linux-kernel - Re: [PATCH V6 1/1] Softirq:avoid large sched delay from the pending softirqs

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Date:   Mon, 14 Sep 2020 16:14:37 +0200
From:   peterz@...radead.org
To:     Qais Yousef <qais.yousef@....com>
Cc:     qianjun.kernel@...il.com, tglx@...utronix.de, will@...nel.org,
        luto@...nel.org, linux-kernel@...r.kernel.org,
        laoar.shao@...il.com, urezki@...il.com,
        John Dias <joaodias@...gle.com>, Wei Wang <wvw@...gle.com>,
        Quentin Perret <qperret@...gle.com>
Subject: Re: [PATCH V6 1/1] Softirq:avoid large sched delay from the pending
 softirqs

On Mon, Sep 14, 2020 at 12:27:35PM +0100, Qais Yousef wrote:
> What does PREEMPT_RT do to deal with softirqs delays?

Makes the lot preemptible, you found the patch below.

> I have tried playing with enabling threadirqs, which AFAIU should make softirqs
> preemptible, right?

Not yet,..

> I realize this patch is still missing from mainline at least:
> 
> 	https://gitlab.com/kalilinux/packages/linux/blob/a17bad0db9da44cd73f594794a58cc5646393b13/debian/patches-rt/softirq-Add-preemptible-softirq.patch
> 
> Would this be a heavy handed approach to make available for non PREEMPT_RT
> kernels?

Not sure, I suspect it relies on migrate_disable(), which is
preempt_disable() on !RT and then we're back to square one.

> I only worry about potential NET_RX throughput issues. Which by the way is
> protected with preempt_disable currently in mainline. See netif_rx_ni().

So preempt_disable() isn't necessairily a problem, you just want it to
terminate soonish after need_resched() becomes true. Also, I'm having a
wee problem getting from net_rx_action() to netif_rx_ni()

> I am guessing here, but I suspect this NET_RX softirq is one source of big
> delays when network activity is high.

Well, one approach is to more agressively limit how long softirq
processing can run. Current measures are very soft in that regard.