linux-kernel - Re: [PATCH v2 2/2] sched/fair: Trigger nohz.next_balance updates when a CPU goes NOHZ-idle

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Date:   Mon, 19 Jul 2021 17:28:44 +0100
From:   Valentin Schneider <valentin.schneider@....com>
To:     Dietmar Eggemann <dietmar.eggemann@....com>,
        linux-kernel@...r.kernel.org
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...nel.org>,
        Vincent Guittot <vincent.guittot@...aro.org>
Subject: Re: [PATCH v2 2/2] sched/fair: Trigger nohz.next_balance updates when a CPU goes NOHZ-idle

On 19/07/21 17:24, Dietmar Eggemann wrote:
> On 19/07/2021 12:31, Valentin Schneider wrote:
>
> [...]
>
>> @@ -10351,6 +10352,9 @@ static void nohz_balancer_kick(struct rq *rq)
>>  unlock:
>>      rcu_read_unlock();
>>  out:
>> +	if (READ_ONCE(nohz.needs_update))
>> +		flags |= NOHZ_NEXT_KICK;
>> +
>
> Since NOHZ_NEXT_KICK is part of NOHZ_KICK_MASK, some conditions above
> will already set it in flags. Is this intended?

So if no kick would be issued (e.g. flags == 0 because nohz.next_balance is
later in the future), then this does the right thing and issues a
NOHZ_NEXT_KICK one.

However you're right to point out that even if nohz.needs_update is false,
we can set NOHZ_NEXT_KICK into the ilb rq's NOHZ flags due to it being
included in NOHZ_KICK_MASK, which I think is a mistake. Looking at it now,
it shouldn't be part of NOHZ_KICK_MASK.

>
>>      if (flags)
>>              kick_ilb(flags);
>>  }
>> @@ -10447,12 +10451,13 @@ void nohz_balance_enter_idle(int cpu)
>>      /*
>>       * Ensures that if nohz_idle_balance() fails to observe our
>>       * @idle_cpus_mask store, it must observe the @has_blocked
>> -	 * store.
>> +	 * and @needs_update stores.
>>       */
>>      smp_mb__after_atomic();
>>
>>      set_cpu_sd_state_idle(cpu);
>>
>> +	WRITE_ONCE(nohz.needs_update, 1);
>>  out:
>>      /*
>>       * Each time a cpu enter idle, we assume that it has blocked load and
>> @@ -10501,13 +10506,17 @@ static void _nohz_idle_balance(struct rq *this_rq, unsigned int flags,
>
> function header would need update to incorporate the new 'update
> nohz.next_balance' functionality. It only talks about 'update of blocked
> load' and 'complete load balance' so far.
>
>>      /*
>>       * We assume there will be no idle load after this update and clear
>>       * the has_blocked flag. If a cpu enters idle in the mean time, it will
>> -	 * set the has_blocked flag and trig another update of idle load.
>> +	 * set the has_blocked flag and trigger another update of idle load.
>>       * Because a cpu that becomes idle, is added to idle_cpus_mask before
>>       * setting the flag, we are sure to not clear the state and not
>>       * check the load of an idle cpu.
>> +	 *
>> +	 * Same applies to idle_cpus_mask vs needs_update.
>>       */
>>      if (flags & NOHZ_STATS_KICK)
>>              WRITE_ONCE(nohz.has_blocked, 0);
>> +	if (flags & NOHZ_NEXT_KICK)
>> +		WRITE_ONCE(nohz.needs_update, 0);
>>
>>      /*
>>       * Ensures that if we miss the CPU, we must see the has_blocked
>> @@ -10531,6 +10540,8 @@ static void _nohz_idle_balance(struct rq *this_rq, unsigned int flags,
>>              if (need_resched()) {
>>                      if (flags & NOHZ_STATS_KICK)
>>                              has_blocked_load = true;
>
> This looks weird now? 'has_blocked_load = true' vs
> 'WRITE_ONCE(nohz.needs_update, 1)'.
>

Well, has_blocked_load lets us factorize the nohz.has_blocked write
(one is needed either when aborting or at the tail of the cpumask
iteration), whereas there is just a single write for nohz.needs_update
(when aborting).

>> +			if (flags & NOHZ_NEXT_KICK)
>> +				WRITE_ONCE(nohz.needs_update, 1);
>>                      goto abort;
>>              }
>>
>>