linux-kernel - Re: [RFC PATCH 36/86] entry: irqentry_exit only preempts TIF_NEED

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Date:   Wed, 8 Nov 2023 10:01:03 +0100
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,
        linux-mm@...ck.org, x86@...nel.org, akpm@...ux-foundation.org,
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        vincent.guittot@...aro.org, willy@...radead.org, mgorman@...e.de,
        jon.grimm@....com, bharata@....com, raghavendra.kt@....com,
        boris.ostrovsky@...cle.com, konrad.wilk@...cle.com,
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        David.Laight@...lab.com, richard@....at, mjguzik@...il.com
Subject: Re: [RFC PATCH 36/86] entry: irqentry_exit only preempts
 TIF_NEED_RESCHED

On Tue, Nov 07, 2023 at 01:57:22PM -0800, Ankur Arora wrote:
> The scheduling policy for RESCHED_lazy (TIF_NEED_RESCHED_LAZY) is
> to let anything running in the kernel run to completion.
> Accordingly, while deciding whether to call preempt_schedule_irq()
> narrow the check to tif_need_resched(RESCHED_eager).
> 
> Also add a comment about why we need to check at all, given that we
> have aleady checked the preempt_count().
> 
> Signed-off-by: Ankur Arora <ankur.a.arora@...cle.com>
> ---
>  kernel/entry/common.c | 10 +++++++++-
>  1 file changed, 9 insertions(+), 1 deletion(-)
> 
> diff --git a/kernel/entry/common.c b/kernel/entry/common.c
> index 0d055c39690b..6433e6c77185 100644
> --- a/kernel/entry/common.c
> +++ b/kernel/entry/common.c
> @@ -384,7 +384,15 @@ void 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())
> +
> +		/*
> +		 * If the scheduler really wants us to preempt while returning
> +		 * to kernel, it would set TIF_NEED_RESCHED.
> +		 * On some archs the flag gets folded in preempt_count, and
> +		 * thus would be covered in the conditional above, but not all
> +		 * archs do that, so check explicitly.
> +		 */
> +		if (tif_need_resched(RESCHED_eager))
>  			preempt_schedule_irq();

See, I'm reading this like if we're eager to preempt, but then it's not
actually eager at all and only wants to preempt when forced.

This naming sucks...