Re: [PATCH] sched/fair: Rate limit calls to update_blocked_averages() for NOHZ

Joel Fernandes
2021-02-03T19:57:21Z

On Fri, Jan 29, 2021 at 06:27:27PM +0100, Vincent Guittot wrote:
[...]
> > update_blocked_averages with preempt and irq off is not a good thing
> > because we don't manage the number of csf_rq to update and I'm going
> > to provide a patchset for this
>
> The patch below moves the update of the blocked load of CPUs outside newidle_balance().
>
> Instead, the update is done with the usual idle load balance update. I'm working on an
> additonnal patch that will select this cpu that is about to become idle, instead of a
> random idle cpu but this 1st step fixe the problem of lot of update in newly idle.
>
> Signed-off-by: Vincent Guittot <vincent.guittot@linaro.org>

I confirmed that with this patch, I don't see the preemptoff issues related
to update_blocked_averages() anymore (tested using preemptoff tracer).

I went through the patch and it looks correct to me, I will further review it
and await further reviews from others as well, and then backport the patch to
our kernels. Thanks Vince and everyone!

Tested-by: Joel Fernandes (Google) <joel@joelfernandes.org>

thanks,

- Joel

> ---
> kernel/sched/fair.c | 32 +++-----------------------------
> 1 file changed, 3 insertions(+), 29 deletions(-)
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 197a51473e0c..8200b1d4df3d 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -7421,8 +7421,6 @@ enum migration_type {
> #define LBF_NEED_BREAK 0x02
> #define LBF_DST_PINNED 0x04
> #define LBF_SOME_PINNED 0x08
> -#define LBF_NOHZ_STATS 0x10
> -#define LBF_NOHZ_AGAIN 0x20
>
> struct lb_env {
> struct sched_domain *sd;
> @@ -8426,9 +8424,6 @@ static inline void update_sg_lb_stats(struct lb_env *env,
> for_each_cpu_and(i, sched_group_span(group), env->cpus) {
> struct rq *rq = cpu_rq(i);
>
> - if ((env->flags & LBF_NOHZ_STATS) && update_nohz_stats(rq, false))
> - env->flags |= LBF_NOHZ_AGAIN;
> -
> sgs->group_load += cpu_load(rq);
> sgs->group_util += cpu_util(i);
> sgs->group_runnable += cpu_runnable(rq);
> @@ -8969,11 +8964,6 @@ static inline void update_sd_lb_stats(struct lb_env *env, struct sd_lb_stats *sd
> struct sg_lb_stats tmp_sgs;
> int sg_status = 0;
>
> -#ifdef CONFIG_NO_HZ_COMMON
> - if (env->idle == CPU_NEWLY_IDLE && READ_ONCE(nohz.has_blocked))
> - env->flags |= LBF_NOHZ_STATS;
> -#endif
> -
> do {
> struct sg_lb_stats *sgs = &tmp_sgs;
> int local_group;
> @@ -9010,15 +9000,6 @@ static inline void update_sd_lb_stats(struct lb_env *env, struct sd_lb_stats *sd
> /* Tag domain that child domain prefers tasks go to siblings first */
> sds->prefer_sibling = child && child->flags & SD_PREFER_SIBLING;
>
> -#ifdef CONFIG_NO_HZ_COMMON
> - if ((env->flags & LBF_NOHZ_AGAIN) &&
> - cpumask_subset(nohz.idle_cpus_mask, sched_domain_span(env->sd))) {
> -
> - WRITE_ONCE(nohz.next_blocked,
> - jiffies + msecs_to_jiffies(LOAD_AVG_PERIOD));
> - }
> -#endif
> -
> if (env->sd->flags & SD_NUMA)
> env->fbq_type = fbq_classify_group(&sds->busiest_stat);
>
> @@ -10547,14 +10528,7 @@ static void nohz_newidle_balance(struct rq *this_rq)
> return;
>
> raw_spin_unlock(&this_rq->lock);
> - /*
> - * This CPU is going to be idle and blocked load of idle CPUs
> - * need to be updated. Run the ilb locally as it is a good
> - * candidate for ilb instead of waking up another idle CPU.
> - * Kick an normal ilb if we failed to do the update.
> - */
> - if (!_nohz_idle_balance(this_rq, NOHZ_STATS_KICK, CPU_NEWLY_IDLE))
> - kick_ilb(NOHZ_STATS_KICK);
> + kick_ilb(NOHZ_STATS_KICK);
> raw_spin_lock(&this_rq->lock);
> }
>
> @@ -10616,8 +10590,6 @@ static int newidle_balance(struct rq *this_rq, struct rq_flags *rf)
> update_next_balance(sd, &next_balance);
> rcu_read_unlock();
>
> - nohz_newidle_balance(this_rq);
> -
> goto out;
> }
>
> @@ -10683,6 +10655,8 @@ static int newidle_balance(struct rq *this_rq, struct rq_flags *rf)
>
> if (pulled_task)
> this_rq->idle_stamp = 0;
> + else
> + nohz_newidle_balance(this_rq);
>
> rq_repin_lock(this_rq, rf);
>
> --
> 2.17.1
>
>
> >
> > > for this.
> > >
> > > > Also update_blocked_averages was supposed called in newlyidle_balance
> > > > when the coming idle duration is expected to be long enough
> > >
> > > No, we do not want the schedule loop to take half a millisecond.
> >
> > keep in mind that you are scaling frequency so everything takes time
> > at lowest frequency/capacity ...
> >
> > >
> > > > > > IIUC, your real problem is that newidle_balance is running whereas a
> > > > > > task is about to wake up on the cpu and we don't abort quickly during
> > > > > > this load_balance
> > > > >
> > > > > Yes.
> > > > >
> > > > > > so we could also try to abort earlier in case of newly idle load balance
> > > > >
> > > > > I think interrupts are disabled when the load balance runs, so there's no way
> > > > > for say an audio interrupt to even run in order to wake up a task. How would
> > > > > you know to abort the new idle load balance?
> > > > >
> > > > > Could you elaborate more also on the drawback of the rate limiting patch we
> > > > > posted? Do you see a side effect?
> > > >
> > > > Your patch just tries to hide your problem and not to solve the root cause.
> > >
> > > Agreed, the solution presented is a band aid and not a proper fix. It
> > > was just intended to illustrate the problem and start a discussion. I
> > > should have marked it RFC for sure.
> > >
> > > thanks!
> > >
> > > - Joel