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From: Frederic Weisbecker <frederic@kernel.org>
To: David Miller <davem@davemloft.net>
Cc: torvalds@linux-foundation.org, pabeni@redhat.com,
	linux-kernel@vger.kernel.org, alexander.levin@verizon.com,
	peterz@infradead.org, mchehab@s-opensource.com,
	hannes@stressinduktion.org, paulmck@linux.vnet.ibm.com,
	wanpeng.li@hotmail.com, dima@arista.com, tglx@linutronix.de,
	akpm@linux-foundation.org, rrendec@arista.com, mingo@kernel.org,
	sgruszka@redhat.com, riel@redhat.com, edumazet@google.com
Subject: Re: [RFC PATCH 0/4] softirq: Per vector threading v3
Date: Wed, 24 Jan 2018 02:57:36 +0100	[thread overview]
Message-ID: <20180124015734.GA11302@lerouge> (raw)
In-Reply-To: <20180123.132424.1035340800864767853.davem@davemloft.net>

On Tue, Jan 23, 2018 at 01:24:24PM -0500, David Miller wrote:
> From: Linus Torvalds <torvalds@linux-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.

  reply	other threads:[~2018-01-24  1:57 UTC|newest]

Thread overview: 39+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2018-01-19 15:46 [RFC PATCH 0/4] softirq: Per vector threading v3 Frederic Weisbecker
2018-01-19 15:46 ` [RFC PATCH 1/4] softirq: Limit vector to a single iteration on IRQ tail Frederic Weisbecker
2018-01-19 16:16   ` David Miller
2018-01-19 18:25     ` Linus Torvalds
2018-01-19 18:47       ` David Miller
2018-01-21 16:30         ` Frederic Weisbecker
2018-01-21 16:57           ` David Miller
2018-01-19 15:46 ` [RFC PATCH 2/4] softirq: Per vector deferment to workqueue Frederic Weisbecker
2018-01-20  8:41   ` Pavan Kondeti
2018-01-21 16:11     ` Frederic Weisbecker
2018-01-21 17:50       ` Pavan Kondeti
2018-01-21 20:48         ` Frederic Weisbecker
2018-02-08 17:44   ` Sebastian Andrzej Siewior
2018-02-08 18:45     ` David Miller
2018-02-08 20:14       ` Dmitry Safonov
2018-02-08 20:22         ` David Miller
2018-02-08 20:30           ` Dmitry Safonov
2018-02-09  4:11             ` Mike Galbraith
2018-02-09 12:35               ` Sebastian Andrzej Siewior
2018-02-15 16:13     ` Frederic Weisbecker
2018-02-15 16:58       ` Sebastian Andrzej Siewior
2018-01-19 15:46 ` [RFC PATCH 3/4] softirq: Defer to workqueue when rescheduling is needed Frederic Weisbecker
2018-01-19 15:46 ` [RFC PATCH 4/4] softirq: Replace ksoftirqd with workqueues entirely Frederic Weisbecker
2018-01-22 19:58 ` [RFC PATCH 0/4] softirq: Per vector threading v3 Mauro Carvalho Chehab
2018-01-23 10:13 ` Paolo Abeni
2018-01-23 12:32   ` Dmitry Safonov
2018-01-24  2:12     ` Frederic Weisbecker
2018-01-23 16:22   ` David Miller
2018-01-23 16:57     ` Paolo Abeni
2018-01-23 17:42       ` Linus Torvalds
2018-01-23 18:01         ` Mike Galbraith
2018-01-23 18:24         ` David Miller
2018-01-24  1:57           ` Frederic Weisbecker [this message]
2018-01-24  2:01             ` Frederic Weisbecker
2018-01-24 14:54         ` Paolo Abeni
2018-01-24 15:05           ` David Miller
2018-01-24 16:11             ` Paolo Abeni
2018-02-07 14:18 ` Mauro Carvalho Chehab
2018-03-01 15:21   ` Frederic Weisbecker

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