sched/nohz: Rewrite and fix load-avg computation -- again
Thanks to Charles Wang for spotting the defects in the current code: - If we go idle during the sample window -- after sampling, we get a negative bias because we can negate our own sample. - If we wake up during the sample window we get a positive bias because we push the sample to a known active period. So rewrite the entire nohz load-avg muck once again, now adding copious documentation to the code. Reported-and-tested-by: Doug Smythies <dsmythies@telus.net> Reported-and-tested-by: Charles Wang <muming.wq@gmail.com> Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl> Cc: Linus Torvalds <torvalds@linux-foundation.org> Cc: Andrew Morton <akpm@linux-foundation.org> Cc: stable@kernel.org Link: http://lkml.kernel.org/r/1340373782.18025.74.camel@twins [ minor edits ] Signed-off-by: Ingo Molnar <mingo@kernel.org>
This commit is contained in:
Peter Zijlstra
Ingo Molnar
parent
164c33c6ad
commit
5167e8d541
5 changed files with 214 additions and 76 deletions
|
|
@ -1909,6 +1909,14 @@ static inline int set_cpus_allowed_ptr(struct task_struct *p,
|
|||
}
|
||||
#endif
|
||||
|
||||
#ifdef CONFIG_NO_HZ
|
||||
void calc_load_enter_idle(void);
|
||||
void calc_load_exit_idle(void);
|
||||
#else
|
||||
static inline void calc_load_enter_idle(void) { }
|
||||
static inline void calc_load_exit_idle(void) { }
|
||||
#endif /* CONFIG_NO_HZ */
|
||||
|
||||
#ifndef CONFIG_CPUMASK_OFFSTACK
|
||||
static inline int set_cpus_allowed(struct task_struct *p, cpumask_t new_mask)
|
||||
{
|
||||
|
|
|
|||
|
|
@ -2161,11 +2161,73 @@ unsigned long this_cpu_load(void)
|
|||
}
|
||||
|
||||
|
||||
/*
|
||||
* Global load-average calculations
|
||||
*
|
||||
* We take a distributed and async approach to calculating the global load-avg
|
||||
* in order to minimize overhead.
|
||||
*
|
||||
* The global load average is an exponentially decaying average of nr_running +
|
||||
* nr_uninterruptible.
|
||||
*
|
||||
* Once every LOAD_FREQ:
|
||||
*
|
||||
* nr_active = 0;
|
||||
* for_each_possible_cpu(cpu)
|
||||
* nr_active += cpu_of(cpu)->nr_running + cpu_of(cpu)->nr_uninterruptible;
|
||||
*
|
||||
* avenrun[n] = avenrun[0] * exp_n + nr_active * (1 - exp_n)
|
||||
*
|
||||
* Due to a number of reasons the above turns in the mess below:
|
||||
*
|
||||
* - for_each_possible_cpu() is prohibitively expensive on machines with
|
||||
* serious number of cpus, therefore we need to take a distributed approach
|
||||
* to calculating nr_active.
|
||||
*
|
||||
* \Sum_i x_i(t) = \Sum_i x_i(t) - x_i(t_0) | x_i(t_0) := 0
|
||||
* = \Sum_i { \Sum_j=1 x_i(t_j) - x_i(t_j-1) }
|
||||
*
|
||||
* So assuming nr_active := 0 when we start out -- true per definition, we
|
||||
* can simply take per-cpu deltas and fold those into a global accumulate
|
||||
* to obtain the same result. See calc_load_fold_active().
|
||||
*
|
||||
* Furthermore, in order to avoid synchronizing all per-cpu delta folding
|
||||
* across the machine, we assume 10 ticks is sufficient time for every
|
||||
* cpu to have completed this task.
|
||||
*
|
||||
* This places an upper-bound on the IRQ-off latency of the machine. Then
|
||||
* again, being late doesn't loose the delta, just wrecks the sample.
