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sched/fair: Remove the rq->cpu_load[] update code
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With LB_BIAS disabled, there is no need to update the rq->cpu_load[idx]
any more.

Signed-off-by: Dietmar Eggemann <dietmar.eggemann@arm.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Acked-by: Rik van Riel <riel@surriel.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Morten Rasmussen <morten.rasmussen@arm.com>
Cc: Patrick Bellasi <patrick.bellasi@arm.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Quentin Perret <quentin.perret@arm.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Valentin Schneider <valentin.schneider@arm.com>
Cc: Vincent Guittot <vincent.guittot@linaro.org>
Link: https://lkml.kernel.org/r/20190527062116.11512-2-dietmar.eggemann@arm.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>
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Dietmar Eggemann authored and Ingo Molnar committed Jun 3, 2019
1 parent f2bedc4 commit 5e83eaf
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Showing 5 changed files with 0 additions and 272 deletions.
8 changes: 0 additions & 8 deletions include/linux/sched/nohz.h
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Expand Up @@ -6,14 +6,6 @@
* This is the interface between the scheduler and nohz/dynticks:
*/

#if defined(CONFIG_SMP) && defined(CONFIG_NO_HZ_COMMON)
extern void cpu_load_update_nohz_start(void);
extern void cpu_load_update_nohz_stop(void);
#else
static inline void cpu_load_update_nohz_start(void) { }
static inline void cpu_load_update_nohz_stop(void) { }
#endif

#if defined(CONFIG_SMP) && defined(CONFIG_NO_HZ_COMMON)
extern void nohz_balance_enter_idle(int cpu);
extern int get_nohz_timer_target(void);
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1 change: 0 additions & 1 deletion kernel/sched/core.c
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Expand Up @@ -3033,7 +3033,6 @@ void scheduler_tick(void)

update_rq_clock(rq);
curr->sched_class->task_tick(rq, curr, 0);
cpu_load_update_active(rq);
calc_global_load_tick(rq);
psi_task_tick(rq);

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255 changes: 0 additions & 255 deletions kernel/sched/fair.c
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Original file line number Diff line number Diff line change
Expand Up @@ -5322,71 +5322,6 @@ DEFINE_PER_CPU(cpumask_var_t, load_balance_mask);
DEFINE_PER_CPU(cpumask_var_t, select_idle_mask);

#ifdef CONFIG_NO_HZ_COMMON
/*
* per rq 'load' arrray crap; XXX kill this.
*/

/*
* The exact cpuload calculated at every tick would be:
*
* load' = (1 - 1/2^i) * load + (1/2^i) * cur_load
*
* If a CPU misses updates for n ticks (as it was idle) and update gets
* called on the n+1-th tick when CPU may be busy, then we have:
*
* load_n = (1 - 1/2^i)^n * load_0
* load_n+1 = (1 - 1/2^i) * load_n + (1/2^i) * cur_load
*
* decay_load_missed() below does efficient calculation of
*
* load' = (1 - 1/2^i)^n * load
*
* Because x^(n+m) := x^n * x^m we can decompose any x^n in power-of-2 factors.
* This allows us to precompute the above in said factors, thereby allowing the
* reduction of an arbitrary n in O(log_2 n) steps. (See also
* fixed_power_int())
*
* The calculation is approximated on a 128 point scale.
*/
#define DEGRADE_SHIFT 7

static const u8 degrade_zero_ticks[CPU_LOAD_IDX_MAX] = {0, 8, 32, 64, 128};
static const u8 degrade_factor[CPU_LOAD_IDX_MAX][DEGRADE_SHIFT + 1] = {
{ 0, 0, 0, 0, 0, 0, 0, 0 },
{ 64, 32, 8, 0, 0, 0, 0, 0 },
{ 96, 72, 40, 12, 1, 0, 0, 0 },
{ 112, 98, 75, 43, 15, 1, 0, 0 },
{ 120, 112, 98, 76, 45, 16, 2, 0 }
};

/*
* Update cpu_load for any missed ticks, due to tickless idle. The backlog
* would be when CPU is idle and so we just decay the old load without
* adding any new load.
*/
static unsigned long
decay_load_missed(unsigned long load, unsigned long missed_updates, int idx)
{
int j = 0;

if (!missed_updates)
return load;

if (missed_updates >= degrade_zero_ticks[idx])
return 0;

if (idx == 1)
return load >> missed_updates;

while (missed_updates) {
if (missed_updates % 2)
load = (load * degrade_factor[idx][j]) >> DEGRADE_SHIFT;

missed_updates >>= 1;
j++;
}
return load;
}

static struct {
cpumask_var_t idle_cpus_mask;
Expand All @@ -5398,201 +5333,12 @@ static struct {

