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