|
||||
*
|
||||
* - cpu_rq()->nr_uninterruptible isn't accurately tracked per-cpu because
|
||||
* this would add another cross-cpu cacheline miss and atomic operation
|
||||
* to the wakeup path. Instead we increment on whatever cpu the task ran
|
||||
* when it went into uninterruptible state and decrement on whatever cpu
|
||||
* did the wakeup. This means that only the sum of nr_uninterruptible over
|
||||
* all cpus yields the correct result.
|
||||
*
|
||||
* This covers the NO_HZ=n code, for extra head-aches, see the comment below.
|
||||
*/
|
||||
|
||||
/* Variables and functions for calc_load */
|
||||
static atomic_long_t calc_load_tasks;
|
||||
static unsigned long calc_load_update;
|
||||
unsigned long avenrun[3];
|
||||
EXPORT_SYMBOL(avenrun);
|
||||
EXPORT_SYMBOL(avenrun); /* should be removed */
|
||||
|
||||
/**
|
||||
* get_avenrun - get the load average array
|
||||
* @loads: pointer to dest load array
|
||||
* @offset: offset to add
|
||||
* @shift: shift count to shift the result left
|
||||
*
|
||||
* These values are estimates at best, so no need for locking.
|
||||
*/
|
||||
void get_avenrun(unsigned long *loads, unsigned long offset, int shift)
|
||||
{
|
||||
loads[0] = (avenrun[0] + offset) << shift;
|
||||
loads[1] = (avenrun[1] + offset) << shift;
|
||||
loads[2] = (avenrun[2] + offset) << shift;
|
||||
}
|
||||
|
||||
static long calc_load_fold_active(struct rq *this_rq)
|
||||
{
|
||||
|
|
@ -2182,6 +2244,9 @@ static long calc_load_fold_active(struct rq *this_rq)
|
|||
return delta;
|
||||
}
|
||||
|
||||
/*
|
||||
* a1 = a0 * e + a * (1 - e)
|
||||
*/
|
||||
static unsigned long
|
||||
calc_load(unsigned long load, unsigned long exp, unsigned long active)
|
||||
{
|
||||
|
|
@ -2193,30 +2258,118 @@ calc_load(unsigned long load, unsigned long exp, unsigned long active)
|
|||
|
||||
#ifdef CONFIG_NO_HZ
|
||||
/*
|
||||
* For NO_HZ we delay the active fold to the next LOAD_FREQ update.
|
||||
* Handle NO_HZ for the global load-average.
|
||||
*
|
||||
* Since the above described distributed algorithm to compute the global
|
||||
* load-average relies on per-cpu sampling from the tick, it is affected by
|
||||
* NO_HZ.
|
||||
*
|
||||
* The basic idea is to fold the nr_active delta into a global idle-delta upon
|
||||
* entering NO_HZ state such that we can include this as an 'extra' cpu delta
|
||||
* when we read the global state.
|
||||
*
|
||||
* Obviously reality has to ruin such a delightfully simple scheme:
|
||||
*
|
||||
* - When we go NO_HZ idle during the window, we can negate our sample
|
||||
* contribution, causing under-accounting.
|
||||
*
|
||||
* We avoid this by keeping two idle-delta counters and flipping them
|
||||
* when the window starts, thus separating old and new NO_HZ load.
|
||||
*
|
||||
* The only trick is the slight shift in index flip for read vs write.
|
||||
*
|
||||
* 0s 5s 10s 15s
|
||||
* +10 +10 +10 +10
|
||||
* |-|-----------|-|-----------|-|-----------|-|
|
||||
* r:0 0 1 1 0 0 1 1 0
|
||||
* w:0 1 1 0 0 1 1 0 0
|
||||
*
|
||||
* This ensures we'll fold the old idle contribution in this window while
|
||||
* accumlating the new one.
|
||||
*
|
||||
* - When we wake up from NO_HZ idle during the window, we push up our
|
||||
* contribution, since we effectively move our sample point to a known
|
||||
* busy state.
