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

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Date:   Fri, 24 Aug 2018 20:41:04 +0200
From:   Frederic Weisbecker <frederic@...nel.org>
To:     Grygorii Strashko <grygorii.strashko@...com>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Greg KH <gregkh@...uxfoundation.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 Fri, Aug 24, 2018 at 11:10:44AM -0500, Grygorii Strashko wrote:
> 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.

That's correct and it's indeed more costly in 4.14 as then the tick is going to be programmed
twice.