Re: [PATCH 2/4] sched: cpufreq: Fix apply_dvfs_headroom() escaping uclamp constraints
From: Vincent Guittot
Date: Thu Sep 28 2023 - 13:50:44 EST
Le dimanche 24 sept. 2023 à 18:23:01 (+0100), Qais Yousef a écrit :
> On 09/24/23 09:58, Vincent Guittot wrote:
>
> > > Shouldn't it be (568+128)*1.25 = 870? Which is almost the 860 above. We calmped
> > > the 812 to 800, with rounding errors that almost accounts for the 10 points
> > > difference between 870 and 860..
> >
> > no I voluntarily use 568 + 128*1.25. I added dvfs headroom for irq
> > just to ensure that you will not raise that I removed the headroom for
> > irq and focus on the use case but it might have created more
> > confusion.
> >
> > My example above demonstrate that only taking care of cases with null
> > irq pressure is not enough and you can still ends up above 800
> >
> > IIUC you point with uclamp_max. It is a performance limit that you
> > don't want to cross because of CFS.This means that we should not go
> > above 800 in my example because of cfs utilization: Irq needs between
> > 128 and CFS asks 568 so the system needs 696 which is below the 800
> > uclamp. Even if you add the dvfs headroom on irq, the system is still
> > below 800. Only when you add dfvs headroom to cfs then you go above
> > 800 but it's not needed because uclamp say that you should not go
>
> Yep, absolutely. It seems we agree that CFS shouldn't go above 800 if it is
> capped even if there's headroom, but the question you have on the way it is
At least I want to ensure that cpufreq has the right information to make a
smart decision. In the example above, it's not needed to go above 800 for
neither cfs nor irq.
> being applied. As long as we agree on this part which is a fundamental behavior
> question that I thought is the pain point, the implementation details are
> certainly something that I can improve on.
>
> > above 800 because of CFS so we should stay at 800 whereas both current
> > formula and your new formula return a value above 800
>
> I'm not sure how to handle irq, rt and dl here to be honest. They seem to have
> been taken as an 'additional' demand on top of CFS. So yes, we'll go above but
> irq, and dl don't have knowledge about ucalmp_max. RT does and will be equally
> capped like CFS. I kept current behavior the same, but I did wonder about them
> too in patch 4.
>
> So in a system where there are active CFS, RT, DL and IRQ and both CFS and RT
> had a cap of 800, then they won't ask for me. But once we add IRQ and DL on
> top, then we'll go above.
>
> You think we shouldn't? See below for a suggestion.
I'm afraid that the answer is not that straight forward
In the example above should we still stay at uclamp value if we now set uclamp
to 680 ?
And what about a uclamp of 160 ?
>
> > > I am still not sure if you mean we are mixing up the code and we need better
> > > abstraction or something else.
> > >
> > > Beside the abstraction problem, which I agree with, I can't see what I am
> > > mixing up yet :( Sorry I think I need more helping hand to see it.
> >
> > There is a mix between actual utilization and performance limit and
> > when we add both we then lose important information as highlighted by
> > my example. If the current formula is not correct because we can go
> > above uclamp_max value, your proposal is not better. And the root
> > cause is mainly coming from adding utilization with performance limit
> > (i.e. uclamp)
> >
> > That's why I said that we need a new interface to enable cpufreq to
> > not blindly apply its headroom but to make smarter decision at cpufreq
> > level
>
> Okay I see. I tend to agree here too. The question is should cpufreq take each
> util (cfs, rt, dl, irq) as input and do the decision on its own. Or should the
> scheduler add them and pass the aggregated value? If the latter, how can
> cpufreq know how to apply the limit? From what I see all these decisions has to
> happen in the same function but not split.