|
||||
*
|
||||
* This is solved by pushing the window forward, and thus skipping the
|
||||
* sample, for this cpu (effectively using the idle-delta for this cpu which
|
||||
* was in effect at the time the window opened). This also solves the issue
|
||||
* of having to deal with a cpu having been in NOHZ idle for multiple
|
||||
* LOAD_FREQ intervals.
|
||||
*
|
||||
* When making the ILB scale, we should try to pull this in as well.
|
||||
*/
|
||||
static atomic_long_t calc_load_tasks_idle;
|
||||
static atomic_long_t calc_load_idle[2];
|
||||
static int calc_load_idx;
|
||||
|
||||
void calc_load_account_idle(struct rq *this_rq)
|
||||
static inline int calc_load_write_idx(void)
|
||||
{
|
||||
int idx = calc_load_idx;
|
||||
|
||||
/*
|
||||
* See calc_global_nohz(), if we observe the new index, we also
|
||||
* need to observe the new update time.
|
||||
*/
|
||||
smp_rmb();
|
||||
|
||||
/*
|
||||
* If the folding window started, make sure we start writing in the
|
||||
* next idle-delta.
|
||||
*/
|
||||
if (!time_before(jiffies, calc_load_update))
|
||||
idx++;
|
||||
|
||||
return idx & 1;
|
||||
}
|
||||
|
||||
static inline int calc_load_read_idx(void)
|
||||
{
|
||||
return calc_load_idx & 1;
|
||||
}
|
||||
|
||||
void calc_load_enter_idle(void)
|
||||
{
|
||||
struct rq *this_rq = this_rq();
|
||||
long delta;
|
||||
|
||||
/*
|
||||
* We're going into NOHZ mode, if there's any pending delta, fold it
|
||||
* into the pending idle delta.
|
||||
*/
|
||||
delta = calc_load_fold_active(this_rq);
|
||||
if (delta)
|
||||
atomic_long_add(delta, &calc_load_tasks_idle);
|
||||
if (delta) {
|
||||
int idx = calc_load_write_idx();
|
||||
atomic_long_add(delta, &calc_load_idle[idx]);
|
||||
}
|
||||
}
|
||||
|
||||
void calc_load_exit_idle(void)
|
||||
{
|
||||
struct rq *this_rq = this_rq();
|
||||
|
||||
/*
|
||||
* If we're still before the sample window, we're done.
|
||||
*/
|
||||
if (time_before(jiffies, this_rq->calc_load_update))
|
||||
return;
|
||||
|
||||
/*
|
||||
* We woke inside or after the sample window, this means we're already
|
||||
* accounted through the nohz accounting, so skip the entire deal and
|
||||
* sync up for the next window.
|
||||
*/
|
||||
this_rq->calc_load_update = calc_load_update;
|
||||
if (time_before(jiffies, this_rq->calc_load_update + 10))
|
||||
this_rq->calc_load_update += LOAD_FREQ;
|
||||
}
|
||||
|
||||
static long calc_load_fold_idle(void)
|
||||
{
|
||||
int idx = calc_load_read_idx();
|
||||
long delta = 0;
|
||||
|
||||
/*
|
||||
* Its got a race, we don't care...
|
||||
*/
|
||||
if (atomic_long_read(&calc_load_tasks_idle))
|
||||
delta = atomic_long_xchg(&calc_load_tasks_idle, 0);
|
||||
if (atomic_long_read(&calc_load_idle[idx]))
|
||||
delta = atomic_long_xchg(&calc_load_idle[idx], 0);
|
||||
|
||||
return delta;
|
||||
}
|
||||
|
|
@ -2302,66 +2455,39 @@ static void calc_global_nohz(void)
|
|||
{
|
||||
long delta, active, n;
|
||||
|
||||
/*
|
||||
* If we crossed a calc_load_update boundary, make sure to fold
|
||||
* any pending idle changes, the respective CPUs might have
|
||||
* missed the tick driven calc_load_account_active() update
|
||||
* due to NO_HZ.
