Re: [PATCH v5 1/3] sched: Stop nohz stats when decayed

From: Vincent Guittot
Date: Thu Feb 15 2018 - 12:06:40 EST

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In reply to: Patrick Bellasi: "Re: [PATCH v5 1/3] sched: Stop nohz stats when decayed"
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Hi Patrick,

On 15 February 2018 at 16:22, Patrick Bellasi <patrick.bellasi@xxxxxxx> wrote:
> Hi Vincent,
> here are some possible small code improvement suggestions from my side.
>
> Cheers Patrick
>
> On 14-Feb 16:26, Vincent Guittot wrote:
>> > Stopped the periodic update of blocked load when all idle CPUs have fully
>> > decayed. We introduce a new nohz.has_blocked that reflect if some idle
>> > CPUs has blocked load that have to be periodiccally updated. nohz.has_blocked
>> > is set everytime that a Idle CPU can have blocked load and it is then clear
>> > when no more blocked load has been detected during an update. We don't need
>> > atomic operation but only to make cure of the right ordering when updating
>> > nohz.idle_cpus_mask and nohz.has_blocked.
>> >
>> > Suggested-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
>> > Signed-off-by: Vincent Guittot <vincent.guittot@xxxxxxxxxx>
>> > ---
>> > kernel/sched/fair.c | 122 ++++++++++++++++++++++++++++++++++++++++++---------
>> > kernel/sched/sched.h | 1 +
>> > 2 files changed, 102 insertions(+), 21 deletions(-)
>> >
>> > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
>> > index 7af1fa9..5a6835e 100644
>> > --- a/kernel/sched/fair.c
>> > +++ b/kernel/sched/fair.c
>> > @@ -5383,8 +5383,9 @@ decay_load_missed(unsigned long load, unsigned long missed_updates, int idx)
>> > static struct {
>> > cpumask_var_t idle_cpus_mask;
>> > atomic_t nr_cpus;
>> > + int has_blocked; /* Idle CPUS has blocked load */
>
> Why not "bool"?
>
> What about "has_blocked_load", which makes it more clear the meaning
> and is also more consistent with the RQ flags "aggregated" by this
> one... as well as some other local variables you already name like
> that?
>
>> unsigned long next_balance; /* in jiffy units */
>> - unsigned long next_stats;
>> + unsigned long next_blocked; /* Next update of blocked load in jiffies */
>> } nohz ____cacheline_aligned;
>>
>> #endif /* CONFIG_NO_HZ_COMMON */
>> @@ -6951,6 +6952,7 @@ enum fbq_type { regular, remote, all };
>> #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;
>> @@ -7335,8 +7337,6 @@ static void attach_tasks(struct lb_env *env)
>> rq_unlock(env->dst_rq, &rf);
>> }
>>
>> -#ifdef CONFIG_FAIR_GROUP_SCHED
>> -
>> static inline bool cfs_rq_is_decayed(struct cfs_rq *cfs_rq)
>> {
>> if (cfs_rq->load.weight)
>> @@ -7354,11 +7354,14 @@ static inline bool cfs_rq_is_decayed(struct cfs_rq *cfs_rq)
>> return true;
>> }
>>
>> +#ifdef CONFIG_FAIR_GROUP_SCHED
>> +
>> static void update_blocked_averages(int cpu)
>> {
>> struct rq *rq = cpu_rq(cpu);
>> struct cfs_rq *cfs_rq, *pos;
>> struct rq_flags rf;
>> + bool done = true;
>>
>> rq_lock_irqsave(rq, &rf);
>> update_rq_clock(rq);
>> @@ -7388,10 +7391,14 @@ static void update_blocked_averages(int cpu)
>> */
>> if (cfs_rq_is_decayed(cfs_rq))
>> list_del_leaf_cfs_rq(cfs_rq);
>> + else
>> + done = false;
>> }
>>
>> #ifdef CONFIG_NO_HZ_COMMON
>> rq->last_blocked_load_update_tick = jiffies;
>> + if (done)
>> + rq->has_blocked_load = 0;
>> #endif
>> rq_unlock_irqrestore(rq, &rf);
>> }
>> @@ -7454,6 +7461,8 @@ static inline void update_blocked_averages(int cpu)
>> update_cfs_rq_load_avg(cfs_rq_clock_task(cfs_rq), cfs_rq);
>> #ifdef CONFIG_NO_HZ_COMMON
>> rq->last_blocked_load_update_tick = jiffies;
>> + if (cfs_rq_is_decayed(cfs_rq))
>> + rq->has_blocked_load = 0;
>> #endif
>> rq_unlock_irqrestore(rq, &rf);
>> }
>> @@ -7789,18 +7798,25 @@ group_type group_classify(struct sched_group *group,
>> return group_other;
>> }
>>
>> -static void update_nohz_stats(struct rq *rq)
>> +static bool update_nohz_stats(struct rq *rq)
>> {
>> #ifdef CONFIG_NO_HZ_COMMON
>> unsigned int cpu = rq->cpu;
>>
>> + if (!rq->has_blocked_load)
>> + return false;
>> +
>> if (!cpumask_test_cpu(cpu, nohz.idle_cpus_mask))
>> - return;
>> + return false;
>>
>> if (!time_after(jiffies, rq->last_blocked_load_update_tick))
>> - return;
>> + return true;
>>
>> update_blocked_averages(cpu);
>> +
>> + return rq->has_blocked_load;
>> +#else
>> + return false;
>> #endif
>> }
>>
>> @@ -7826,8 +7842,8 @@ 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);
>> + if ((env->flags & LBF_NOHZ_STATS) && update_nohz_stats(rq))
>> + env->flags |= LBF_NOHZ_AGAIN;
>>
>> /* Bias balancing toward cpus of our domain */
>> if (local_group)
>> @@ -7979,18 +7995,17 @@ static inline void update_sd_lb_stats(struct lb_env *env, struct sd_lb_stats *sd
>> struct sg_lb_stats *local = &sds->local_stat;
>> struct sg_lb_stats tmp_sgs;
>> int load_idx, prefer_sibling = 0;
>> +#ifdef CONFIG_NO_HZ_COMMON
>> + int has_blocked = READ_ONCE(nohz.has_blocked);
>> +#endif
>> bool overload = false;
>>
>> if (child && child->flags & SD_PREFER_SIBLING)
>> prefer_sibling = 1;
>>
>> #ifdef CONFIG_NO_HZ_COMMON
>> - if (env->idle == CPU_NEWLY_IDLE) {
>> + if (env->idle == CPU_NEWLY_IDLE && has_blocked)
>
> Why using a local "has_blocked" here?
>
> Is not better to inline directly the READ_ONCE, we avoid the above
> idfed and also (maybe?) save the load if the previous checks should fail?