#endif /* CONFIG_NO_HZ_COMMON */

/**
* __cpu_load_update - update the rq->cpu_load[] statistics
* @this_rq: The rq to update statistics for
* @this_load: The current load
* @pending_updates: The number of missed updates
*
* Update rq->cpu_load[] statistics. This function is usually called every
* scheduler tick (TICK_NSEC).
*
* This function computes a decaying average:
*
* load[i]' = (1 - 1/2^i) * load[i] + (1/2^i) * load
*
* Because of NOHZ it might not get called on every tick which gives need for
* the @pending_updates argument.
*
* load[i]_n = (1 - 1/2^i) * load[i]_n-1 + (1/2^i) * load_n-1
* = A * load[i]_n-1 + B ; A := (1 - 1/2^i), B := (1/2^i) * load
* = A * (A * load[i]_n-2 + B) + B
* = A * (A * (A * load[i]_n-3 + B) + B) + B
* = A^3 * load[i]_n-3 + (A^2 + A + 1) * B
* = A^n * load[i]_0 + (A^(n-1) + A^(n-2) + ... + 1) * B
* = A^n * load[i]_0 + ((1 - A^n) / (1 - A)) * B
* = (1 - 1/2^i)^n * (load[i]_0 - load) + load
*
* In the above we've assumed load_n := load, which is true for NOHZ_FULL as
* any change in load would have resulted in the tick being turned back on.
*
* For regular NOHZ, this reduces to:
*
* load[i]_n = (1 - 1/2^i)^n * load[i]_0
*
* see decay_load_misses(). For NOHZ_FULL we get to subtract and add the extra
* term.
*/
static void cpu_load_update(struct rq *this_rq, unsigned long this_load,
unsigned long pending_updates)
{
unsigned long __maybe_unused tickless_load = this_rq->cpu_load[0];
int i, scale;

this_rq->nr_load_updates++;

/* Update our load: */
this_rq->cpu_load[0] = this_load; /* Fasttrack for idx 0 */
for (i = 1, scale = 2; i < CPU_LOAD_IDX_MAX; i++, scale += scale) {
unsigned long old_load, new_load;

/* scale is effectively 1 << i now, and >> i divides by scale */

old_load = this_rq->cpu_load[i];
#ifdef CONFIG_NO_HZ_COMMON
old_load = decay_load_missed(old_load, pending_updates - 1, i);
if (tickless_load) {
old_load -= decay_load_missed(tickless_load, pending_updates - 1, i);
/*
* old_load can never be a negative value because a
* decayed tickless_load cannot be greater than the
* original tickless_load.
*/
old_load += tickless_load;
}
#endif
new_load = this_load;
/*
* Round up the averaging division if load is increasing. This
* prevents us from getting stuck on 9 if the load is 10, for
* example.
*/
if (new_load > old_load)
new_load += scale - 1;

this_rq->cpu_load[i] = (old_load * (scale - 1) + new_load) >> i;
}
}

/* Used instead of source_load when we know the type == 0 */
static unsigned long weighted_cpuload(struct rq *rq)
{
return cfs_rq_runnable_load_avg(&rq->cfs);
}

#ifdef CONFIG_NO_HZ_COMMON
/*
* There is no sane way to deal with nohz on smp when using jiffies because the
* CPU doing the jiffies update might drift wrt the CPU doing the jiffy reading
* causing off-by-one errors in observed deltas; {0,2} instead of {1,1}.
*
* Therefore we need to avoid the delta approach from the regular tick when
* possible since that would seriously skew the load calculation. This is why we
* use cpu_load_update_periodic() for CPUs out of nohz. However we'll rely on
* jiffies deltas for updates happening while in nohz mode (idle ticks, idle
* loop exit, nohz_idle_balance, nohz full exit...)
*
* This means we might still be one tick off for nohz periods.
*/

static void cpu_load_update_nohz(struct rq *this_rq,
unsigned long curr_jiffies,
unsigned long load)
{
unsigned long pending_updates;

pending_updates = curr_jiffies - this_rq->last_load_update_tick;
if (pending_updates) {
this_rq->last_load_update_tick = curr_jiffies;
/*
* In the regular NOHZ case, we were idle, this means load 0.
* In the NOHZ_FULL case, we were non-idle, we should consider
* its weighted load.
*/
cpu_load_update(this_rq, load, pending_updates);
}
}

