linux-kernel - [PATCH 08/30] entry: irqentry_exit only preempts for TIF_NEED

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Date: Mon, 12 Feb 2024 21:55:32 -0800
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, akpm@...ux-foundation.org, luto@...nel.org,
        bp@...en8.de, dave.hansen@...ux.intel.com, hpa@...or.com,
        mingo@...hat.com, juri.lelli@...hat.com, vincent.guittot@...aro.org,
        willy@...radead.org, mgorman@...e.de, jpoimboe@...nel.org,
        mark.rutland@....com, jgross@...e.com, andrew.cooper3@...rix.com,
        bristot@...nel.org, mathieu.desnoyers@...icios.com,
        geert@...ux-m68k.org, glaubitz@...sik.fu-berlin.de,
        anton.ivanov@...bridgegreys.com, mattst88@...il.com,
        krypton@...ich-teichert.org, rostedt@...dmis.org,
        David.Laight@...LAB.COM, richard@....at, mjguzik@...il.com,
        jon.grimm@....com, bharata@....com, raghavendra.kt@....com,
        boris.ostrovsky@...cle.com, konrad.wilk@...cle.com,
        Ankur Arora <ankur.a.arora@...cle.com>
Subject: [PATCH 08/30] entry: irqentry_exit only preempts for TIF_NEED_RESCHED

Use tif_need_resched(NR_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 already has 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 | 10 +++++++++-
 1 file changed, 9 insertions(+), 1 deletion(-)

diff --git a/kernel/entry/common.c b/kernel/entry/common.c
index 5c5befb05410..e1e6d7c92ca2 100644
--- a/kernel/entry/common.c
+++ b/kernel/entry/common.c
@@ -301,7 +301,15 @@ 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 archs don't
+		 * fold TIF_NEED_RESCHED in the preempt_count. For archs that
+		 * do, this is already covered in the conditinal above.
+		 */
+		if (tif_need_resched(NR_now))
 			preempt_schedule_irq();
 	}
 }
-- 
2.31.1