netdev - Re: dvb usb issues since kernel 4.9

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Date:   Wed, 10 Jan 2018 04:02:48 +0100
From:   Mike Galbraith <efault@....de>
To:     Jesper Dangaard Brouer <jbrouer@...hat.com>,
        Mauro Carvalho Chehab <mchehab@...pensource.com>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Alan Stern <stern@...land.harvard.edu>,
        Ingo Molnar <mingo@...nel.org>,
        Josef Griebichler <griebichler.josef@....at>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        USB list <linux-usb@...r.kernel.org>,
        Eric Dumazet <edumazet@...gle.com>,
        Rik van Riel <riel@...hat.com>,
        Paolo Abeni <pabeni@...hat.com>,
        Hannes Frederic Sowa <hannes@...hat.com>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        netdev <netdev@...r.kernel.org>,
        Jonathan Corbet <corbet@....net>,
        LMML <linux-media@...r.kernel.org>,
        David Miller <davem@...emloft.net>
Subject: Re: dvb usb issues since kernel 4.9

On Tue, 2018-01-09 at 22:26 +0100, Jesper Dangaard Brouer wrote:
> 
> I've previously experienced that you can be affected by the scheduler
> granularity, which is adjustable (with CONFIG_SCHED_DEBUG=y):
> 
>  $ grep -H . /proc/sys/kernel/sched_*_granularity_ns
>  /proc/sys/kernel/sched_min_granularity_ns:2250000
>  /proc/sys/kernel/sched_wakeup_granularity_ns:3000000
> 
> The above numbers were confirmed on the RPi2 (see[2]). With commit
> 4cd13c21b207 ("softirq: Let ksoftirqd do its job"), I expect/assume that
> softirq processing latency is bounded by the sched_wakeup_granularity_ns,
> which with 3 ms is not good enough for their use-case.

Note of caution wrt twiddling sched_wakeup_granularity_ns: it must
remain < sched_latency_ns/2 else you effectively disable wakeup
preemption completely, turning CFS into a tick granularity scheduler.

	-Mike