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

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Date:   Mon, 20 Feb 2023 11:53:05 +0100
From:   Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To:     "Chang, Junxiao" <junxiao.chang@...el.com>
Cc:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-rt-users@...r.kernel.org" <linux-rt-users@...r.kernel.org>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
        "rostedt@...dmis.org" <rostedt@...dmis.org>,
        "Peh, Hock Zhang" <hock.zhang.peh@...el.com>
Subject: Re: RE: [PATCH] softirq: wake up ktimer thread in softirq context

------->8------

From: Junxiao Chang <junxiao.chang@...el.com>
Date: Mon, 20 Feb 2023 09:12:20 +0100
Subject: [PATCH] softirq: Wake ktimers thread also in softirq.

If the hrtimer is raised while a softirq is processed then it does not
wake the corresponding ktimers thread. This is due to the optimisation in the
irq-exit path which is also used to wake the ktimers thread. For the other
softirqs, this is okay because the additional softirq bits will be handled by
the currently running softirq handler.
The timer related softirq bits are added to a different variable and rely on
the ktimers thread.
As a consuequence the wake up of ktimersd is delayed until the next timer tick.

Always wake the ktimers thread if a timer related softirq is pending.

Reported-by: Peh, Hock Zhang <hock.zhang.peh@...el.com>
Signed-off-by: Junxiao Chang <junxiao.chang@...el.com>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
---
 kernel/softirq.c |   11 +++++------
 1 file changed, 5 insertions(+), 6 deletions(-)

--- 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();
 }