linux-kernel - Re: [PATCH v4 6/7] sched: add function nr_running_cpu to expose number of tasks running on cpu

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Date:	Mon, 14 Jul 2014 09:10:14 -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>,
	linux-crypto@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4 6/7] sched: add function nr_running_cpu to expose
 number of tasks running on cpu

On Mon, 2014-07-14 at 12:16 +0200, Peter Zijlstra wrote:
> On Fri, Jul 11, 2014 at 01:33:04PM -0700, Tim Chen wrote:
> > This function will help a thread decide if it wants to to do work
> > that can be delayed, to accumulate more tasks for more efficient
> > batch processing later.
> > 
> > However, if no other tasks are running on the cpu, it can take
> > advantgae of the available cpu cycles to complete the tasks
> > for immediate processing to minimize delay, otherwise it will yield.
> 
> Ugh.. and ignore topology and everything else.
> 
> Yet another scheduler on top of the scheduler.
> 
> We have the padata muck, also only ever used by crypto.
> We have the workqueue nonsense, used all over the place
> And we have btrfs doing their own padata like muck.
> And I'm sure there's at least one more out there, just because.
> 
> Why do we want yet another thing?
> 
> I'm inclined to go NAK and get people to reduce the amount of async
> queueing and processing crap.

The mult-buffer class of crypto algorithms is by nature
asynchronous.  The algorithm gathers several crypto jobs, and
put the buffer from each job in a data lane of the SIMD register.
This allows for parallel processing and increases throughput.
The gathering of the crypto jobs is an async process and
queuing is necessary for this class of algorithm.

Tim

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