linux-kernel - Re: [PATCH] softirq: wake up ktimer thread in softirq context

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Date:   Fri, 17 Feb 2023 18:30:23 +0100
From:   Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To:     Junxiao Chang <junxiao.chang@...el.com>
Cc:     linux-kernel@...r.kernel.org, linux-rt-users@...r.kernel.org,
        tglx@...utronix.de, rostedt@...dmis.org, hock.zhang.peh@...el.com
Subject: Re: [PATCH] softirq: wake up ktimer thread in softirq context

On 2022-12-08 15:56:04 [+0800], Junxiao Chang wrote:
> Occiaionally timer interrupt might be triggered in softirq context,
> ktimer thread should be woken up with RT kernel, or else ktimer
> thread might stay in sleep state although timer interrupt has been
> triggered.
> 
> This change fixes a latency issue that timer handler is delayed for
> more than 4ms in network related test.

Sorry for keeping you waiting. Your observation and patch is correct. I'm
going to apply a slightly modified version of the patch (see below)
after I reworded the commit message on Monday.

diff --git a/kernel/softirq.c b/kernel/softirq.c
index ab1fe34326bab..82f3e68fbe220 100644
--- a/kernel/softirq.c
+++ b/kernel/softirq.c
@@ -664,13 +664,12 @@ static inline void __irq_exit_rcu(void)
 #endif
 	account_hardirq_exit(current);
 	preempt_count_sub(HARDIRQ_OFFSET);
-	if (!in_interrupt()) {
-		if (local_softirq_pending())
-			invoke_softirq();
+	if (!in_interrupt() && local_softirq_pending())
+		invoke_softirq();
 
-		if (IS_ENABLED(CONFIG_PREEMPT_RT) && local_pending_timers())
-			wake_timersd();
-	}
+	if (IS_ENABLED(CONFIG_PREEMPT_RT) && local_pending_timers() &&
+	    !(in_nmi() | in_hardirq()))
+		wake_timersd();
 
 	tick_irq_exit();
 }

Sebastian