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

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Message-ID: <1518121820.2849.17.camel@arista.com>
Date:   Thu, 08 Feb 2018 20:30:20 +0000
From:   Dmitry Safonov <dima@...sta.com>
To:     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 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..

> 
> There is a set of vectors which are "overloaded" and ksoftirqd
> processes
> them one by one.
> 
> The only difference with what happens now is that one softirq being
> overloaded doesn't defer the processing of all softirqs to ksoftirqd.