39Z

Hi,

On 3 August 2015 at 11:55, Vincent Guittot <vincent.guittot@linaro.org> 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 <vincent.guittot@linaro.org>
> ---
>
> change since v1:
> - add #ifdef CONFIG_NO_HZ_COMMON for accessing nohz structure
> - fix some typos
>
> kernel/sched/fair.c | 35 +++++++++++++++++++++++++++++++----
> 1 file changed, 31 insertions(+), 4 deletions(-)
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 587a2f6..581378a 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -7779,8 +7779,23 @@ 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;
> +
> +#ifdef CONFIG_NO_HZ_COMMON
> + /*
> + * If this cpu has been elected to perform the nohz idle
> + * balance. Other idle cpus have already rebalanced with
> + * nohz_idle_balance and the nohz.next_balance has been
> + * updated accordingly. This cpu is now running 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;
> +#endif
> + }
> }
>
> #ifdef CONFIG_NO_HZ_COMMON
> @@ -7793,6 +7808,9 @@ static void nohz_idle_balance(struct rq *this_rq, enum cpu_idle_type idle)
> int this_cpu = this_rq->cpu;
> struct rq *rq;
> int balance_cpu;
> + /* Earliest time when we have to do rebalance again */
> + unsigned long next_balance = jiffies + 60*HZ;
> + int update_next_balance = 0;
>
> if (idle != CPU_IDLE ||
> !test_bit(NOHZ_BALANCE_KICK, nohz_flags(this_cpu)))
> @@ -7824,10 +7842,19 @@ static void nohz_idle_balance(struct rq *this_rq, enum cpu_idle_type idle)
> rebalance_domains(rq, CPU_IDLE);
> }
>
> - if (time_after(this_rq->next_balance, rq->next_balance))
> - this_rq->next_balance = rq->next_balance;
> + if (time_after(next_balance, rq->next_balance)) {
> + next_balance = rq->next_balance;
> + update_next_balance = 1;
> + }
> }
> - nohz.next_balance = this_rq->next_balance;
> +
> + /*
> + * next_balance will be updated only when there is a need.
> + * When the cpu is attached to null domain for ex, it will not be
> + * updated.
> + */
> + if (likely(update_next_balance))
> + nohz.next_balance = next_balance;
> end:
> clear_bit(NOHZ_BALANCE_KICK, nohz_flags(this_cpu));
> }
> --
> 1.9.1
>

Gentle ping

Regards,