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SubjectRe: [PATCH v2 1/3] sched: Stop nohz stats when decayed
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On 02/08/2018 01:36 PM, Vincent Guittot wrote:
> On 8 February 2018 at 13:46, Valentin Schneider
> <valentin.schneider@arm.com> wrote:
>> On 02/06/2018 07:23 PM, Vincent Guittot wrote:
>>> [...]
>>> @@ -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 +7995,15 @@ 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;
>>> + int has_blocked = READ_ONCE(nohz.has_blocked);
>>> 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)
>>> 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 +8059,15 @@ 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));
>>
>> Here we push the stats update forward if we visited all the nohz CPUs but they
>> still have blocked load. IMO we should also clear the nohz.has_blocked flag
>> if we visited all the nohz CPUs and none had blocked load left.
>>
>> If we don't do that, we could very well have cleared all of the nohz blocked
>> load in idle_balance and successfully pulled a task, but the flag isn't
>> cleared so we'll end up doing a _nohz_idle_balance() later on for nothing.
>>
>>
>> As I said in a previous comment, we also have this problem with periodic
>> load balance: if a CPU goes nohz (nohz.has_blocked is raised) but wakes up,
>> e.g. before the next nohz.next_blocked, we should stop kicking ILBs
>>
>> Now I'd need to test this, but I think it can actually get worse: if that
>> CPU keeps generating blocked load after this short idle period, no matter
>> how many _nohz_idle_balance() we go through we will never reach a point
>> where nohz.has_blocked gets cleared, and we'll keep kicking those ILBs to
>> update blocked load that already gets updated in the periodic balance.
>>
>> I think that's where a nohz blocked load cpumask can also help: on top of
>> skipping nohz CPUs that don't need an update, we can stop the whole remote
>> update machinery when the last nohz cpu with blocked load wakes up, or say
>> when it goes through its first periodic balance.
>
> The main question is : Do we want to remove all useless kicks to ilb
> or useless calls to _nohz_idle_balance at the cost of adding more
> latency in the idle/wakeup path because of the additional atomic
> operations to keep track of which CPUs are idle, tickless with blocked
> load or do we accept to kick ilb or call _nohz_idle_balance uselessly
> from time to time for some use cases
>
> I agree with you that there are some useless calls with the proposal
> which can be removed with additional checks, variables and cpumask
> manipulation. Which use case will benefits from these additional
> checks and does it worth ?
>
> Vincent

For now I've been using those made-up rt-app workloads to stress specific
bits of code, but I agree it would be really nice to have a "real" workload
to see how both power (additional kick_ilb()'s) and performance (additional
atomic ops) are affected. As Vincent suggested offline, it could be worth
giving it a shot on Android...


In the meantime I've done some more accurate profiling with cpumasks on my
Juno r2 (patch at the bottom). As a sidenote, I realised I don't have a test
case for the load update through load_balance() in idle_balance() - I only
test the !overload segment. Will think about that.

In summary:

20 iterations per test case
All non-mentioned CPUs are idling

---------------------
kick_ilb() test case:
---------------------

a, b = 100% rt-app tasks
- = idling

Accumulating load before sleeping
^
^
CPU1| a a a - - - a
CPU2| - - b b b b b
v
v > Periodically kicking ILBs to update nohz blocked load

Baseline:
_nohz_idle_balance() takes 39µs in average
nohz_balance_enter_idle() takes 233ns in average

W/ cpumask:
_nohz_idle_balance() takes 33µs in average
nohz_balance_enter_idle() takes 283ns in average

Diff:
_nohz_idle_balance() -6µs in average (-16%)
nohz_balance_enter_idle() +50ns in average (+21%)


---------------------------------------------------
Local _nohz_idle_balance() for NEWLY_IDLE test case:
---------------------------------------------------

a = 100% rt-app task
b = periodic rt-app task
- = idling

Accumulating load before sleeping
^
^
CPU1| a a a - - - - - a
CPU2| - - b - b - b - b
v
v > Periodically updating nohz blocked load through local
_nohz_idle_balance() in idle_balance()


(execution times are x2 as fast as previous test case because CPU2 is a
big CPU, whereas CPU0 - the ILB 99% of the time in the previous test - is a
LITTLE CPU ~ half as powerful).

