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

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Message-Id: <20231107215742.363031-37-ankur.a.arora@oracle.com>
Date:   Tue,  7 Nov 2023 13:57:22 -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,
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        Ankur Arora <ankur.a.arora@...cle.com>
Subject: [RFC PATCH 36/86] entry: irqentry_exit only preempts TIF_NEED_RESCHED

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();
 	}
 }
-- 
2.31.1