linux-kernel - Re: [PATCH v2 2/2] sched/core: Drop spinlocks on contention iff kernel is preemptible

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Date: Fri, 26 Apr 2024 11:41:25 +0800
From: Chen Yu <yu.c.chen@...el.com>
To: Sean Christopherson <seanjc@...gle.com>
CC: Jonathan Corbet <corbet@....net>, Ingo Molnar <mingo@...hat.com>, "Peter
 Zijlstra" <peterz@...radead.org>, Juri Lelli <juri.lelli@...hat.com>,
	"Vincent Guittot" <vincent.guittot@...aro.org>, Will Deacon
	<will@...nel.org>, <linux-doc@...r.kernel.org>,
	<linux-kernel@...r.kernel.org>, "Valentin Schneider"
	<valentin.schneider@....com>, Marco Elver <elver@...gle.com>, Frederic
 Weisbecker <frederic@...nel.org>, David Matlack <dmatlack@...gle.com>,
	Friedrich Weber <f.weber@...xmox.com>, Ankur Arora
	<ankur.a.arora@...cle.com>, Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH v2 2/2] sched/core: Drop spinlocks on contention iff
 kernel is preemptible

On 2024-04-25 at 09:47:52 -0700, Sean Christopherson wrote:
> On Thu, Apr 25, 2024, Chen Yu wrote:
> > Hi Sean,
> > 
> > On 2024-03-12 at 12:39:11 -0700, Sean Christopherson wrote:
> > > Use preempt_model_preemptible() to detect a preemptible kernel when
> > > deciding whether or not to reschedule in order to drop a contended
> > > spinlock or rwlock.  Because PREEMPT_DYNAMIC selects PREEMPTION, kernels
> > 
> > It took me a while to wonder why PREEMPT_DYNAMIC selects PREEMPTION
> > in Kconfig, then I assume that you mean the static config is CONFIG_PREEMPTION,
> > but the live preemption model is "none" or "voluntary", which makes the
> > static check of CONFIG_PREEMPTION in spin_needbreak() and rwlock_needbreak()
> > invalid?
> 
> Yep, exactly.
> 
> > > diff --git a/include/linux/spinlock.h b/include/linux/spinlock.h
> > > index 3fcd20de6ca8..63dd8cf3c3c2 100644
> > > --- a/include/linux/spinlock.h
> > > +++ b/include/linux/spinlock.h
> > > @@ -462,11 +462,10 @@ static __always_inline int spin_is_contended(spinlock_t *lock)
> > >   */
> > >  static inline int spin_needbreak(spinlock_t *lock)
> > >  {
> > > -#ifdef CONFIG_PREEMPTION
> > > +	if (!preempt_model_preemptible())
> > 
> > The old version checks against static CONFIG_PREEMPTION, now we check
> > the live CONFIG_PREEMPTION and static CONFIG_PREEMPT_RT, just wonder
> > if the rt check is needed here?
> 
> It's required, as CONFIG_PREEMPT_RT=y doesn't imply CONFIG_PREEMPT, and
> CONFIG_PREEMPT_RT=y is mutually exclusive with CONFIG_PREEMPT_DYNAMIC.  I.e. a
> CONFIG_PREEMPT_RT=y kernel will look yield:
> 
>   CONFIG_PREEMPT_RT=y
>   CONFIG_PREEMPT_DYNAMIC=n
>   CONFIG_PREEMPT=n
> 
> which in turn generates:
> 
>   static inline bool preempt_model_full(void)
>   {
> 	return IS_ENABLED(CONFIG_PREEMPT);
>   }
> 
> and so just checking preempt_model_full() would incorrectly return false for
> CONFIG_PREEMPT_RT=y.

You are right,  I missunderstood the definition of preempt_model_full(). For my
understanding of this patch:

Reviewed-by: Chen Yu <yu.c.chen@...el.com>

thanks,
Chenyu