linux-kernel - Re: [tip: sched/core] sched/fair: Skip sched_balance

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Message-ID: <20251118095606.GH4068168@noisy.programming.kicks-ass.net>
Date: Tue, 18 Nov 2025 10:56:06 +0100
From: Peter Zijlstra <peterz@...radead.org>
To: Tim Chen <tim.c.chen@...ux.intel.com>, nathan@...nel.org
Cc: Shrikanth Hegde <sshegde@...ux.ibm.com>, linux-kernel@...r.kernel.org,
	linux-tip-commits@...r.kernel.org, Chen Yu <yu.c.chen@...el.com>,
	Vincent Guittot <vincent.guittot@...aro.org>,
	K Prateek Nayak <kprateek.nayak@....com>,
	Srikar Dronamraju <srikar@...ux.ibm.com>,
	Mohini Narkhede <mohini.narkhede@...el.com>, x86@...nel.org
Subject: Re: [tip: sched/core] sched/fair: Skip sched_balance_running cmpxchg
 when balance is not due


And now with Nathan as a recipient..

On Tue, Nov 18, 2025 at 10:54:33AM +0100, Peter Zijlstra wrote:
> On Mon, Nov 17, 2025 at 10:55:07AM -0800, Tim Chen wrote:
> 
> > >          if (!need_unlock && (sd->flags & SD_SERIALIZE)) {
> > > -               if (!atomic_try_cmpxchg_acquire(&sched_balance_running, 0, 1))
> > 
> > The second argument of atomic_try_cmpxchg_acquire is "int *old" while that of atomic_cmpxchg_acquire
> > is "int old". So the above check would result in NULL pointer access.  Probably have
> > to do something like the following to use atomic_try_cmpxchg_acquire()
> > 
> > 		int zero = 0;
> > 		if (!atomic_try_cmpxchg_acquire(&sched_balance_running, &zero, 1))
> > 		
> > Otherwise we should do atomic_cmpxchg_acquire() as below
> 
> Yes, and I'm all mightily miffed all the compilers accept 0 (which is
> int) for an 'int *' argument without so much as a warning :/
> 
> Nathan, you looked into this a bit yesterday, afaict there is:
> 
>   -Wzero-as-null-pointer-constant
> 
> which is supposed to issue a warn here, but I can't get clang-22 to
> object :/ (GCC doesn't take that warning for C mode, only C++, perhaps
> that's the problem?).
> 
> And then there is modernize-use-nullptr, but that objects to using NULL,
> although I suppose we could do:
> 
>   #define NULL nullptr
> 
> to get around that. Except I also cannot get clangd to report on the
> issue.
> 
> Help?