I'm not in favor of showing all details to cpufreq because it will have to
follow the internal changes. In instead, I was thinking of something like:
/* Function name to be changed */
unsigned_long effective_cpu_util(int cpu, unsigned int *min, unsigned int *max)
The function returns the actual utilization of the CPU and some minimum and
maximum limits with the possibility to have the min and/or Actual values > Max
because the min would be a hard minimum value whereas max only a soft maximum
value.
Min would be the minimum perf to provide to the cpu : typically DL_bw + irq
Actual would be the actual utilization of the cpu: cfs+rt+dl+irq (after scaling
everything in the normal range)
Max would be the maximum needed performance for normal work: typically the
minimum between uclamp and capacity
Then cpufreq can use these 3 values to compute a performance level and it
will know up to which perf level it should go and if it is worth it.
Something likr:
/* get scheduler statistic */
target = effective_cpu_util(cpu, util, &min, &max)
/* ensure min perf for dl and irq + some margin for others */
target = min + headroom
/* check if we really need to go to max */
if ((actual + headroom) < max)
max = actual + headroom
/* use max of the 2 values */
target = max(target, max)
I put all this in the below quick patch which only compiled but not tested
---
include/linux/energy_model.h | 1 -
kernel/sched/core.c | 98 +++++++++++++++++---------------
kernel/sched/cpufreq_schedutil.c | 6 +-
kernel/sched/fair.c | 4 +-
kernel/sched/sched.h | 7 ++-
5 files changed, 61 insertions(+), 55 deletions(-)
diff --git a/include/linux/energy_model.h b/include/linux/energy_model.h
index e4cf9baf5f9e..c424a1bcec38 100644
--- a/include/linux/energy_model.h
+++ b/include/linux/energy_model.h
@@ -261,7 +261,6 @@ static inline unsigned long em_cpu_energy(struct em_perf_domain *pd,
scale_cpu = arch_scale_cpu_capacity(cpu);
ref_freq = em_get_capacity_ref_freq(cpu, pd);
- max_util = map_util_perf(max_util);
max_util = min(max_util, allowed_cpu_cap);
freq = map_util_freq(max_util, ref_freq, scale_cpu);
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 6560392f2f83..030564f5be24 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -7404,18 +7404,13 @@ int sched_core_idle_cpu(int cpu)
* required to meet deadlines.
*/
unsigned long effective_cpu_util(int cpu, unsigned long util_cfs,
- enum cpu_util_type type,
- struct task_struct *p)
+ unsigned long *min,
+ unsigned long *max)
{
- unsigned long dl_util, util, irq, max;
+ unsigned long dl_util, util, irq, scale;
struct rq *rq = cpu_rq(cpu);
- max = arch_scale_cpu_capacity(cpu);
-
- if (!uclamp_is_used() &&
- type == FREQUENCY_UTIL && rt_rq_is_runnable(&rq->rt)) {
- return max;
- }
+ scale = arch_scale_cpu_capacity(cpu);
/*
* Early check to see if IRQ/steal time saturates the CPU, can be
@@ -7423,9 +7418,16 @@ unsigned long effective_cpu_util(int cpu, unsigned long util_cfs,
* update_irq_load_avg().
*/
irq = cpu_util_irq(rq);
- if (unlikely(irq >= max))
- return max;
+ if (unlikely(irq >= scale)) {
+ if (min)
+ *min = scale;
+ if (max)
+ *max = scale;
+ return scale;
+ }
+ if (min)
+ min = irq + cpu_bw_dl(rq);
/*
* Because the time spend on RT/DL tasks is visible as 'lost' time to
* CFS tasks and we use the same metric to track the effective
@@ -7439,29 +7441,13 @@ unsigned long effective_cpu_util(int cpu, unsigned long util_cfs,
* frequency will be gracefully reduced with the utilization decay.
