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

Qais Yousef
2021-01-28T16:59:04Z

On 01/28/21 10:09, Joel Fernandes wrote:
> > > > Qais mentioned half of the time being used by
> > > > sugov_next_freq_shared(). Are there any frequency changes resulting in
> > > > this call ?
> > >
> > > I do not see a frequency update happening at the time of the problem. However
> > > note that sugov_iowait_boost() does run even if frequency is not being
> > > updated. IIRC, this function is also not that light weight and I am not sure
> > > if it is a good idea to call this that often.
> >
> > Scheduler can't make any assumption about how often schedutil/cpufreq
> > wants to be called. Some are fast and straightforward and can be
> > called very often to adjust frequency; Others can't handle much
> > updates. The rate limit mechanism in schedutil and io-boost should be
> > there for such purpose.
>
> Sure, I know that's the intention.

How about the below patch to help with reducing the impact of
sugov_update_shared()?

I basically made sure it'll bail out immediately if it gets 2 calls within the
defined rate_limit_us value.

And tweaked the way we call cpufreq_update_util() from
update_blocked_averages() too so that we first update blocked load on all cpus,
then we ask for the frequency update. Combined with above this should result to
a single call to sugov_update_shared() for each policy. Each call should see
the final state of what is the load really is.

Only compile tested!

Thanks

--
Qais Yousef

----->8-----

diff --git a/kernel/sched/cpufreq_schedutil.c b/kernel/sched/cpufreq_schedutil.c
index 6931f0cdeb80..bd83e9343749 100644
--- a/kernel/sched/cpufreq_schedutil.c
+++ b/kernel/sched/cpufreq_schedutil.c
@@ -109,7 +109,6 @@ static bool sugov_update_next_freq(struct sugov_policy *sg_policy, u64 time,
return false;

sg_policy->next_freq = next_freq;
- sg_policy->last_freq_update_time = time;

return true;
}
@@ -554,20 +553,23 @@ sugov_update_shared(struct update_util_data *hook, u64 time, unsigned int flags)

raw_spin_lock(&sg_policy->update_lock);

+ ignore_dl_rate_limit(sg_cpu, sg_policy);
+
+ if (!sugov_should_update_freq(sg_policy, time))
+ goto out;
+
sugov_iowait_boost(sg_cpu, time, flags);
sg_cpu->last_update = time;

- ignore_dl_rate_limit(sg_cpu, sg_policy);
-
- if (sugov_should_update_freq(sg_policy, time)) {
- next_f = sugov_next_freq_shared(sg_cpu, time);
+ next_f = sugov_next_freq_shared(sg_cpu, time);

- if (sg_policy->policy->fast_switch_enabled)
- sugov_fast_switch(sg_policy, time, next_f);
- else
- sugov_deferred_update(sg_policy, time, next_f);
- }
+ if (sg_policy->policy->fast_switch_enabled)
+ sugov_fast_switch(sg_policy, time, next_f);
+ else
+ sugov_deferred_update(sg_policy, time, next_f);

+ sg_policy->last_freq_update_time = time;
+out:
raw_spin_unlock(&sg_policy->update_lock);
}

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 04a3ce20da67..47bd8be03a6c 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -8033,7 +8033,7 @@ static unsigned long task_h_load(struct task_struct *p)
}
#endif

-static void update_blocked_averages(int cpu)
+static void update_blocked_averages(int cpu, bool update_freq)
{
bool decayed = false, done = true;
struct rq *rq = cpu_rq(cpu);
@@ -8046,7 +8046,7 @@ static void update_blocked_averages(int cpu)
decayed |= __update_blocked_fair(rq, &done);

update_blocked_load_status(rq, !done);
- if (decayed)
+ if (decayed && update_freq)
cpufreq_update_util(rq, 0);
rq_unlock_irqrestore(rq, &rf);
}
@@ -8384,7 +8384,7 @@ static bool update_nohz_stats(struct rq *rq, bool force)
if (!force && !time_after(jiffies, rq->last_blocked_load_update_tick))
return true;

- update_blocked_averages(cpu);
+ update_blocked_averages(cpu, false);

return rq->has_blocked_load;
#else
@@ -8454,6 +8454,15 @@ static inline void update_sg_lb_stats(struct lb_env *env,
}
}

+ /*
+ * We did a bunch a blocked average updates, let cpufreq know about
+ * them in one go. For system with a lot of cpus on a frequency domain
+ * this will make sure we take all the changes that affects the policy
+ * in one go.
+ */
+ for_each_cpu_and(i, sched_group_span(group), env->cpus)
+ cpufreq_update_util(cpu_rq(i), 0);
+
/* Check if dst CPU is idle and preferred to this group */
if (env->sd->flags & SD_ASYM_PACKING &&
env->idle != CPU_NOT_IDLE &&
@@ -10464,7 +10473,7 @@ static bool _nohz_idle_balance(struct rq *this_rq, unsigned int flags,

/* Newly idle CPU doesn't need an update */
if (idle != CPU_NEWLY_IDLE) {
- update_blocked_averages(this_cpu);
+ update_blocked_averages(this_cpu, false);
has_blocked_load |= this_rq->has_blocked_load;
}

@@ -10478,6 +10487,15 @@ static bool _nohz_idle_balance(struct rq *this_rq, unsigned int flags,
ret = true;

abort:
+ /*
+ * We did a bunch a blocked average updates, let cpufreq know about
+ * them in one go. For system with a lot of cpus on a frequency domain
+ * this will make sure we take all the changes that affects the policy
+ * in one go.
+ */
+ for_each_cpu(balance_cpu, nohz.idle_cpus_mask)
+ cpufreq_update_util(cpu_rq(balance_cpu), 0);
+
/* There is still blocked load, enable periodic update */
if (has_blocked_load)
WRITE_ONCE(nohz.has_blocked, 1);
@@ -10603,7 +10621,7 @@ static int newidle_balance(struct rq *this_rq, struct rq_flags *rf)

raw_spin_unlock(&this_rq->lock);

- update_blocked_averages(this_cpu);
+ update_blocked_averages(this_cpu, true);
rcu_read_lock();
for_each_domain(this_cpu, sd) {
int continue_balancing = 1;
@@ -10691,7 +10709,7 @@ static __latent_entropy void run_rebalance_domains(struct softirq_action *h)
return;

/* normal load balance */
- update_blocked_averages(this_rq->cpu);
+ update_blocked_averages(this_rq->cpu, true);
rebalance_domains(this_rq, idle);
}