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?
