linux-kernel - Re: [PATCH] nohz: Fix missing tick reprog while interrupting inline timer softirq

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Date:   Fri, 24 Aug 2018 09:01:02 +0200 (CEST)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Greg KH <gregkh@...uxfoundation.org>
cc:     Grygorii Strashko <grygorii.strashko@...com>,
        Frederic Weisbecker <frederic@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Ingo Molnar <mingo@...nel.org>,
        Anna-Maria Gleixner <anna-maria@...utronix.de>,
        stable@...r.kernel.org
Subject: Re: [PATCH] nohz: Fix missing tick reprog while interrupting inline
 timer softirq

On Fri, 24 Aug 2018, Greg KH wrote:
> On Thu, Aug 23, 2018 at 05:57:06PM -0500, Grygorii Strashko wrote:
> > This patch was back ported to the Stable linux-4.14.y and It causes regression -
> >  flood of "NOHZ: local_softirq_pending" messages on all TI boards during boot (NFS boot):
> > 
> > [    4.179796] NOHZ: local_softirq_pending 2c2 in sirq 256
> > [    4.185051] NOHZ: local_softirq_pending 2c2 in sirq 256

This printout is weird. Did you add something here?

> > the same is not reproducible with LKML - seems due to changes in tick-sched.c 
> > __tick_nohz_idle_enter()/tick_nohz_irq_exit().
> 
> What changes do you think fixed this?
> 
> > I've generated backtrace from  can_stop_idle_tick() (see below) and seems this
> > patch makes tick_nohz_irq_exit() call unconditional in case of nested interrupt:
> > 
> > gic_handle_irq
> >  |- irq_exit
> >     |- preempt_count_sub(HARDIRQ_OFFSET); <-- [1]
> >     |-__do_softirq 
> > 	<irqs enabled>
> > 	|- gic_handle_irq()
> > 	   |- irq_exit()
> > 		|- tick_irq_exit()
> > 		   if (!in_irq()) <-- My understanding is that this condition will be always true due to [1]

Correct, but that's not the problem. The issue is that this happens in a
softirq disabled region. Does the below fix it?

Thanks,

	tglx

8<--------------------
diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c
index 5b33e2f5c0ed..6aab9d54a331 100644
--- a/kernel/time/tick-sched.c
+++ b/kernel/time/tick-sched.c
@@ -888,7 +888,7 @@ static bool can_stop_idle_tick(int cpu, struct tick_sched *ts)
 	if (unlikely(local_softirq_pending() && cpu_online(cpu))) {
 		static int ratelimit;
 
-		if (ratelimit < 10 &&
+		if (ratelimit < 10 && !in_softirq() &&
 		    (local_softirq_pending() & SOFTIRQ_STOP_IDLE_MASK)) {
 			pr_warn("NOHZ: local_softirq_pending %02x\n",
 				(unsigned int) local_softirq_pending());