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    Subject[tip:sched/core] sched/nohz: Rewrite and fix load-avg computation -- again
    Commit-ID:  5167e8d5417bf5c322a703d2927daec727ea40dd
    Gitweb: http://git.kernel.org/tip/5167e8d5417bf5c322a703d2927daec727ea40dd
    Author: Peter Zijlstra <a.p.zijlstra@chello.nl>
    AuthorDate: Fri, 22 Jun 2012 15:52:09 +0200
    Committer: Ingo Molnar <mingo@kernel.org>
    CommitDate: Thu, 5 Jul 2012 20:58:13 +0200

    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>
    ---
    include/linux/sched.h | 8 ++
    kernel/sched/core.c | 275 ++++++++++++++++++++++++++++++++++------------
    kernel/sched/idle_task.c | 1 -
    kernel/sched/sched.h | 2 -
    kernel/time/tick-sched.c | 2 +
    5 files changed, 213 insertions(+), 75 deletions(-)

    diff --git a/include/linux/sched.h b/include/linux/sched.h
    index 4059c0f..20cb749 100644
    --- a/include/linux/sched.h
    +++ b/include/linux/sched.h
    @@ -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)
    {
    diff --git a/kernel/sched/core.c b/kernel/sched/core.c
    index d5594a4..bb84040 100644
    --- a/kernel/sched/core.c
    +++ b/kernel/sched/core.c
    @@ -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]);
    + }
    }

    -static long calc_load_fold_idle(void)
    +void calc_load_exit_idle(void)
    {
    - long delta = 0;
    + 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;

    /*
    - * Its got a race, we don't care...
    + * 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.
    */
    - if (atomic_long_read(&calc_load_tasks_idle))
    - delta = atomic_long_xchg(&calc_load_tasks_idle, 0);
    + 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;
    +
    + 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);
    -
    - /*
    - * It could be the one fold was all it took, we done!
    - */
    - 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);
    + 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;
    + 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);
    + 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)
    -{
    -}
    + calc_load_update += n * LOAD_FREQ;
    + }

    -static inline long calc_load_fold_idle(void)
    -{
    - return 0;
    + /*
    + * 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.
    + */
    + smp_wmb();
    + calc_load_idx++;
    }
    +#else /* !CONFIG_NO_HZ */

    -static void calc_global_nohz(void)
    -{
    -}
    -#endif
    +static inline long calc_load_fold_idle(void) { return 0; }
    +static inline void calc_global_nohz(void) { }

    -/**
    - * 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();
    }
    @@ -2406,7 +2534,6 @@ 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);

    @@ -2414,6 +2541,10 @@ static void calc_load_account_active(struct rq *this_rq)
    }

    /*
    + * 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
    *
    diff --git a/kernel/sched/idle_task.c b/kernel/sched/idle_task.c
    index b44d604..b6baf37 100644
    --- a/kernel/sched/idle_task.c
    +++ b/kernel/sched/idle_task.c
    @@ -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;
    }

    diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
    index 6d52cea..55844f2 100644
    --- a/kernel/sched/sched.h
    +++ b/kernel/sched/sched.h
    @@ -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

    /*
    diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c
    index 8699978..4a08472 100644
    --- a/kernel/time/tick-sched.c
    +++ b/kernel/time/tick-sched.c
    @@ -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

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