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Date:   Wed, 21 Jun 2023 13:17:21 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Chen Yu <yu.c.chen@intel.com>
Cc:     Vincent Guittot <vincent.guittot@linaro.org>,
        Ingo Molnar <mingo@redhat.com>,
        Juri Lelli <juri.lelli@redhat.com>,
        Tim Chen <tim.c.chen@intel.com>,
        Mel Gorman <mgorman@techsingularity.net>,
        Dietmar Eggemann <dietmar.eggemann@arm.com>,
        K Prateek Nayak <kprateek.nayak@amd.com>,
        Abel Wu <wuyun.abel@bytedance.com>,
        "Gautham R . Shenoy" <gautham.shenoy@amd.com>,
        Len Brown <len.brown@intel.com>,
        Chen Yu <yu.chen.surf@gmail.com>,
        Yicong Yang <yangyicong@hisilicon.com>,
        linux-kernel@vger.kernel.org
Subject: Re: [RFC PATCH 3/4] sched/fair: Calculate the scan depth for idle
 balance based on system utilization
Message-ID: <20230621111721.GA2053369@hirez.programming.kicks-ass.net>
References: <cover.1686554037.git.yu.c.chen@intel.com>
 <bc96b53efb712337a645e2c2a340975545ed5a28.1686554037.git.yu.c.chen@intel.com>
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On Tue, Jun 13, 2023 at 12:18:57AM +0800, Chen Yu wrote:
> When CPU is about to enter idle, it invokes newidle_balance() to pull
> some tasks from other runqueues. Although there is per domain
> max_newidle_lb_cost to throttle the newidle_balance(), it would be
> good to further limit the scan based on overall system utilization.
> The reason is that there is no limitation for newidle_balance() to
> launch this balance simultaneously on multiple CPUs. Since each
> newidle_balance() has to traverse all the CPUs to calculate the
> statistics one by one, this total time cost on newidle_balance()
> could be O(n^2). This is not good for performance or power saving.

Another possible solution is to keep struct sg_lb_stats in
sd->child->shared (below the NUMA domains) and put a lock around it.

Then have update_sd_lb_stats() do something like:

	struct sg_lb_stats *sgs = &sds->sgs;

	if (raw_spin_trylock(&sds->sg_lock)) {
		struct sg_lb_stats tmp;

		... collect tmp

		sds->sgs = tmp;
		raw_spin_unlock(&sds->sg_lock);
	}

	... use sgs

Then you know you've always got a 'recent' copy but avoid the concurrent
updates.