linux-kernel - Re: [PATCH -next v7 5/7] arm64: entry: Refactor preempt_schedule

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Message-ID: <4242ef37-efeb-446f-abbc-0104b805ad12@arm.com>
Date: Mon, 11 Aug 2025 17:02:42 +0100
From: Ada Couprie Diaz <ada.coupriediaz@....com>
To: Jinjie Ruan <ruanjinjie@...wei.com>
Cc: catalin.marinas@....com, will@...nel.org, oleg@...hat.com,
 sstabellini@...nel.org, mark.rutland@....com, puranjay@...nel.org,
 broonie@...nel.org, mbenes@...e.cz, ryan.roberts@....com,
 akpm@...ux-foundation.org, chenl311@...natelecom.cn,
 anshuman.khandual@....com, kristina.martsenko@....com,
 liaochang1@...wei.com, ardb@...nel.org, leitao@...ian.org,
 linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
 xen-devel@...ts.xenproject.org, Ada Couprie Diaz <ada.coupriediaz@....com>
Subject: Re: [PATCH -next v7 5/7] arm64: entry: Refactor
 preempt_schedule_irq() check code

On 29/07/2025 02:54, Jinjie Ruan wrote:

> ARM64 requires an additional check whether to reschedule on return
> from interrupt. So add arch_irqentry_exit_need_resched() as the default
> NOP implementation and hook it up into the need_resched() condition in
> raw_irqentry_exit_cond_resched(). This allows ARM64 to implement
> the architecture specific version for switching over to
> the generic entry code.
>
> To align the structure of the code with irqentry_exit_cond_resched()
> from the generic entry code, hoist the need_irq_preemption()
> and IS_ENABLED() check earlier. And different preemption check functions
> are defined based on whether dynamic preemption is enabled.
>
> Suggested-by: Mark Rutland <mark.rutland@....com>
> Suggested-by: Kevin Brodsky <kevin.brodsky@....com>
> Suggested-by: Thomas Gleixner <tglx@...utronix.de>
> Signed-off-by: Jinjie Ruan <ruanjinjie@...wei.com>
> ---
Unrelated to the other thread : I noticed that compiling this patch
with `allnoconfig` would fail :
- `raw_irqentry_exit_cond_resched` has no previous prototype,
   as it is defined within `#ifdef CONFIG_PREEMPTION`
- `irqentry_exit_cond_resched()` is not declared, as it is also within
   `#ifdef CONFIG_PREEMPTION`

The patch below fixes the issue, but introduces merge conflicts in
patches 6 and 7, plus the `#ifdef` needs to be moved accordingly
in patch 6 and the empty "without preemption" `irq_exit_cond_resched()`
needs to be removed in patch 7.

I hope this can be useful,
Ada

---
diff --git a/arch/arm64/include/asm/preempt.h 
b/arch/arm64/include/asm/preempt.h
index 0f0ba250efe8..d9aba8b1e466 100644
--- a/arch/arm64/include/asm/preempt.h
+++ b/arch/arm64/include/asm/preempt.h
@@ -103,6 +103,8 @@ void dynamic_irqentry_exit_cond_resched(void);
  #define irqentry_exit_cond_resched() raw_irqentry_exit_cond_resched()

  #endif /* CONFIG_PREEMPT_DYNAMIC */
+#else /* CONFIG_PREEMPTION */
+#define irqentry_exit_cond_resched() {}
  #endif /* CONFIG_PREEMPTION */

  #endif /* __ASM_PREEMPT_H */
diff --git a/arch/arm64/kernel/entry-common.c 
b/arch/arm64/kernel/entry-common.c
index 4f92664fd46c..abd7a315145e 100644
--- a/arch/arm64/kernel/entry-common.c
+++ b/arch/arm64/kernel/entry-common.c
@@ -661,6 +661,7 @@ static __always_inline void __el1_pnmi(struct 
pt_regs *regs,
         arm64_exit_nmi(regs, state);
  }

+#ifdef CONFIG_PREEMPTION
  void raw_irqentry_exit_cond_resched(void)
  {
         if (!preempt_count()) {
@@ -668,6 +669,7 @@ void raw_irqentry_exit_cond_resched(void)
                         preempt_schedule_irq();
         }
  }
+#endif

  #ifdef CONFIG_PREEMPT_DYNAMIC
  DEFINE_STATIC_KEY_TRUE(sk_dynamic_irqentry_exit_cond_resched);