|
||||
*/
|
||||
delta = calc_load_fold_idle();
|
||||
if (delta)
|
||||
atomic_long_add(delta, &calc_load_tasks);
|
||||
if (!time_before(jiffies, calc_load_update + 10)) {
|
||||
/*
|
||||
* Catch-up, fold however many we are behind still
|
||||
*/
|
||||
delta = jiffies - calc_load_update - 10;
|
||||
n = 1 + (delta / LOAD_FREQ);
|
||||
|
||||
active = atomic_long_read(&calc_load_tasks);
|
||||
active = active > 0 ? active * FIXED_1 : 0;
|
||||
|
||||
avenrun[0] = calc_load_n(avenrun[0], EXP_1, active, n);
|
||||
avenrun[1] = calc_load_n(avenrun[1], EXP_5, active, n);
|
||||
avenrun[2] = calc_load_n(avenrun[2], EXP_15, active, n);
|
||||
|
||||
calc_load_update += n * LOAD_FREQ;
|
||||
}
|
||||
|
||||
/*
|
||||
* It could be the one fold was all it took, we done!
|
||||
* Flip the idle index...
|
||||
*
|
||||
* Make sure we first write the new time then flip the index, so that
|
||||
* calc_load_write_idx() will see the new time when it reads the new
|
||||
* index, this avoids a double flip messing things up.
|
||||
*/
|
||||
if (time_before(jiffies, calc_load_update + 10))
|
||||
return;
|
||||
|
||||
/*
|
||||
* Catch-up, fold however many we are behind still
|
||||
*/
|
||||
delta = jiffies - calc_load_update - 10;
|
||||
n = 1 + (delta / LOAD_FREQ);
|
||||
|
||||
active = atomic_long_read(&calc_load_tasks);
|
||||
active = active > 0 ? active * FIXED_1 : 0;
|
||||
|
||||
avenrun[0] = calc_load_n(avenrun[0], EXP_1, active, n);
|
||||
avenrun[1] = calc_load_n(avenrun[1], EXP_5, active, n);
|
||||
avenrun[2] = calc_load_n(avenrun[2], EXP_15, active, n);
|
||||
|
||||
calc_load_update += n * LOAD_FREQ;
|
||||
}
|
||||
#else
|
||||
void calc_load_account_idle(struct rq *this_rq)
|
||||
{
|
||||
smp_wmb();
|
||||
calc_load_idx++;
|
||||
}
|
||||
#else /* !CONFIG_NO_HZ */
|
||||
|
||||
static inline long calc_load_fold_idle(void)
|
||||
{
|
||||
return 0;
|
||||
}
|
||||
static inline long calc_load_fold_idle(void) { return 0; }
|
||||
static inline void calc_global_nohz(void) { }
|
||||
|
||||
static void calc_global_nohz(void)
|
||||
{
|
||||
}
|
||||
#endif
|
||||
|
||||
/**
|
||||
* get_avenrun - get the load average array
|
||||
* @loads: pointer to dest load array
|
||||
* @offset: offset to add
|
||||
* @shift: shift count to shift the result left
|
||||
*
|
||||
* These values are estimates at best, so no need for locking.
|
||||
*/
|
||||
void get_avenrun(unsigned long *loads, unsigned long offset, int shift)
|
||||
{
|
||||
loads[0] = (avenrun[0] + offset) << shift;
|
||||
loads[1] = (avenrun[1] + offset) << shift;
|
||||
loads[2] = (avenrun[2] + offset) << shift;
|
||||
}
|
||||
#endif /* CONFIG_NO_HZ */
|
||||
|
||||
/*
|
||||
* calc_load - update the avenrun load estimates 10 ticks after the
|
||||
|
|
@ -2369,11 +2495,18 @@ void get_avenrun(unsigned long *loads, unsigned long offset, int shift)
|
|||
*/
|
||||
void calc_global_load(unsigned long ticks)
|
||||
{
|
||||
long active;
|
||||
long active, delta;
|
||||
|
||||
if (time_before(jiffies, calc_load_update + 10))
|
||||
return;
|
||||
|
||||
/*
|
||||
* Fold the 'old' idle-delta to include all NO_HZ cpus.
