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: Mon, 4 Mar 2024 10:24:52 +0100
From: Ingo Molnar <mingo@...nel.org>
To: Shrikanth Hegde <sshegde@...ux.ibm.com>
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


* Shrikanth Hegde <sshegde@...ux.ibm.com> wrote:

> 
> 
> 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? 

This code path should never really be 'contended': SD_SERIALIZE is only set 
for the outermost, largest domains (NUMA and up) that get balanced 
infrequently. This change is more for the sake of readability: a flag 
disguised as a spinlock.

Thanks,

	Ingo