Baseline:
_nohz_idle_balance() takes 20µs in average
nohz_balance_enter_idle() takes 183ns in average

W/ cpumask:
_nohz_idle_balance() takes 13µs in average
nohz_balance_enter_idle() takes 217ns in average

Diff:
_nohz_idle_balance() -7µs in average (-38%)
nohz_balance_enter_idle() +34ns in average (+18%)



For more details: https://gist.github.com/valschneider/78099acee87a18057d56cc6cc03978b1

---
kernel/sched/fair.c | 82 +++++++++++++++++++++++++----------------------------
1 file changed, 39 insertions(+), 43 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 3abb3bc..4042025 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -5404,8 +5404,8 @@ decay_load_missed(unsigned long load, unsigned long missed_updates, int idx)

static struct {
cpumask_var_t idle_cpus_mask;
+ cpumask_var_t stale_cpus_mask; /* Idle CPUs with blocked load */
atomic_t nr_cpus;
- int has_blocked; /* Idle CPUS has blocked load */
unsigned long next_balance; /* in jiffy units */
unsigned long next_blocked; /* Next update of blocked load in jiffies */
} nohz ____cacheline_aligned;
@@ -6968,7 +6968,6 @@ 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;
@@ -7827,25 +7826,24 @@ group_type group_classify(struct sched_group *group,
return group_other;
}

-static bool update_nohz_stats(struct rq *rq)
+static void 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 false;
+ if (!cpumask_test_cpu(cpu, nohz.stale_cpus_mask))
+ return;

if (!time_after(jiffies, rq->last_blocked_load_update_tick))
- return true;
+ return;

update_blocked_averages(cpu);

- return rq->has_blocked_load;
+ if (!rq->has_blocked_load)
+ cpumask_clear_cpu(cpu, nohz.stale_cpus_mask);
#else
- return false;
+ return;
#endif
}

@@ -7871,8 +7869,8 @@ 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))
- env->flags |= LBF_NOHZ_AGAIN;
+ if (env->flags & LBF_NOHZ_STATS)
+ update_nohz_stats(rq);

/* Bias balancing toward cpus of our domain */
if (local_group)
@@ -8024,7 +8022,8 @@ 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;
- int has_blocked = READ_ONCE(nohz.has_blocked);
+ int has_blocked = cpumask_intersects(sched_domain_span(env->sd),
+ nohz.stale_cpus_mask);
bool overload = false;

if (child && child->flags & SD_PREFER_SIBLING)
@@ -8089,8 +8088,13 @@ static inline void update_sd_lb_stats(struct lb_env *env, struct sd_lb_stats *sd
} 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))) {
+ /*
+ * All nohz CPUs with blocked load were visited but some haven't fully
+ * decayed. Visit them again later.
+ */
+ if ((env->flags & LBF_NOHZ_STATS) &&
+ cpumask_subset(nohz.idle_cpus_mask, sched_domain_span(env->sd)) &&
+ !cpumask_empty(nohz.stale_cpus_mask)) {

WRITE_ONCE(nohz.next_blocked,
jiffies + msecs_to_jiffies(LOAD_AVG_PERIOD));
@@ -8882,7 +8886,7 @@ static int idle_balance(struct rq *this_rq, struct rq_flags *rf)

if (this_rq->avg_idle < sysctl_sched_migration_cost ||
!this_rq->rd->overload) {
- unsigned long has_blocked = READ_ONCE(nohz.has_blocked);
+ unsigned long has_blocked = !cpumask_empty(nohz.stale_cpus_mask);
unsigned long next_blocked = READ_ONCE(nohz.next_blocked);

