linux-kernel - Re: [PATCH 01/10] sched/balancing: Switch the 'DEFINE_SPINLOCK(balancing)' spinlock into an 'atomic_t sched_balance

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Date: Fri, 15 Mar 2024 14:55:52 +0530
From: Shrikanth Hegde <sshegde@...ux.ibm.com>
To: Ingo Molnar <mingo@...nel.org>
Cc: Dietmar Eggemann <dietmar.eggemann@....com>,
        Peter Zijlstra <peterz@...radead.org>,
        Valentin Schneider <vschneid@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 01/10] sched/balancing: Switch the
 'DEFINE_SPINLOCK(balancing)' spinlock into an 'atomic_t
 sched_balance_running' flag



On 3/8/24 4:28 PM, Ingo Molnar wrote:
> The 'balancing' spinlock added in:
> 

> Also document the flag a bit.
> 
> No change in functionality intended.
> 

> -static DEFINE_SPINLOCK(balancing);
> +/*
> + * This flag serializes load-balancing passes over large domains
> + * (above the NODE topology level) - only one load-balancing instance
> + * may run at a time, to reduce overhead on very large systems with
> + * lots of CPUs and large NUMA distances.
> + *
> + * - Note that load-balancing passes triggered while another one
> + *   is executing are skipped and not re-tried.
> + *
> + * - Also note that this does not serialize rebalance_domains()

nit: please change rebalance_domains to sched_balance_domains. 

> + *   execution, as non-SD_SERIALIZE domains will still be
> + *   load-balanced in parallel.
> + */
> +static atomic_t sched_balance_running = ATOMIC_INIT(0);
>  
>  /*
>   * Scale the max load_balance interval with the number of CPUs in the system.
> @@ -11711,7 +11724,7 @@ static void rebalance_domains(struct rq *rq, enum cpu_idle_type idle)
>  
>  		need_serialize = sd->flags & SD_SERIALIZE;
>  		if (need_serialize) {