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

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Message-ID: <20180124015734.GA11302@lerouge>
Date:   Wed, 24 Jan 2018 02:57:36 +0100
From:   Frederic Weisbecker <frederic@...nel.org>
To:     David Miller <davem@...emloft.net>
Cc:     torvalds@...ux-foundation.org, pabeni@...hat.com,
        linux-kernel@...r.kernel.org, alexander.levin@...izon.com,
        peterz@...radead.org, mchehab@...pensource.com,
        hannes@...essinduktion.org, paulmck@...ux.vnet.ibm.com,
        wanpeng.li@...mail.com, dima@...sta.com, tglx@...utronix.de,
        akpm@...ux-foundation.org, rrendec@...sta.com, mingo@...nel.org,
        sgruszka@...hat.com, riel@...hat.com, edumazet@...gle.com
Subject: Re: [RFC PATCH 0/4] softirq: Per vector threading v3

On Tue, Jan 23, 2018 at 01:24:24PM -0500, David Miller wrote:
> From: Linus Torvalds <torvalds@...ux-foundation.org>
> Date: Tue, 23 Jan 2018 09:42:32 -0800
> 
> > 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..
> 
> If a single cpu's softirq deferral can end up running on multiple
> workqueue threads, indeed that's a serious problem.
> 
> So if we're in a workqueue and it does a:
> 
> 	schedule_work_on(this_cpu, currently_executing_work);
> 
> it'll potentially make a new thread?
> 
> That's exactly the code path that will get exercised during a UDP
> flood the way that vector_work_func() is implemented.

It shouldn't create a new thread unless all other workers in the CPU
are voluntarily sleeping while executing a work. Also since softirqs
can't sleep, we shouldn't even have two vectors running concurrently
on workqueues.