On 1 February 2018 at 17:52, Peter Zijlstra <pet...@infradead.org> wrote:
> 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.
Ok
>
> 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?
Yes. we can do like that
