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

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Date:   Wed, 10 Jan 2018 18:09:56 -0800
From:   Subhra Mazumdar <subhra.mazumdar@...cle.com>
To:     Steven Sistare <steven.sistare@...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 01/09/2018 06:50 AM, Steven Sistare wrote:
> 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.
OK, I have moved it to pick_next_task_idle/put_prev_task_idle. Will send 
out the v3.

Thanks,
Subhra
>
> - Steve