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

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Date: Mon, 27 May 2024 17:34:56 -0700
From: Ankur Arora <ankur.a.arora@...cle.com>
To: linux-kernel@...r.kernel.org
Cc: tglx@...utronix.de, peterz@...radead.org, 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, Ankur Arora <ankur.a.arora@...cle.com>,
        Andy Lutomirski <luto@...nel.org>
Subject: [PATCH v2 10/35] entry: irqentry_exit only preempts for TIF_NEED_RESCHED

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();
 	}
 }
-- 
2.31.1