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

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Date:   Fri, 11 Sep 2020 20:28:32 +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 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.