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Date:   Tue, 11 Jul 2023 12:18:32 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     David Vernet <void@manifault.com>
Cc:     linux-kernel@vger.kernel.org, mingo@redhat.com,
        juri.lelli@redhat.com, vincent.guittot@linaro.org,
        dietmar.eggemann@arm.com, rostedt@goodmis.org, bsegall@google.com,
        mgorman@suse.de, bristot@redhat.com, vschneid@redhat.com,
        gautham.shenoy@amd.com, kprateek.nayak@amd.com, aaron.lu@intel.com,
        clm@meta.com, tj@kernel.org, roman.gushchin@linux.dev,
        kernel-team@meta.com
Subject: Re: [PATCH v2 5/7] sched: Implement shared runqueue in CFS
Message-ID: <20230711101832.GF3062772@hirez.programming.kicks-ass.net>
References: <20230710200342.358255-1-void@manifault.com>
 <20230710200342.358255-6-void@manifault.com>
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On Mon, Jul 10, 2023 at 03:03:40PM -0500, David Vernet wrote:

> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index 1451f5aa82ac..3ad437d4ea3d 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c

> @@ -9842,6 +9843,7 @@ void __init sched_init_smp(void)
>  
>  	init_sched_rt_class();
>  	init_sched_dl_class();
> +	init_sched_fair_class_late();
>  
>  	sched_smp_initialized = true;
>  }

> @@ -12854,3 +12999,34 @@ __init void init_sched_fair_class(void)
>  #endif /* SMP */
>  
>  }
> +
> +__init void init_sched_fair_class_late(void)
> +{
> +#ifdef CONFIG_SMP
> +	int i;
> +	struct shared_runq *shared_runq;
> +	struct rq *rq;
> +	struct rq *llc_rq;
> +
> +	for_each_possible_cpu(i) {
> +		if (per_cpu(sd_llc_id, i) == i) {
> +			llc_rq = cpu_rq(i);
> +
> +			shared_runq = kzalloc_node(sizeof(struct shared_runq),
> +					       GFP_KERNEL, cpu_to_node(i));
> +			INIT_LIST_HEAD(&shared_runq->list);
> +			spin_lock_init(&shared_runq->lock);
> +			llc_rq->cfs.shared_runq = shared_runq;
> +		}
> +	}
> +
> +	for_each_possible_cpu(i) {
> +		rq = cpu_rq(i);
> +		llc_rq = cpu_rq(per_cpu(sd_llc_id, i));
> +
> +		if (rq == llc_rq)
> +			continue;
> +		rq->cfs.shared_runq = llc_rq->cfs.shared_runq;
> +	}
> +#endif /* SMP */
> +}

I don't think this is right; the llc_id thing depends on the online
mask, not on possible mask. So if you boot with a number of CPUs offline
and late bring them online, you're screwy (IIRC this is actually a very
common thing in virt land).

Additionally, llc_id depends on the sched_domain tree, if someone were
to go create partitions, they can split an LLC and llc_id would split
right along with it.

I think you need to move this into sched/topology.c and handle hotplug /
domain (re) creation.

And yes, that's going to be a pain, because you might need to re-hash
existing lists.