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    SubjectRe: [RFC PATCH 2/5] sched: Add NOHZ_STATS_KICK
    On Mon, Jan 15, 2018 at 09:26:09AM +0100, Vincent Guittot wrote:

    Would've probably been easier to read if you'd not included the revert
    of that timer patch...

    > @@ -9258,21 +9255,11 @@ void nohz_balance_enter_idle(int cpu)
    > set_cpu_sd_state_idle(cpu);
    >
    > /*
    > - * implies a barrier such that if the stats_state update is observed
    > - * the above updates are also visible. Pairs with stuff in
    > - * update_sd_lb_stats() and nohz_idle_balance().
    > + * Each time a cpu enter idle, we assume that it has blocked load and
    > + * enable the periodic update of the load of idle cpus
    > */
    > - val = atomic_read(&nohz.stats_state);
    > - do {
    > - new = val + 2;
    > - new |= 1;
    > - } while (!atomic_try_cmpxchg(&nohz.stats_state, &val, new));
    > + atomic_set(&nohz.stats_state, 1);
    >

    > @@ -9422,7 +9408,6 @@ static bool nohz_idle_balance(struct rq *this_rq, enum cpu_idle_type idle)
    > return false;
    > }
    >
    > - stats_seq = atomic_read(&nohz.stats_state);
    > /*
    > * barrier, pairs with nohz_balance_enter_idle(), ensures ...
    > */
    > @@ -9432,6 +9417,16 @@ static bool nohz_idle_balance(struct rq *this_rq, enum cpu_idle_type idle)
    >
    > SCHED_WARN_ON((flags & NOHZ_KICK_MASK) == NOHZ_BALANCE_KICK);
    >
    > + /*
    > + * We assume there will be no idle load after this update and clear
    > + * the stats state. If a cpu enters idle in the mean time, it will
    > + * set the stats state and trig another update of idle load.
    > + * Because a cpu that becomes idle, is added to idle_cpus_mask before
    > + * setting the stats state, we are sure to not clear the state and not
    > + * check the load of an idle cpu.
    > + */
    > + atomic_set(&nohz.stats_state, 0);
    > +
    > for_each_cpu(balance_cpu, nohz.idle_cpus_mask) {
    > if (balance_cpu == this_cpu || !idle_cpu(balance_cpu))
    > continue;
    > @@ -9441,8 +9436,10 @@ static bool nohz_idle_balance(struct rq *this_rq, enum cpu_idle_type idle)
    > * work being done for other cpus. Next load
    > * balancing owner will pick it up.
    > */
    > - if (need_resched())
    > + if (need_resched()) {
    > + has_blocked_load = true;
    > break;
    > + }
    >
    > rq = cpu_rq(balance_cpu);
    >
    > @@ -9477,12 +9474,12 @@ static bool nohz_idle_balance(struct rq *this_rq, enum cpu_idle_type idle)
    > if (flags & NOHZ_BALANCE_KICK)
    > rebalance_domains(this_rq, CPU_IDLE);
    >
    > - if (has_blocked_load ||
    > - !atomic_try_cmpxchg(&nohz.stats_state, &stats_seq, 0)) {
    > - WRITE_ONCE(nohz.next_stats,
    > - now + msecs_to_jiffies(LOAD_AVG_PERIOD));
    > - mod_timer(&nohz.timer, nohz.next_stats);
    > - }
    > + WRITE_ONCE(nohz.next_stats,
    > + now + msecs_to_jiffies(LOAD_AVG_PERIOD));
    > +
    > + /* There is still blocked load, enable periodic update */
    > + if (has_blocked_load)
    > + atomic_set(&nohz.stats_state, 1);
    >
    > /*
    > * next_balance will be updated only when there is a need.

    After this there is no point for stats_state to be atomic anymore. Also
    a better name.

    Maybe if I drop the last two patches (and you re-introduce the bits
    from: Subject: sched: Optimize nohz stats, that you do need) this all
    becomes more readable?

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