/*
* Called from nohz_idle_balance() to update the load ratings before doing the
* idle balance.
*/
static void cpu_load_update_idle(struct rq *this_rq)
{
/*
* bail if there's load or we're actually up-to-date.
*/
if (weighted_cpuload(this_rq))
return;

cpu_load_update_nohz(this_rq, READ_ONCE(jiffies), 0);
}

/*
* Record CPU load on nohz entry so we know the tickless load to account
* on nohz exit. cpu_load[0] happens then to be updated more frequently
* than other cpu_load[idx] but it should be fine as cpu_load readers
* shouldn't rely into synchronized cpu_load[*] updates.
*/
void cpu_load_update_nohz_start(void)
{
struct rq *this_rq = this_rq();

/*
* This is all lockless but should be fine. If weighted_cpuload changes
* concurrently we'll exit nohz. And cpu_load write can race with
* cpu_load_update_idle() but both updater would be writing the same.
*/
this_rq->cpu_load[0] = weighted_cpuload(this_rq);
}

/*
* Account the tickless load in the end of a nohz frame.
*/
void cpu_load_update_nohz_stop(void)
{
unsigned long curr_jiffies = READ_ONCE(jiffies);
struct rq *this_rq = this_rq();
unsigned long load;
struct rq_flags rf;

if (curr_jiffies == this_rq->last_load_update_tick)
return;

load = weighted_cpuload(this_rq);
rq_lock(this_rq, &rf);
update_rq_clock(this_rq);
cpu_load_update_nohz(this_rq, curr_jiffies, load);
rq_unlock(this_rq, &rf);
}
#else /* !CONFIG_NO_HZ_COMMON */
static inline void cpu_load_update_nohz(struct rq *this_rq,
unsigned long curr_jiffies,
unsigned long load) { }
#endif /* CONFIG_NO_HZ_COMMON */

static void cpu_load_update_periodic(struct rq *this_rq, unsigned long load)
{
#ifdef CONFIG_NO_HZ_COMMON
/* See the mess around cpu_load_update_nohz(). */
this_rq->last_load_update_tick = READ_ONCE(jiffies);
#endif
cpu_load_update(this_rq, load, 1);
}

/*
* Called from scheduler_tick()
*/
void cpu_load_update_active(struct rq *this_rq)
{
unsigned long load = weighted_cpuload(this_rq);

if (tick_nohz_tick_stopped())
cpu_load_update_nohz(this_rq, READ_ONCE(jiffies), load);
else
cpu_load_update_periodic(this_rq, load);
}

/*
* Return a low guess at the load of a migration-source CPU weighted
* according to the scheduling class and "nice" value.
Expand Down Expand Up @@ -9876,7 +9622,6 @@ static bool _nohz_idle_balance(struct rq *this_rq, unsigned int flags,

rq_lock_irqsave(rq, &rf);
update_rq_clock(rq);
cpu_load_update_idle(rq);
rq_unlock_irqrestore(rq, &rf);

if (flags & NOHZ_BALANCE_KICK)
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6 changes: 0 additions & 6 deletions kernel/sched/sched.h
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Expand Up @@ -96,12 +96,6 @@ extern atomic_long_t calc_load_tasks;
extern void calc_global_load_tick(struct rq *this_rq);
extern long calc_load_fold_active(struct rq *this_rq, long adjust);

#ifdef CONFIG_SMP
extern void cpu_load_update_active(struct rq *this_rq);
#else
static inline void cpu_load_update_active(struct rq *this_rq) { }
#endif

/*
* Helpers for converting nanosecond timing to jiffy resolution
*/
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2 changes: 0 additions & 2 deletions kernel/time/tick-sched.c
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Original file line number Diff line number Diff line change
Expand Up @@ -782,7 +782,6 @@ static void tick_nohz_stop_tick(struct tick_sched *ts, int cpu)
*/
if (!ts->tick_stopped) {
calc_load_nohz_start();
cpu_load_update_nohz_start();
quiet_vmstat();

ts->last_tick = hrtimer_get_expires(&ts->sched_timer);
Expand Down Expand Up @@ -829,7 +828,6 @@ static void tick_nohz_restart_sched_tick(struct tick_sched *ts, ktime_t now)
{
/* Update jiffies first */
tick_do_update_jiffies64(now);
cpu_load_update_nohz_stop();

/*
* Clear the timer idle flag, so we avoid IPIs on remote queueing and
Expand Down

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