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Date: Thu, 25 Apr 2013 22:59:05 +0200 (CEST)
From: Thomas Gleixner <tglx@linutronix.de>
To: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
cc: Borislav Petkov <bp@alien8.de>, linux-kernel@vger.kernel.org,
        mingo@elte.hu, sbw@mit.edu, Frederic Weisbecker <fweisbec@gmail.com>,
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        Arjan van de Ven <arjan@linux.intel.com>,
        Kevin Hilman <khilman@linaro.org>, Christoph Lameter <cl@linux.com>,
        Olivier Baetz <olivier.baetz@novasparks.com>
Subject: Re: [PATCH documentation 2/2] kthread: Document ways of reducing OS
 jitter due to per-CPU kthreads
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On Thu, 25 Apr 2013, Paul E. McKenney wrote:
> On Thu, Apr 25, 2013 at 12:23:12PM +0200, Borislav Petkov wrote:
> > On Mon, Apr 22, 2013 at 09:03:29PM -0700, Paul E. McKenney wrote:
> > > > > +Name: ehca_comp/%u
> > > > > +Purpose: Periodically process Infiniband-related work.
> > > > > +To reduce corresponding OS jitter, do any of the following:
> > > > > +1.	Don't use EHCA Infiniband hardware.  This will prevent these
> > > > 
> > > > Sounds like this particular hardware is slow and its IRQ handler/softirq
> > > > needs a lot of time. Yes, no?
> > > > 
> > > > Can we have a reason why people shouldn't use that hw.
> > > 
> > > Because it has per-CPU kthreads that can cause OS jitter.  ;-)
> > 
> > Yeah, I stumbled over this specific brand of Infiniband hw. It looks
> > like this particular Infiniband driver uses per-CPU kthreads and the
> > others in drivers/infiniband/hw/ don't?
> > 
> > I hope this explains my head-scratching moment here...
> 
> Ah!  I rewrote the first sentence to read:
> 
> 	Don't use eHCA Infiniband hardware, instead choosing hardware
> 	that does not require per-CPU kthreads.

Another option would be to teach that eHCA driver to be configurable
on which cpus kthreads are desired and on which not. I can't see a
reason (aside of throughput) why that hardware can't cope with a
single thread.

Thanks,

	tglx