linux-kernel - Re: [RFC PATCH V2] sched: Improve scalability of select_idle

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Date:   Tue, 9 Jan 2018 09:50:44 -0500
From:   Steven Sistare <steven.sistare@...cle.com>
To:     subhra mazumdar <subhra.mazumdar@...cle.com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        linux-kernel@...r.kernel.org, mingo@...hat.com,
        dhaval.giani@...cle.com
Subject: Re: [RFC PATCH V2] sched: Improve scalability of select_idle_sibling
 using SMT balance

On 1/8/2018 5:18 PM, Peter Zijlstra wrote:
> On Mon, Jan 08, 2018 at 02:12:37PM -0800, subhra mazumdar wrote:
>> @@ -2751,6 +2763,31 @@ context_switch(struct rq *rq, struct task_struct *prev,
>>  	       struct task_struct *next, struct rq_flags *rf)
>>  {
>>  	struct mm_struct *mm, *oldmm;
>> +	int this_cpu = rq->cpu;
>> +	struct sched_domain *sd;
>> +	int prev_busy, next_busy;
>> +
>> +	if (rq->curr_util == UTIL_UNINITIALIZED)
>> +		prev_busy = 0;
>> +	else
>> +		prev_busy = (prev != rq->idle);
>> +	next_busy = (next != rq->idle);
>> +
>> +	/*
>> +	 * From sd_llc downward update the SMT utilization.
>> +	 * Skip the lowest level 0.
>> +	 */
>> +	sd = rcu_dereference_sched(per_cpu(sd_llc, this_cpu));
>> +	if (next_busy != prev_busy) {
>> +		for_each_lower_domain(sd) {
>> +			if (sd->level == 0)
>> +				break;
>> +			sd_context_switch(sd, rq, next_busy - prev_busy);
>> +		}
>> +	}
>> +
> 
> No, we're not going to be adding atomic ops here. We've been arguing
> over adding a single memory barrier to this path, atomic are just not
> going to happen.
> 
> Also this is entirely the wrong way to do this, we already have code
> paths that _know_ if they're going into or coming out of idle.

Yes, it would be more efficient to adjust the busy-cpu count of each level 
of the hierarchy in pick_next_task_idle and put_prev_task_idle.

- Steve