linux-kernel - Re: [RFC PATCH 0/4] softirq: Per vector threading v3

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Date:   Tue, 23 Jan 2018 09:42:32 -0800
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Paolo Abeni <pabeni@...hat.com>
Cc:     David Miller <davem@...emloft.net>,
        Frederic Weisbecker <frederic@...nel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Sasha Levin <alexander.levin@...izon.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Mauro Carvalho Chehab <mchehab@...pensource.com>,
        Hannes Frederic Sowa <hannes@...essinduktion.org>,
        Paul McKenney <paulmck@...ux.vnet.ibm.com>,
        Wanpeng Li <wanpeng.li@...mail.com>,
        Dmitry Safonov <dima@...sta.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Radu Rendec <rrendec@...sta.com>,
        Ingo Molnar <mingo@...nel.org>,
        Stanislaw Gruszka <sgruszka@...hat.com>,
        Rik van Riel <riel@...hat.com>,
        Eric Dumazet <edumazet@...gle.com>
Subject: Re: [RFC PATCH 0/4] softirq: Per vector threading v3

On Tue, Jan 23, 2018 at 8:57 AM, Paolo Abeni <pabeni@...hat.com> wrote:
>
>> Or is it that the workqueue execution is simply not yielding for some
>> reason?
>
> It's like that.
>
> I spent little time on it, so I haven't many data point. I'll try to
> investigate the scenario later this week.

Hmm. workqueues seem to use cond_resched_rcu_qs(), which does a
cond_resched() (and a RCU quiescent note).

But I wonder if the test triggers the "lets run lots of workqueue
threads", and then the single-threaded user space just gets blown out
of the water by many kernel threads. Each thread gets its own "fair"
amount of CPU, but..

                   Linus