linux-kernel - [PATCH 1/2] x86/entry: Use should_resched() in idtentry_exit_cond

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Date:   Tue, 30 Jun 2020 12:22:08 +0200
From:   Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To:     linux-kernel@...r.kernel.org
Cc:     Andy Lutomirski <luto@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        x86@...nel.org, "H . Peter Anvin " <hpa@...or.com>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Subject: [PATCH 1/2] x86/entry: Use should_resched() in idtentry_exit_cond_resched()

The TIF_NEED_RESCHED bit is inlined on x86 into the preemption counter.
So instead checking the preemption counter against zero via
preempt_count() and later checking the TIF_NEED_RESCHED bit via
need_resched() we could use should_resched() which does both checks in
one go.
The functional difference is that we don't enter the if statement with
preempt_count == 0 and TIF_NEED_RESCHED not set.

Use should_resched() instead need_resched() + preempt_count().

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
---
 arch/x86/entry/common.c | 5 ++---
 1 file changed, 2 insertions(+), 3 deletions(-)

diff --git a/arch/x86/entry/common.c b/arch/x86/entry/common.c
index bd3f14175193c..212382f61b8e4 100644
--- a/arch/x86/entry/common.c
+++ b/arch/x86/entry/common.c
@@ -612,13 +612,12 @@ bool noinstr idtentry_enter_cond_rcu(struct pt_regs *regs)
 
 static void idtentry_exit_cond_resched(struct pt_regs *regs, bool may_sched)
 {
-	if (may_sched && !preempt_count()) {
+	if (may_sched && should_resched(0)) {
 		/* Sanity check RCU and thread stack */
 		rcu_irq_exit_check_preempt();
 		if (IS_ENABLED(CONFIG_DEBUG_ENTRY))
 			WARN_ON_ONCE(!on_thread_stack());
-		if (need_resched())
-			preempt_schedule_irq();
+		preempt_schedule_irq();
 	}
 	/* Covers both tracing and lockdep */
 	trace_hardirqs_on();
-- 
2.27.0