I agree that saving ifdef could be interesting although it make the
condition less readable but that's just an opinion
About saving the load, I expect the compiler to optimize that
correctly and i'm not sure that inlining the READ_ONCE will prevent
any reordering and will really save the load

>
>> env->flags |= LBF_NOHZ_STATS;
>> -
>> - if (cpumask_subset(nohz.idle_cpus_mask, sched_domain_span(env->sd)))
>> - nohz.next_stats = jiffies + msecs_to_jiffies(LOAD_AVG_PERIOD);
>> - }
>> #endif
>>
>> load_idx = get_sd_load_idx(env->sd, env->idle);
>> @@ -8046,6 +8061,15 @@ static inline void update_sd_lb_stats(struct lb_env *env, struct sd_lb_stats *sd
>> sg = sg->next;
>> } while (sg != env->sd->groups);
>>
>> +#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
>> +
>
> Moreover, the above two ifdef maybe can be better moved into compile
> time inline functions to keep update_sg_lb_stats more streamlined,
> e.g. something like:
>
> ---8<---
> #ifdef CONFIG_NO_HZ_COMMON
> static inline void nohz_update_next_stats(struct lb_env *env)
> {
> if (env->idle != CPU_NEWLY_IDLE || READ_ONCE(nohz.has_blocked))
> return;
> eenv->flags |= LBF_NOHZ_STATS;
> etc...
> }

IMHO, Having inline function for 2 lines of code doesn't make it more
readable as we then have to looks for the content of the function to
understand what it does

