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

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Message-ID: <20200914112735.6ivyy2iqlyqatzn7@e107158-lin.cambridge.arm.com>
Date:   Mon, 14 Sep 2020 12:27:35 +0100
From:   Qais Yousef <qais.yousef@....com>
To:     peterz@...radead.org
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 09/11/20 20:28, peterz@...radead.org wrote:
> On Fri, Sep 11, 2020 at 05:46:45PM +0100, Qais Yousef wrote:
> > On 09/09/20 17:09, qianjun.kernel@...il.com wrote:
> > > From: jun qian <qianjun.kernel@...il.com>
> > > 
> > > When get the pending softirqs, it need to process all the pending
> > > softirqs in the while loop. If the processing time of each pending
> > > softirq is need more than 2 msec in this loop, or one of the softirq
> > > will running a long time, according to the original code logic, it
> > > will process all the pending softirqs without wakeuping ksoftirqd,
> > > which will cause a relatively large scheduling delay on the
> > > corresponding CPU, which we do not wish to see. The patch will check
> > > the total time to process pending softirq, if the time exceeds 2 ms
> > > we need to wakeup the ksofirqd to aviod large sched delay.
> > > 
> > > Signed-off-by: jun qian <qianjun.kernel@...il.com>
> > 
> > In Android there's a patch that tries to avoid schedling an RT task on a cpu
> > that is running softirqs. I wonder if this patch helps with this case.
> > 
> > https://android.googlesource.com/kernel/msm/+/5c3f54c34acf4d9ed01530288d4a98acff815d79%5E%21/#F0
> > 
> > John, Wei, is this something of interest to you?
> 
> Urgh.. that's pretty gross. I think the sane approach is indeed getting
> softirqs to react to need_resched() better.

What does PREEMPT_RT do to deal with softirqs delays?

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

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?

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().

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

Thanks

--
Qais Yousef