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Date:   Fri, 7 Apr 2023 12:23:23 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Ankur Arora <ankur.a.arora@oracle.com>
Cc:     linux-kernel@vger.kernel.org, linux-mm@kvack.org, x86@kernel.org,
        torvalds@linux-foundation.org, akpm@linux-foundation.org,
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        tglx@linutronix.de, vincent.guittot@linaro.org, jon.grimm@amd.com,
        bharata@amd.com, boris.ostrovsky@oracle.com, konrad.wilk@oracle.com
Subject: Re: [PATCH 8/9] irqentry: define irqentry_exit_allow_resched()
Message-ID: <20230407102323.GA430894@hirez.programming.kicks-ass.net>
References: <20230403052233.1880567-1-ankur.a.arora@oracle.com>
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 <87o7o1lzko.fsf@oracle.com>
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 <87lej4ifvc.fsf@oracle.com>
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On Thu, Apr 06, 2023 at 07:29:59PM -0700, Ankur Arora wrote:
> Peter Zijlstra <peterz@infradead.org> writes:

> > Something like the below perhaps?
> >
> > ---
> >  include/linux/entry-common.h |  6 ++++++
> >  kernel/entry/common.c        | 23 +++++++++++++++++++++--
> >  2 files changed, 27 insertions(+), 2 deletions(-)
> >
> > diff --git a/include/linux/entry-common.h b/include/linux/entry-common.h
> > index d95ab85f96ba..0c365dc1f1c2 100644
> > --- a/include/linux/entry-common.h
> > +++ b/include/linux/entry-common.h
> > @@ -415,10 +415,16 @@ irqentry_state_t noinstr irqentry_enter(struct pt_regs *regs);
> >   * Conditional reschedule with additional sanity checks.
> >   */
> >  void raw_irqentry_exit_cond_resched(void);
> > +void irqentry_exit_cond_resched_tif(void);
> > +
> >  #ifdef CONFIG_PREEMPT_DYNAMIC
> >  #if defined(CONFIG_HAVE_PREEMPT_DYNAMIC_CALL)
> >  #define irqentry_exit_cond_resched_dynamic_enabled	raw_irqentry_exit_cond_resched
> > +#ifdef TIF_RESCHED_ALLOW
> > +#define irqentry_exit_cond_resched_dynamic_disabled	irqentry_exit_cond_resched_tif
> > +#else
> >  #define irqentry_exit_cond_resched_dynamic_disabled	NULL
> > +#endif
> 
> So this is clever. Essentially this would toggle between the two kinds
> for the preempt_model_preemptible()/!preempt_model_preemptible() dynamic
> case. Do I have that right?

Exactly!

> >  DECLARE_STATIC_CALL(irqentry_exit_cond_resched, raw_irqentry_exit_cond_resched);
> >  #define irqentry_exit_cond_resched()	static_call(irqentry_exit_cond_resched)()
> >  #elif defined(CONFIG_HAVE_PREEMPT_DYNAMIC_KEY)
> > diff --git a/kernel/entry/common.c b/kernel/entry/common.c
> > index be61332c66b5..211d118aa672 100644
> > --- a/kernel/entry/common.c
> > +++ b/kernel/entry/common.c
> > @@ -390,6 +390,21 @@ void raw_irqentry_exit_cond_resched(void)
> >  			preempt_schedule_irq();
> >  	}
> >  }
> > +
> > +void irqentry_exit_cond_resched_tif(void)
> > +{
> > +#ifdef TIF_RESCHED_ALLOW
> > +	if (resched_allowed()) {
> > +		/* Sanity check RCU and thread stack */
> > +		rcu_irq_exit_check_preempt();
> > +		if (IS_ENABLED(CONFIG_DEBUG_ENTRY))
> > +			WARN_ON_ONCE(!on_thread_stack());
> > +		if (need_resched())
> > +			preempt_schedule_irq();
> > +	}
> > +#endif
> > +}
> > +
> >  #ifdef CONFIG_PREEMPT_DYNAMIC
> >  #if defined(CONFIG_HAVE_PREEMPT_DYNAMIC_CALL)
> >  DEFINE_STATIC_CALL(irqentry_exit_cond_resched, raw_irqentry_exit_cond_resched);
> > @@ -397,8 +412,10 @@ DEFINE_STATIC_CALL(irqentry_exit_cond_resched, raw_irqentry_exit_cond_resched);
> >  DEFINE_STATIC_KEY_TRUE(sk_dynamic_irqentry_exit_cond_resched);
> >  void dynamic_irqentry_exit_cond_resched(void)
> >  {
> > -	if (!static_branch_unlikely(&sk_dynamic_irqentry_exit_cond_resched))
> > -		return;
> > +	if (!static_branch_unlikely(&sk_dynamic_irqentry_exit_cond_resched)) {
> > +		irqentry_exit_cond_resched_tif();
> > +		return
> > +	}
> >  	raw_irqentry_exit_cond_resched();
> >  }
> >  #endif
> > @@ -431,6 +448,8 @@ noinstr void irqentry_exit(struct pt_regs *regs, irqentry_state_t state)
> >  		instrumentation_begin();
> >  		if (IS_ENABLED(CONFIG_PREEMPTION))
> >  			irqentry_exit_cond_resched();
> > +		else
> > +			irqentry_exit_cond_resched_tif();
> 
> And, if we choose between the two resched modes at compile time then this
> would work.

Just so.

> Might massage the names a little but this should work as is.

Yeah, I'm not liking the naming either, perhaps your
irqentry_exit_allow_resched() would've been better, dunno, see what
works.

> Okay if I use your Codeveloped-by/Suggested-by on this patch?

Yep.