>
> static inline void nohz_update_next_blocked(struct lb_env *env)
> {
> if (!(env->flags & LBF_NOHZ_AGAIN)
> return
> if (!cpumask_subset(nohz.idle_cpus_mask,
> sched_domain_span(env->sd)))
> return
> WRITE_ONCE();

ditto

> }
> #else
> #define nohz_update_next_stats(env) do {} while(false);
> #define nohz_update_blocked(env) do {} while(false);
> #endif
> ---8<---
>
>> if (env->sd->flags & SD_NUMA)
>> env->fbq_type = fbq_classify_group(&sds->busiest_stat);
>>
>> @@ -9069,6 +9093,8 @@ static void nohz_balancer_kick(struct rq *rq)
>> struct sched_domain *sd;
>> int nr_busy, i, cpu = rq->cpu;
>> unsigned int flags = 0;
>> + unsigned long has_blocked = READ_ONCE(nohz.has_blocked);
>> + unsigned long next_blocked = READ_ONCE(nohz.next_blocked);
>>
>> if (unlikely(rq->idle_balance))
>> return;
>> @@ -9086,7 +9112,7 @@ static void nohz_balancer_kick(struct rq *rq)
>> if (likely(!atomic_read(&nohz.nr_cpus)))
>> return;
>>
>> - if (time_after(now, nohz.next_stats))
>> + if (time_after(now, next_blocked) && has_blocked)
>
> Why not using READ_ONCE right here, these are the only two usages in
> this function, isn't it?

It' s mainly a matter of keeping the condition on 1 line to make it
more readable.

> This would allow also to skip (some of) the loads if some of the
> checks should fails. Ins't it?

As mentioned previously I expect the compiler to re order all data
access to the most optimized way

