linux-kernel - Re: [PATCH documentation 2/2] kthread: Document ways of reducing OS jitter due to per-CPU kthreads

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Message-ID: <20130425212336.GN3427@linux.vnet.ibm.com>
Date:	Thu, 25 Apr 2013 14:23:36 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Thomas Gleixner <tglx@...utronix.de>
Cc:	Borislav Petkov <bp@...en8.de>, linux-kernel@...r.kernel.org,
	mingo@...e.hu, sbw@....edu,
	Frederic Weisbecker <fweisbec@...il.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Arjan van de Ven <arjan@...ux.intel.com>,
	Kevin Hilman <khilman@...aro.org>,
	Christoph Lameter <cl@...ux.com>,
	Olivier Baetz <olivier.baetz@...asparks.com>
Subject: Re: [PATCH documentation 2/2] kthread: Document ways of reducing OS
 jitter due to per-CPU kthreads

On Thu, Apr 25, 2013 at 10:59:05PM +0200, Thomas Gleixner wrote:
> 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.

Good point!  I have added a third item to the eHCA list:

	Rework the eHCA driver so that its per-CPU kthreads are
	provisioned only on selected CPUs.

							Thanx, Paul

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