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

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Date:   Fri, 24 Aug 2018 11:10:44 -0500
From:   Grygorii Strashko <grygorii.strashko@...com>
To:     Thomas Gleixner <tglx@...utronix.de>,
        Greg KH <gregkh@...uxfoundation.org>
CC:     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>, Tero Kristo <t-kristo@...com>
Subject: Re: [PATCH] nohz: Fix missing tick reprog while interrupting inline
 timer softirq



On 08/24/2018 02:01 AM, Thomas Gleixner wrote:
> 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?

yes. 

ff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c
index da74d2f..a5fad1c 100644
--- a/kernel/time/tick-sched.c
+++ b/kernel/time/tick-sched.c
@@ -910,8 +910,9 @@ static bool can_stop_idle_tick(int cpu, struct tick_sched *ts)
 
                if (ratelimit < 100 &&
                    (local_softirq_pending() & SOFTIRQ_STOP_IDLE_MASK)) {
-                       pr_warn("NOHZ: local_softirq_pending %02x in sirq %d\n",
+                       pr_warn("NOHZ: local_softirq_pending %02x in sirq %lu\n",
                                (unsigned int) local_softirq_pending(), in_softirq());
+                       WARN_ON_ONCE(true);
                        ratelimit++;
                }


> 
>>> 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());
> 
> 

Yes. i do not see local_softirq_pending messages any more

But one question, just to clarify, after patch "nohz: Fix missing tick reprog while interrupting inline timer softirq"
the tick_nohz_irq_exit() will be called few times in case of nested interrupts (min 2):
gic_handle_irq
 |- irq_exit
    |- preempt_count_sub(HARDIRQ_OFFSET); 
    |-__do_softirq 
	<irqs enabled>
	|- gic_handle_irq()
	   |- irq_exit()
		|- tick_irq_exit()
		   if (!in_irq())
			tick_nohz_irq_exit(); <-- [1]
    |- tick_irq_exit()
	if (!in_irq())
		tick_nohz_irq_exit(); <-- [2]

Is it correct? in 4.14 tick_nohz_irq_exit() is much more complex then in LKML now,
and this is hot path.


-- 
regards,
-grygorii