linux-kernel - Re: [PATCH 1/7] sched/balancing: Switch the 'DEFINE_SPINLOCK(balancing)' spinlock into an 'atomic_t sched_balance

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Date: Fri, 1 Mar 2024 21:05:02 +0530
From: Shrikanth Hegde <sshegde@...ux.ibm.com>
To: Ingo Molnar <mingo@...nel.org>
Cc: Peter Zijlstra <peterz@...radead.org>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Valentin Schneider <vschneid@...hat.com>,
        Nicholas Piggin
 <npiggin@...il.com>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/7] sched/balancing: Switch the
 'DEFINE_SPINLOCK(balancing)' spinlock into an 'atomic_t
 sched_balance_running' flag



On 3/1/24 4:39 PM, Ingo Molnar wrote:
> The 'balancing' spinlock added in:

Hi Ingo. 

> 
>   08c183f31bdb ("[PATCH] sched: add option to serialize load balancing")
>

 
[...]

>  
>  		need_serialize = sd->flags & SD_SERIALIZE;
>  		if (need_serialize) {
> -			if (!spin_trylock(&balancing))
> +			if (atomic_cmpxchg_acquire(&sched_balance_running, 0, 1))

Thinking from very little I know, I may be completely wrong. 

Is it possible that arch_spin_trylock, which would be called from spin_trylock is 
faster in some architectures? Maybe in contended case? 

For example, in powerpc, queued_spin_trylock, uses more optimal ll/sc style access patterns 
rather than cmpxchg. 
https://lore.kernel.org/all/20221126095932.1234527-4-npiggin@gmail.com/

+nick 


>  				goto out;
>  		}
>  
> @@ -11729,7 +11742,7 @@ static void rebalance_domains(struct rq *rq, enum cpu_idle_type idle)
>  			interval = get_sd_balance_interval(sd, busy);
>  		}
>  		if (need_serialize)
> -			spin_unlock(&balancing);
> +			atomic_set_release(&sched_balance_running, 0);
>  out:
>  		if (time_after(next_balance, sd->last_balance + interval)) {
>  			next_balance = sd->last_balance + interval;