Messages in this thread | | | Date | Mon, 14 Sep 2020 12:27:35 +0100 | From | Qais Yousef <> | Subject | Re: [PATCH V6 1/1] Softirq:avoid large sched delay from the pending softirqs |
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On 09/11/20 20:28, peterz@infradead.org wrote: > On Fri, Sep 11, 2020 at 05:46:45PM +0100, Qais Yousef wrote: > > On 09/09/20 17:09, qianjun.kernel@gmail.com wrote: > > > From: jun qian <qianjun.kernel@gmail.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@gmail.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
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