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Subject: Re: Softirq priority inversion from "softirq: reduce latencies"
To: Eric Dumazet <eric.dumazet@gmail.com>
References: <56D1E8B6.6090003@hurleysoftware.com>
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Cc: Mike Galbraith <umgwanakikbuti@gmail.com>,
        Francois Romieu <romieu@fr.zoreil.com>,
        Eric Dumazet <edumazet@google.com>, David Miller <davem@davemloft.net>,
        netdev@vger.kernel.org, linux-kernel@vger.kernel.org,
        Greg KH <gregkh@linuxfoundation.org>, dmaengine@vger.kernel.org,
        John Ogness <john.ogness@linutronix.de>,
        Sebastian Andrzej Siewior <bigeasy@linutronix.de>,
        Andrew Morton <akpm@linux-foundation.org>,
        Thomas Gleixner <tglx@linutronix.de>
From: Peter Hurley <peter@hurleysoftware.com>
Message-ID: <56D48869.1000106@hurleysoftware.com>
Date: Mon, 29 Feb 2016 10:05:29 -0800
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On 02/29/2016 08:21 AM, Eric Dumazet wrote:
> On lun., 2016-02-29 at 07:54 -0800, Peter Hurley wrote:
> 
>>  The current kernel is HZ=250 but this would occur on HZ=1000 as well.
> 
> Right. But the problem with HZ=100 and HZ=250 is that the detection can
> happens because jiffy granularity is too coarse, since 
> 
> msecs_to_jiffies(2) -> 1
> 
> Following patch might reduce the probability, but wont really fix your
> problem.
> 
> Fact that ksoftirqd prio is not what you want is completely orthogonal.
> 
> diff --git a/kernel/softirq.c b/kernel/softirq.c
> index 479e443..f7cc594 100644
> --- a/kernel/softirq.c
> +++ b/kernel/softirq.c
> @@ -180,7 +180,7 @@ EXPORT_SYMBOL(__local_bh_enable_ip);
>  
>  /*
>   * We restart softirq processing for at most MAX_SOFTIRQ_RESTART times,
> - * but break the loop if need_resched() is set or after 2 ms.
> + * but break the loop if need_resched() is set or after 2 ms/ticks.
>   * The MAX_SOFTIRQ_TIME provides a nice upper bound in most cases, but in
>   * certain cases, such as stop_machine(), jiffies may cease to
>   * increment and so we need the MAX_SOFTIRQ_RESTART limit as
> @@ -191,7 +191,7 @@ EXPORT_SYMBOL(__local_bh_enable_ip);
>   * we want to handle softirqs as soon as possible, but they
>   * should not be able to lock up the box.
>   */
> -#define MAX_SOFTIRQ_TIME  msecs_to_jiffies(2)
> +#define MAX_SOFTIRQ_TIME  (1 + msecs_to_jiffies(2))
>  #define MAX_SOFTIRQ_RESTART 10
>  
>  #ifdef CONFIG_TRACE_IRQFLAGS

While I appreciate the attempt, that's not the problem.

Just to be clear

		if (time_before(jiffies, end) && !need_resched() &&
		    --max_restart)
			goto restart;

aborts softirq *even if 0ns have elapsed*, if NET_RX has woken a process.