linux-kernel - Re: [PATCH v2 10/35] entry: irqentry_exit only preempts for TIF_NEED

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Message-ID: <20240528161816.GF26599@noisy.programming.kicks-ass.net>
Date: Tue, 28 May 2024 18:18:16 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: Ankur Arora <ankur.a.arora@...cle.com>
Cc: linux-kernel@...r.kernel.org, tglx@...utronix.de,
	torvalds@...ux-foundation.org, paulmck@...nel.org,
	rostedt@...dmis.org, mark.rutland@....com, juri.lelli@...hat.com,
	joel@...lfernandes.org, raghavendra.kt@....com,
	sshegde@...ux.ibm.com, boris.ostrovsky@...cle.com,
	konrad.wilk@...cle.com, Andy Lutomirski <luto@...nel.org>
Subject: Re: [PATCH v2 10/35] entry: irqentry_exit only preempts for
 TIF_NEED_RESCHED

On Mon, May 27, 2024 at 05:34:56PM -0700, Ankur Arora wrote:
> Use __tif_need_resched(RESCHED_NOW) instead of need_resched() to be
> explicit that this path only reschedules if it is needed imminently.
> 
> Also, add a comment about why we need a need-resched check here at
> all, given that the top level conditional has already checked the
> preempt_count().
> 
> Cc: Peter Zijlstra <peterz@...radead.org>
> Cc: Andy Lutomirski <luto@...nel.org>
> Originally-by: Thomas Gleixner <tglx@...utronix.de>
> Link: https://lore.kernel.org/lkml/87jzshhexi.ffs@tglx/
> Signed-off-by: Ankur Arora <ankur.a.arora@...cle.com>
> ---
>  kernel/entry/common.c | 11 ++++++++++-
>  1 file changed, 10 insertions(+), 1 deletion(-)
> 
> diff --git a/kernel/entry/common.c b/kernel/entry/common.c
> index bcb23c866425..c684385921de 100644
> --- a/kernel/entry/common.c
> +++ b/kernel/entry/common.c
> @@ -307,7 +307,16 @@ void raw_irqentry_exit_cond_resched(void)
>  		rcu_irq_exit_check_preempt();
>  		if (IS_ENABLED(CONFIG_DEBUG_ENTRY))
>  			WARN_ON_ONCE(!on_thread_stack());
> -		if (need_resched())
> +
> +		/*
> +		 * Check if we need to preempt eagerly.
> +		 *
> +		 * Note: we need an explicit check here because some
> +		 * architectures don't fold TIF_NEED_RESCHED in the
> +		 * preempt_count. For archs that do, this is already covered
> +		 * in the conditional above.
> +		 */
> +		if (__tif_need_resched(RESCHED_NOW))
>  			preempt_schedule_irq();

Seeing how you introduced need_resched_lazy() and kept need_resched() to
be the NOW thing, I really don't see the point of using the long form
here?