>
>> flags = NOHZ_STATS_KICK;
>>
>> if (time_before(now, nohz.next_balance))
>> @@ -9207,13 +9233,26 @@ void nohz_balance_enter_idle(int cpu)
>> if (!housekeeping_cpu(cpu, HK_FLAG_SCHED))
>> return;
>>
>> + /*
>> + * Can be set safely without rq->lock held
>> + * If a clear happens, it will have evaluated last additions because
>> + * rq->lock is held during the check and the clear
>> + */
>> + rq->has_blocked_load = 1;
>> +
>> + /*
>> + * The tick is still stopped but load could have been added in the
>> + * meantime. We set the nohz.has_blocked flag to trig a check of the
>> + * *_avg. The CPU is already part of nohz.idle_cpus_mask so the clear
>> + * of nohz.has_blocked can only happen after checking the new load
>> + */
>> if (rq->nohz_tick_stopped)
>> - return;
>> + goto out;
>>
>> /*
>> * If we're a completely isolated CPU, we don't play.
>> */
>> - if (on_null_domain(cpu_rq(cpu)))
>> + if (on_null_domain(rq))
>> return;
>>
>> rq->nohz_tick_stopped = 1;
>> @@ -9221,7 +9260,21 @@ void nohz_balance_enter_idle(int cpu)
>> cpumask_set_cpu(cpu, nohz.idle_cpus_mask);
>> atomic_inc(&nohz.nr_cpus);
>>
>> + /*
>> + * Ensures that if nohz_idle_balance() fails to observe our
>> + * @idle_cpus_mask store, it must observe the @has_blocked
>> + * store.
>> + */
>> + smp_mb__after_atomic();
>> +
>> set_cpu_sd_state_idle(cpu);
>> +
>> +out:
>> + /*
>> + * Each time a cpu enter idle, we assume that it has blocked load and
>> + * enable the periodic update of the load of idle cpus
>> + */
>> + WRITE_ONCE(nohz.has_blocked, 1);
>> }
>> #else
>> static inline void nohz_balancer_kick(struct rq *rq) { }
>> @@ -9355,7 +9408,7 @@ static bool nohz_idle_balance(struct rq *this_rq, enum cpu_idle_type idle)
>> /* Earliest time when we have to do rebalance again */
>> unsigned long now = jiffies;
>> unsigned long next_balance = now + 60*HZ;
>> - unsigned long next_stats = now + msecs_to_jiffies(LOAD_AVG_PERIOD);
>> + bool has_blocked_load = false;
>> int update_next_balance = 0;
>> int this_cpu = this_rq->cpu;
>> unsigned int flags;
>> @@ -9374,6 +9427,22 @@ static bool nohz_idle_balance(struct rq *this_rq, enum cpu_idle_type idle)
>>
>> SCHED_WARN_ON((flags & NOHZ_KICK_MASK) == NOHZ_BALANCE_KICK);
>>
>> + /*
>> + * We assume there will be no idle load after this update and clear
>> + * the has_blocked flag. If a cpu enters idle in the mean time, it will
>> + * set the has_blocked flag and trig another update of idle load.
>> + * Because a cpu that becomes idle, is added to idle_cpus_mask before
>> + * setting the flag, we are sure to not clear the state and not
>> + * check the load of an idle cpu.
>> + */
>> + WRITE_ONCE(nohz.has_blocked, 0);
>> +
>> + /*
>> + * Ensures that if we miss the CPU, we must see the has_blocked
>> + * store from nohz_balance_enter_idle().
>> + */
>> + smp_mb();
>> +
>> for_each_cpu(balance_cpu, nohz.idle_cpus_mask) {
>> if (balance_cpu == this_cpu || !idle_cpu(balance_cpu))
>> continue;
>> @@ -9383,11 +9452,16 @@ static bool nohz_idle_balance(struct rq *this_rq, enum cpu_idle_type idle)
>> * work being done for other cpus. Next load
>> * balancing owner will pick it up.
>> */
>> - if (need_resched())
>> - break;
>> + if (need_resched()) {
>> + has_blocked_load = true;
>> + goto abort;
>> + }
>>
>> rq = cpu_rq(balance_cpu);
>>
>> + update_blocked_averages(rq->cpu);
>> + has_blocked_load |= rq->has_blocked_load;
>> +
>> /*
>> * If time for next balance is due,
>> * do the balance.
>> @@ -9400,7 +9474,6 @@ static bool nohz_idle_balance(struct rq *this_rq, enum cpu_idle_type idle)
>> cpu_load_update_idle(rq);
>> rq_unlock_irq(rq, &rf);
>>
>> - update_blocked_averages(rq->cpu);
>> if (flags & NOHZ_BALANCE_KICK)
>> rebalance_domains(rq, CPU_IDLE);
>> }
>> @@ -9415,7 +9488,13 @@ static bool nohz_idle_balance(struct rq *this_rq, enum cpu_idle_type idle)
>> if (flags & NOHZ_BALANCE_KICK)
>> rebalance_domains(this_rq, CPU_IDLE);
>>
>> - nohz.next_stats = next_stats;
>> + WRITE_ONCE(nohz.next_blocked,
>> + now + msecs_to_jiffies(LOAD_AVG_PERIOD));
>
> Is it not slightly more clear to still use a local? I.e.
>
> next_stats = now + msecs_to_jiffies(LOAD_AVG_PERIOD);

next_stats doesn't exist anymore with this patch

> WRITE_ONCE(nohz.next_blocked, next_stats);
>
>> +
>> +abort:
>> + /* There is still blocked load, enable periodic update */
>> + if (has_blocked_load)
>> + WRITE_ONCE(nohz.has_blocked, 1);
>>
>> /*
>> * next_balance will be updated only when there is a need.
>> @@ -10046,6 +10125,7 @@ __init void init_sched_fair_class(void)
>>
>> #ifdef CONFIG_NO_HZ_COMMON
>> nohz.next_balance = jiffies;
>> + nohz.next_blocked = jiffies;
>> zalloc_cpumask_var(&nohz.idle_cpus_mask, GFP_NOWAIT);
>> #endif
>> #endif /* SMP */
>> diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
>> index e200045..ad9b929 100644
>> --- a/kernel/sched/sched.h
>> +++ b/kernel/sched/sched.h
>> @@ -723,6 +723,7 @@ struct rq {
>> #ifdef CONFIG_SMP
>> unsigned long last_load_update_tick;
>> unsigned long last_blocked_load_update_tick;
>> + unsigned int has_blocked_load;
>> #endif /* CONFIG_SMP */
>> unsigned int nohz_tick_stopped;
>> atomic_t nohz_flags;
>> > --
>> > 2.7.4
>> >
>
> --
> #include <best/regards.h>
>
> Patrick Bellasi

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