rcu_read_lock();
@@ -9137,7 +9141,7 @@ 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 has_blocked = !cpumask_empty(nohz.stale_cpus_mask);
unsigned long next_blocked = READ_ONCE(nohz.next_blocked);

if (unlikely(rq->idle_balance))
@@ -9297,10 +9301,10 @@ void nohz_balance_enter_idle(int 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
+ * Each time a cpu enters idle, we assume that it has blocked load and
+ * enable the periodic update of its blocked load
*/
- WRITE_ONCE(nohz.has_blocked, 1);
+ cpumask_set_cpu(cpu, nohz.stale_cpus_mask);
}
#else
static inline void nohz_balancer_kick(struct rq *rq) { }
@@ -9437,7 +9441,6 @@ static bool _nohz_idle_balance(struct rq *this_rq, unsigned int flags, enum cpu_
/* Earliest time when we have to do rebalance again */
unsigned long now = jiffies;
unsigned long next_balance = now + 60*HZ;
- bool has_blocked_load = false;
int update_next_balance = 0;
int this_cpu = this_rq->cpu;
int balance_cpu;
@@ -9447,16 +9450,6 @@ static bool _nohz_idle_balance(struct rq *this_rq, unsigned int flags, enum cpu_

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);
-
for_each_cpu(balance_cpu, nohz.idle_cpus_mask) {
u64 t0, domain_cost;

@@ -9466,29 +9459,34 @@ static bool _nohz_idle_balance(struct rq *this_rq, unsigned int flags, enum cpu_
continue;

/*
+ * When using the nohz balance to only update blocked load,
+ * we can skip nohz CPUs which no longer have blocked load.
+ */
+ if ((flags & NOHZ_KICK_MASK) == NOHZ_STATS_KICK &&
+ !cpumask_test_cpu(balance_cpu, nohz.stale_cpus_mask))
+ continue;
+
+ /*
* If this cpu gets work to do, stop the load balancing
* work being done for other cpus. Next load
* balancing owner will pick it up.
*/
- if (need_resched()) {
- has_blocked_load = true;
+ if (need_resched())
goto abort;
- }

/*
* If the update is done while CPU becomes idle, we abort
* the update when its cost is higher than the average idle
* time in orde to not delay a possible wake up.
*/
- if (idle == CPU_NEWLY_IDLE && this_rq->avg_idle < curr_cost) {
- has_blocked_load = true;
+ if (idle == CPU_NEWLY_IDLE && this_rq->avg_idle < curr_cost)
goto abort;
- }

rq = cpu_rq(balance_cpu);

update_blocked_averages(rq->cpu);
- has_blocked_load |= rq->has_blocked_load;
+ if (!rq->has_blocked_load)
+ cpumask_clear_cpu(balance_cpu, nohz.stale_cpus_mask);

/*
* If time for next balance is due,
@@ -9519,7 +9517,8 @@ static bool _nohz_idle_balance(struct rq *this_rq, unsigned int flags, enum cpu_
/* Newly idle CPU doesn't need an update */
if (idle != CPU_NEWLY_IDLE) {
update_blocked_averages(this_cpu);
- has_blocked_load |= this_rq->has_blocked_load;
+ if (!this_rq->has_blocked_load)
+ cpumask_clear_cpu(this_cpu, nohz.stale_cpus_mask);
}

if (flags & NOHZ_BALANCE_KICK)
@@ -9532,10 +9531,6 @@ static bool _nohz_idle_balance(struct rq *this_rq, unsigned int flags, enum cpu_
ret = true;

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.
* When the CPU is attached to null domain for ex, it will not be
@@ -10196,6 +10191,7 @@ __init void init_sched_fair_class(void)
nohz.next_balance = jiffies;
nohz.next_blocked = jiffies;
zalloc_cpumask_var(&nohz.idle_cpus_mask, GFP_NOWAIT);
+ zalloc_cpumask_var(&nohz.stale_cpus_mask, GFP_NOWAIT);
#endif
#endif /* SMP */

--
2.7.4

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