linux-kernel - Re: [RFC PATCH v3 06/10] sched: Add over-utilization/tipping point indicator

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Date:   Tue, 19 Jun 2018 12:26:35 +0200
From:   Dietmar Eggemann <dietmar.eggemann@....com>
To:     Pavan Kondeti <pkondeti@...eaurora.org>,
        Quentin Perret <quentin.perret@....com>
Cc:     peterz@...radead.org, rjw@...ysocki.net,
        gregkh@...uxfoundation.org, linux-kernel@...r.kernel.org,
        linux-pm@...r.kernel.org, mingo@...hat.com,
        morten.rasmussen@....com, chris.redpath@....com,
        patrick.bellasi@....com, valentin.schneider@....com,
        vincent.guittot@...aro.org, thara.gopinath@...aro.org,
        viresh.kumar@...aro.org, tkjos@...gle.com, joelaf@...gle.com,
        smuckle@...gle.com, adharmap@...cinc.com, skannan@...cinc.com,
        juri.lelli@...hat.com, edubezval@...il.com,
        srinivas.pandruvada@...ux.intel.com, currojerez@...eup.net,
        javi.merino@...nel.org
Subject: Re: [RFC PATCH v3 06/10] sched: Add over-utilization/tipping point
 indicator

On 06/19/2018 09:01 AM, Pavan Kondeti wrote:
> On Mon, May 21, 2018 at 03:25:01PM +0100, Quentin Perret wrote:

[...]

>> @@ -8152,6 +8176,9 @@ static inline void update_sg_lb_stats(struct lb_env *env,
>>   		if (nr_running > 1)
>>   			*overload = true;
>>   
>> +		if (cpu_overutilized(i))
>> +			*overutilized = 1;
>> +
> 
> There is no need to check if every CPU is overutilized or not once
> *overutilized is marked as true, right?

True, so you want to check *overutilized before calling 
cpu_overutilized() to save a little bit on compute?

[...]

>> @@ -8586,6 +8621,10 @@ static struct sched_group *find_busiest_group(struct lb_env *env)
>>   	 * this level.
>>   	 */
>>   	update_sd_lb_stats(env, &sds);
>> +
>> +	if (sched_energy_enabled() && !READ_ONCE(env->dst_rq->rd->overutilized))
>> +		goto out_balanced;
>> +
> 
> Is there any reason for sending no-hz idle kicks but bailing out here when
> system is not overutilized?

Even if a system is not-overutilized, we want to update stale cpu 
blocked load and utilization so NOHZ_STATS_KICK have to get through.

So calling find_busiest_group() -> update_sd_lb_stats() -> 
update_sg_lb_stats() to possibly execute update_nohz_stats() is IMHO the 
right thing to do.