linux-kernel - Re: [RFC PATCH 2/4] softirq: Per vector deferment to workqueue

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Date:   Fri, 09 Feb 2018 05:11:08 +0100
From:   Mike Galbraith <efault@....de>
To:     Dmitry Safonov <dima@...sta.com>,
        David Miller <davem@...emloft.net>
Cc:     bigeasy@...utronix.de, frederic@...nel.org,
        linux-kernel@...r.kernel.org, alexander.levin@...izon.com,
        peterz@...radead.org, mchehab@...pensource.com,
        torvalds@...ux-foundation.org, hannes@...essinduktion.org,
        paulmck@...ux.vnet.ibm.com, wanpeng.li@...mail.com,
        tglx@...utronix.de, akpm@...ux-foundation.org, pabeni@...hat.com,
        rrendec@...sta.com, mingo@...nel.org, sgruszka@...hat.com,
        riel@...hat.com, edumazet@...gle.com
Subject: Re: [RFC PATCH 2/4] softirq: Per vector deferment to workqueue

On Thu, 2018-02-08 at 20:30 +0000, Dmitry Safonov wrote:
> On Thu, 2018-02-08 at 15:22 -0500, David Miller wrote:
> > From: Dmitry Safonov <dima@...sta.com>
> > Date: Thu, 08 Feb 2018 20:14:55 +0000
> > 
> > > On Thu, 2018-02-08 at 13:45 -0500, David Miller wrote:
> > >> From: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
> > >> Date: Thu, 8 Feb 2018 18:44:52 +0100
> > >> 
> > >> > May I instead suggest to stick to ksoftirqd? So you run in
> > softirq
> > >> > context (after return from IRQ) and if takes too long, you
> > offload
> > >> the
> > >> > vector to ksoftirqd instead. You may want to play with the
> > metric
> > >> on
> > >> > which you decide when you want switch to ksoftirqd / account how
> > >> long a
> > >> > vector runs.
> > >> 
> > >> Having read over this stuff for the past few weeks this is how I
> > feel
> > >> as well.  Just make ksofbitrq do what we want (only execute the
> > >> overloaded softirq vectors).
> > >> 
> > >> The more I look at the workqueue stuff, the more complications and
> > >> weird behavioral artifacts we are getting for questionable gain.
> > > 
> > > What about creating several ksoftirqd threads per-cpu?
> > > Like I did with boot parameter to specify how many threads and
> > which
> > > softirqs to serve.
> > 
> > Why do we need more than one per cpu?
> 
> Ugh, yeah, I remember why I did it - I tried to reuse scheduler for
> each ksoftirqd thread to decide if it need to run now or later.
> That would give an admin a way to prioritise softirqs with nice.
> Not sure if it's a nice idea at all..

For RT that can be handy, but for the general case it's a waste of
cycles, so would want to be opt-in.

	-Mike