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

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Date:   Wed, 24 Jan 2018 03:01:37 +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

2018-01-24 2:57 UTC+01:00, Frederic Weisbecker <frederic@...nel.org>:
> 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.
>

...unless I misunderstood workqueue internals or missed something, in
which case we may try ordered workqueue.