netdev - Re: [PATCH 1/3] vhost: replace vhost_workqueue with per-vhost kthread

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Message-ID: <20100531153104.GA9452@redhat.com>
Date:	Mon, 31 May 2010 17:31:04 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Tejun Heo <tj@...nel.org>
Cc:	"Michael S. Tsirkin" <mst@...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 1/3] vhost: replace vhost_workqueue with per-vhost
	kthread

On 05/31, Tejun Heo wrote:
>
> On 05/31/2010 04:39 PM, Oleg Nesterov wrote:
>
> > What I can't understand is why we do have ->queue_seq and ->done_seq.
> >
> > Isn't the single "bool poll->active" enough? vhost_poll_queue() sets
> > ->active == T, vhost_poller() clears it before wake_up_all(poll->done).
>
> I might have slightly over engineered this part not knowing the
> expected workload.  ->queue_seq/->done_seq pair is to guarantee that
> flushers never get starved.

Ah, indeed.

Well, afaics we do not need 2 counters anyway, both vhost_poll_queue()
and vhost_poller() could increment the single counter and the flusher
can take bit 0 into account. But I agree 2 counters are much more clean.

> >> +static int vhost_poller(void *data)
> >> +{
> >> +	struct vhost_dev *dev = data;
> >> +	struct vhost_poll *poll;
> >> +
> >> +repeat:
> >> +	set_current_state(TASK_INTERRUPTIBLE);	/* mb paired w/ kthread_stop */
> >
> > I don't understand the comment... why do we need this barrier?
>
> So that either kthread_stop()'s should_stop = 1 in kthread_stop() is
> visible to kthread_should_stop() or task state is set to RUNNING.

Of course, you are right. I am really surprized I asked this question ;)

Thanks,

Oleg.

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