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Message-ID: <1456759643.648.65.camel@edumazet-ThinkPad-T530>
Subject: Re: Softirq priority inversion from "softirq: reduce latencies"
From: Eric Dumazet <eric.dumazet@gmail.com>
To: Peter Hurley <peter@hurleysoftware.com>
Cc: Mike Galbraith <umgwanakikbuti@gmail.com>,
        Francois Romieu <romieu@fr.zoreil.com>,
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        John Ogness <john.ogness@linutronix.de>,
        Sebastian Andrzej Siewior <bigeasy@linutronix.de>,
        Andrew Morton <akpm@linux-foundation.org>,
        Thomas Gleixner <tglx@linutronix.de>
Date: Mon, 29 Feb 2016 07:27:23 -0800
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On lun., 2016-02-29 at 07:03 -0800, Peter Hurley wrote:

> The reason why Eric's change is so effective for Eric's workload is
> that it fixes the problem where NET_RX keeps getting new network packets
> so it keeps looping, servicing more NET_RX softirq.

You have very little idea of what is happening in networking land.

Once hard irq for RX has triggered, we arm a NAPI (NET_RX softirq), and
no more irq will come unless the napi handler ran. Then when NAPI is
complete, we re-allow interrupt to be delivered when a new packet is
coming.

Yes, ksoftirqd runs under load, and this is _wanted_.

Sure, it might add a latency if some high prio task is wanting the same
cpu, but this is exactly the purpose of having multi tasking.