*/
util = util_cfs + cpu_util_rt(rq);
- if (type == FREQUENCY_UTIL)
- util = uclamp_rq_util_with(rq, util, p);
+ util += cpu_util_dl(rq);
- dl_util = cpu_util_dl(rq);
-
- /*
- * For frequency selection we do not make cpu_util_dl() a permanent part
- * of this sum because we want to use cpu_bw_dl() later on, but we need
- * to check if the CFS+RT+DL sum is saturated (ie. no idle time) such
- * that we select f_max when there is no idle time.
- *
- * NOTE: numerical errors or stop class might cause us to not quite hit
- * saturation when we should -- something for later.
- */
- if (util + dl_util >= max)
- return max;
-
- /*
- * OTOH, for energy computation we need the estimated running time, so
- * include util_dl and ignore dl_bw.
- */
- if (type == ENERGY_UTIL)
- util += dl_util;
+ if (util >= scale) {
+ if (max)
+ *max = scale;
+ return scale;
+ }
/*
* There is still idle time; further improve the number by using the
@@ -7472,28 +7458,48 @@ unsigned long effective_cpu_util(int cpu, unsigned long util_cfs,
* U' = irq + --------- * U
* max
*/
- util = scale_irq_capacity(util, irq, max);
+ util = scale_irq_capacity(util, irq, scale);
util += irq;
+ if (max)
+ *max = uclamp_rq_util_with(rq, util, NULL);
+
+ return min(scale, util);
+}
+
+
+/*
+ * TODO: move this in cpufreq
+ */
+unsigned long effective_cpu_perf(int cpu, unsigned long util_cfs,
+ struct task_struct *p)
+{
+ unsigned long actual, target;
+
+ /* Get utilization stats */
+ actual = effective_cpu_util(cpu, util_cfs, &min, &max);
+
/*
- * Bandwidth required by DEADLINE must always be granted while, for
- * FAIR and RT, we use blocked utilization of IDLE CPUs as a mechanism
- * to gracefully reduce the frequency when no tasks show up for longer
- * periods of time.
- *
- * Ideally we would like to set bw_dl as min/guaranteed freq and util +
- * bw_dl as requested freq. However, cpufreq is not yet ready for such
- * an interface. So, we only do the latter for now.
+ * Provide at least enough capacity for DL + irq plus some headroom
+ * for other activities
*/
- if (type == FREQUENCY_UTIL)
- util += cpu_bw_dl(rq);
+ target = map_util_perf(min);
- return min(max, util);
+ actual = map_util_perf(actual);
+ /* Actually we don't need to target the max performance */
+ if (actual < max)
+ max = actual;
+
+ /*
+ * Ensure at least minimum perf target while providing more computa capacity when
+ * possible
+ */
+ target = max(target,max);
}
unsigned long sched_cpu_util(int cpu)
{
- return effective_cpu_util(cpu, cpu_util_cfs(cpu), ENERGY_UTIL, NULL);
+ return effective_cpu_util(cpu, cpu_util_cfs(cpu), NULL, NULL);
}
#endif /* CONFIG_SMP */
diff --git a/kernel/sched/cpufreq_schedutil.c b/kernel/sched/cpufreq_schedutil.c
index e2b9c8c3d69a..ef6b4b09ac12 100644
--- a/kernel/sched/cpufreq_schedutil.c
+++ b/kernel/sched/cpufreq_schedutil.c
@@ -162,7 +162,6 @@ static unsigned int get_next_freq(struct sugov_policy *sg_policy,
unsigned int freq;
struct cpufreq_policy *policy = sg_policy->policy;
- util = map_util_perf(util);
freq = get_capacity_ref_freq(policy);
freq = map_util_freq(util, freq, max);
@@ -179,8 +178,7 @@ static void sugov_get_util(struct sugov_cpu *sg_cpu)
struct rq *rq = cpu_rq(sg_cpu->cpu);
sg_cpu->bw_dl = cpu_bw_dl(rq);
- sg_cpu->util = effective_cpu_util(sg_cpu->cpu, util,
- FREQUENCY_UTIL, NULL);
+ sg_cpu->util = effective_cpu_perf(sg_cpu->cpu, util, NULL);
