RE: [PATCH] softirq: wake up ktimer thread in softirq context

["Chang, Junxiao" <junxiao.chang@xxxxxxxxx>]

Any comment? This patch is for 6.1-rt, issue could be reproduced with 5.19-rt kernel as well.

This issue is easier to reproduced when there is heavy network workload which introduces a lot of softirq events. If hrtimer interrupt is triggered in softirq context, with current RT kernel, it will not wake up ktimers thread which handles hrtimer event because in function __irq_exit_rcu, "in_interrupt()" is true:

static inline void __irq_exit_rcu(void)
{
...
        preempt_count_sub(HARDIRQ_OFFSET);
        if (!in_interrupt()) {
                if (local_softirq_pending())
                        invoke_softirq();

                if (IS_ENABLED(CONFIG_PREEMPT_RT) && local_pending_timers())
                        wake_timersd();
        }
...
}

Then ktimers threads stays in sleep state, hrtimer function will not be called although hrtimer interrupt has been triggered. Ktimers thread might be woken up in next timer interrupt which introduces long delay.

Any comments are welcome.

Regards,
Junxiao

-----Original Message-----
From: Chang, Junxiao <junxiao.chang@xxxxxxxxx> 
Sent: Thursday, December 8, 2022 3:56 PM
To: linux-kernel@xxxxxxxxxxxxxxx
Cc: linux-rt-users@xxxxxxxxxxxxxxx; bigeasy@xxxxxxxxxxxxx; tglx@xxxxxxxxxxxxx; rostedt@xxxxxxxxxxx; Chang, Junxiao <junxiao.chang@xxxxxxxxx>; Peh, Hock Zhang <hock.zhang.peh@xxxxxxxxx>
Subject: [PATCH] softirq: wake up ktimer thread in softirq context

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.

Fixes: 2165d27554e8 ("softirq: Use a dedicated thread for timer wakeups.")
Reported-by: Peh, Hock Zhang <hock.zhang.peh@xxxxxxxxx>
Signed-off-by: Junxiao Chang <junxiao.chang@xxxxxxxxx>
---
 kernel/softirq.c | 11 ++++-------
 1 file changed, 4 insertions(+), 7 deletions(-)

diff --git a/kernel/softirq.c b/kernel/softirq.c index ab1fe34326bab..34ae39e4a3d10 100644
--- a/kernel/softirq.c
+++ b/kernel/softirq.c
@@ -664,13 +664,10 @@ 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 (IS_ENABLED(CONFIG_PREEMPT_RT) && local_pending_timers())
-			wake_timersd();
-	}
+	if (!in_interrupt() && local_softirq_pending())
+		invoke_softirq();
+	if (!(in_nmi() || in_hardirq()) && local_pending_timers())
+		wake_timersd();
 
 	tick_irq_exit();
 }
--
2.25.1