|
||||
*/
|
||||
delta = calc_load_fold_idle();
|
||||
if (delta)
|
||||
atomic_long_add(delta, &calc_load_tasks);
|
||||
|
||||
active = atomic_long_read(&calc_load_tasks);
|
||||
active = active > 0 ? active * FIXED_1 : 0;
|
||||
|
||||
|
|
@ -2384,12 +2517,7 @@ void calc_global_load(unsigned long ticks)
|
|||
calc_load_update += LOAD_FREQ;
|
||||
|
||||
/*
|
||||
* Account one period with whatever state we found before
|
||||
* folding in the nohz state and ageing the entire idle period.
|
||||
*
|
||||
* This avoids loosing a sample when we go idle between
|
||||
* calc_load_account_active() (10 ticks ago) and now and thus
|
||||
* under-accounting.
|
||||
* In case we idled for multiple LOAD_FREQ intervals, catch up in bulk.
|
||||
*/
|
||||
calc_global_nohz();
|
||||
}
|
||||
|
|
@ -2406,13 +2534,16 @@ static void calc_load_account_active(struct rq *this_rq)
|
|||
return;
|
||||
|
||||
delta = calc_load_fold_active(this_rq);
|
||||
delta += calc_load_fold_idle();
|
||||
if (delta)
|
||||
atomic_long_add(delta, &calc_load_tasks);
|
||||
|
||||
this_rq->calc_load_update += LOAD_FREQ;
|
||||
}
|
||||
|
||||
/*
|
||||
* End of global load-average stuff
|
||||
*/
|
||||
|
||||
/*
|
||||
* The exact cpuload at various idx values, calculated at every tick would be
|
||||
* load = (2^idx - 1) / 2^idx * load + 1 / 2^idx * cur_load
|
||||
|
|
|
|||
|
|
@ -25,7 +25,6 @@ static void check_preempt_curr_idle(struct rq *rq, struct task_struct *p, int fl
|
|||
static struct task_struct *pick_next_task_idle(struct rq *rq)
|
||||
{
|
||||
schedstat_inc(rq, sched_goidle);
|
||||
calc_load_account_idle(rq);
|
||||
return rq->idle;
|
||||
}
|
||||
|
||||
|
|
|
|||
|
|
@ -942,8 +942,6 @@ static inline u64 sched_avg_period(void)
|
|||
return (u64)sysctl_sched_time_avg * NSEC_PER_MSEC / 2;
|
||||
}
|
||||
|
||||
void calc_load_account_idle(struct rq *this_rq);
|
||||
|
||||
#ifdef CONFIG_SCHED_HRTICK
|
||||
|
||||
/*
|
||||
|
|
|
|||
|
|
@ -406,6 +406,7 @@ static void tick_nohz_stop_sched_tick(struct tick_sched *ts)
|
|||
*/
|
||||
if (!ts->tick_stopped) {
|
||||
select_nohz_load_balancer(1);
|
||||
calc_load_enter_idle();
|
||||
|
||||
ts->idle_tick = hrtimer_get_expires(&ts->sched_timer);
|
||||
ts->tick_stopped = 1;
|
||||
|
|
@ -597,6 +598,7 @@ void tick_nohz_idle_exit(void)
|
|||
account_idle_ticks(ticks);
|
||||
#endif
|
||||
|
||||
calc_load_exit_idle();
|
||||
touch_softlockup_watchdog();
|
||||
/*
|
||||
* Cancel the scheduled timer and restore the tick
|
||||
|
|
|
|||
Loading…
Add table
Add a link
Reference in a new issue