}
/**
@@ -427,7 +425,7 @@ static void sugov_update_single_perf(struct update_util_data *hook, u64 time,
sg_cpu->util = prev_util;
cpufreq_driver_adjust_perf(sg_cpu->cpu, map_util_perf(sg_cpu->bw_dl),
- map_util_perf(sg_cpu->util), max_cap);
+ sg_cpu->util, max_cap);
sg_cpu->sg_policy->last_freq_update_time = time;
}
diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 06d6d0dde48a..50568e2fa1ba 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -7570,7 +7570,7 @@ static inline void eenv_pd_busy_time(struct energy_env *eenv,
for_each_cpu(cpu, pd_cpus) {
unsigned long util = cpu_util(cpu, p, -1, 0);
- busy_time += effective_cpu_util(cpu, util, ENERGY_UTIL, NULL);
+ busy_time += effective_cpu_util(cpu, util, NULL, NULL);
}
eenv->pd_busy_time = min(eenv->pd_cap, busy_time);
@@ -7602,7 +7602,7 @@ eenv_pd_max_util(struct energy_env *eenv, struct cpumask *pd_cpus,
* NOTE: in case RT tasks are running, by default the
* FREQUENCY_UTIL's utilization can be max OPP.
*/
- eff_util = effective_cpu_util(cpu, util, FREQUENCY_UTIL, tsk);
+ eff_util = effective_cpu_perf(cpu, util, tsk);
max_util = max(max_util, eff_util);
}
diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
index 17ae151e90c0..e79cb1110e8d 100644
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -2988,10 +2988,13 @@ enum cpu_util_type {
ENERGY_UTIL,
};
-unsigned long effective_cpu_util(int cpu, unsigned long util_cfs,
- enum cpu_util_type type,
+unsigned long effective_cpu_perf(int cpu, unsigned long util_cfs,
struct task_struct *p);
+unsigned long effective_cpu_util(int cpu, unsigned long util_cfs,
+ unsigned long *min,
+ unsigned long *max)
+
/*
* Verify the fitness of task @p to run on @cpu taking into account the
* CPU original capacity and the runtime/deadline ratio of the task.
--
2.34.1
>
> It seems the sticking point is how we interpret irq pressure with uclamp. It
> seems you think we should apply any uclamp capping to this, which I think would
> make sense.
>
> And DL bandwidth we need to max() with the aggregated value.
>
> So I think the formula could be
>
> util = cfs + rt pressure + irq pressure
>
> unsigned long cpufreq_convert_util_to_freq(rq, util, dl_bw)
> {
> eff_util = apply_dvfs_headroom(util);
> eff_util = uclamp_rq_util_with(rq, util, NULL);
>
> eff_util = max(eff_util, dl_bw);
> }
>
> so we add the utilization of cfs, rt and irq (as per current formula). And then
> let cpufreq do the headroom and limit management.
>
> I changed the way we handle dl_bw as it is actually requesting to run at
> a specific level and not really a pressure. So we max() it with eff_util.
>
> If there's a DL task on the rq then it'd be running and the frequency it
> needs is defined by its bandwidth.
>
> We could also keep it as it is with
>
> unsigned long cpufreq_convert_util_to_freq(rq, util, dl_bw)
> {
> eff_util = apply_dvfs_headroom(util);
> eff_util = uclamp_rq_util_with(rq, util, NULL);
>
> eff_util += dl_bw;
> }
>
> RT has uclamp knowledge so it'll either run at max or whatever value it might
> have requested via uclamp_min. But DL doesn't set any uclamp_min and must be
> either added or max()ed. I'm not sure which is more correct yet, but maybe
> adding actually is better to ensure the CPU runs higher to handle all the tasks
> on the rq.
>
> What do you think?
>
>
> Thanks!
>
> --
> Qais Yousef