netdev - Re: [PATCH 2/3] workqueue: Add an API to create a singlethread workqueue attached to the current task's cgroup

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Message-ID: <20100527173207.GA21880@redhat.com>
Date:	Thu, 27 May 2010 20:32:07 +0300
From:	"Michael S. Tsirkin" <mst@...hat.com>
To:	Tejun Heo <tj@...nel.org>
Cc:	Oleg Nesterov <oleg@...hat.com>,
	Sridhar Samudrala <sri@...ibm.com>,
	netdev <netdev@...r.kernel.org>,
	lkml <linux-kernel@...r.kernel.org>,
	"kvm@...r.kernel.org" <kvm@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Dmitri Vorobiev <dmitri.vorobiev@...ial.com>,
	Jiri Kosina <jkosina@...e.cz>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...e.hu>, Andi Kleen <ak@...ux.intel.com>
Subject: Re: [PATCH 2/3] workqueue: Add an API to create a singlethread
	workqueue attached to the current task's cgroup

On Thu, May 27, 2010 at 06:56:20PM +0200, Tejun Heo wrote:
> Hello,
> 
> On 05/27/2010 06:39 PM, Michael S. Tsirkin wrote:
> >> Unless you're gonna convert every driver to use this
> >> special kind of workqueue (and what happens when multiple tasks from
> >> different cgroups share the driver?),
> > 
> > We'll then create a workqueue per task. Each workqueue will have the
> > right cgroup. But we are not trying to selve the problem for
> > every driver.
> 
> Ah... I see.  You're gonna use multiple workqueues.  Once concern that
> I have is that this is abuse of workqueue interface to certain level
> and depends on the implementation detail of workqueue rather than its
> intended usage model.

Well, this is why I proposed adding a new API for creating
workqueue within workqueue.c, rather than exposing the task
and attaching it to cgroups in our driver: so that workqueue
maintainers can fix the implementation if it ever changes.

And after all, it's an internal API, we can always change
it later if we need.

> stop_machine() was a similar case and in the
> end it was better served by a different mechanism built on kthread
> directly (cpu_stop).  Wouldn't it be cleaner to use kthread directly
> for your case too?  You're basically trying to use workqueue as a
> frontend to kthread, so...
> 
> Thanks.

Well, yes but we are using APIs like flush_work etc. These are very
handy.  It seems much easier than rolling our own queue on top of kthread.

Makes sense?

> -- 
> tejun
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