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Date:	Fri, 25 Jul 2014 09:25:21 -0700
From:	Tim Chen <tim.c.chen@...ux.intel.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Herbert Xu <herbert@...dor.apana.org.au>,
	"H. Peter Anvin" <hpa@...or.com>,
	"David S.Miller" <davem@...emloft.net>,
	Ingo Molnar <mingo@...nel.org>,
	Chandramouli Narayanan <mouli@...ux.intel.com>,
	Vinodh Gopal <vinodh.gopal@...el.com>,
	James Guilford <james.guilford@...el.com>,
	Wajdi Feghali <wajdi.k.feghali@...el.com>,
	Jussi Kivilinna <jussi.kivilinna@....fi>,
	Thomas Gleixner <tglx@...utronix.de>,
	Tadeusz Struk <tadeusz.struk@...el.com>, tkhai@...dex.ru,
	linux-crypto@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v5 2/7] crypto: SHA1 multibuffer crypto hash
 infrastructure

On Fri, 2014-07-25 at 09:08 +0200, Peter Zijlstra wrote:
> On Tue, Jul 22, 2014 at 03:09:32PM -0700, Tim Chen wrote:
> > +/* Called in workqueue context, do one real cryption work (via
> > + * req->complete) and reschedule itself if there are more work to
> > + * do. */
> 
> You seem to manage the 'normal' comment style in other places, this one
> 'special' for a reason?

Yes, I'll need to correct it.

> 
> > +static void mcryptd_queue_worker(struct work_struct *work)
> > +{
> > +	struct mcryptd_cpu_queue *cpu_queue;
> > +	struct crypto_async_request *req, *backlog;
> > +	int i;
> > +
> > +	/*
> > +	 * Need to loop through more than once for multi-buffer to
> > +	 * be effective.
> > +	 */
> > +
> > +	cpu_queue = container_of(work, struct mcryptd_cpu_queue, work);
> > +	i = 0;
> > +	while (i < MCRYPTD_BATCH || single_task_running()) {
> > +		/*
> > +		 * preempt_disable/enable is used to prevent
> > +		 * being preempted by mcryptd_enqueue_request()
> > +		 */
> > +		local_bh_disable();
> > +		preempt_disable();
> > +		backlog = crypto_get_backlog(&cpu_queue->queue);
> > +		req = crypto_dequeue_request(&cpu_queue->queue);
> > +		preempt_enable();
> > +		local_bh_enable();
> > +
> > +		if (!req)
> > +			return;
> > +
> > +		if (backlog)
> > +			backlog->complete(backlog, -EINPROGRESS);
> > +		req->complete(req, 0);
> > +		if (!cpu_queue->queue.qlen)
> > +			return;
> > +		++i;
> > +	}
> > +	if (cpu_queue->queue.qlen)
> > +		queue_work(kcrypto_wq, &cpu_queue->work);
> > +}
> 
> Right, so I don't think you need that single_task_running() thing for
> this. Also its a rather inconclusive test, a task can come in while
> processing the request, preempt the processing, complete and by the time
> we're back to check if we want to continue the loop its only us again.
> 
> So its actually misleading..
> 
> You could do that, but then you have to keep preemption disabled across
> the entire thing, and break out on need_resched(), that would entirely
> obviate the need for single_task_running(), but would increase the
> preemption latency by however long the req processing takes, so that's
> bad too.
> 
> But you can get similar 'fuzzy' semantics as above by using something
> like:
> 
> static inline bool did_context_switch(unsigned long *nivcs)
> {
> 	if (*nivcs != current->nivcs) {
> 		*nivcs = current->nivcs;
> 		return true;
> 	}
> 	return false;
> }
> 
> static void mcryptd_queue_worker()
> {
> 	...
> 	unsigned long nivcs = current->nivcs;
> 
> 	...
> 
> 	while (!did_context_switch(&nivcs) || i < MCRYPTD_BATCH)

Won't I need something like 

while ((!did_context_switch(&nivcs) && single_task_running()) 
	|| i < MCRYPTD_BATCH)

in case I got some new task in my run queue but I did not get
pre-empted?  I should not continue to run in that case.

> 
> 	...
> }
> 
> It'll terminate earlier I suppose, its the inverse test. But I think you
> want to terminate early if there's any activity.

I have some doubts here.
If I'm still the only task running, why not continue, even if I've been
pre-empted?  Since there's nothing else to run, why not take
advantage of the cpu?

Thanks.

Tim

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