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

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Message-Id: <20180123.132424.1035340800864767853.davem@davemloft.net>
Date:   Tue, 23 Jan 2018 13:24:24 -0500 (EST)
From:   David Miller <davem@...emloft.net>
To:     torvalds@...ux-foundation.org
Cc:     pabeni@...hat.com, frederic@...nel.org,
        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

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.