Message-ID: <1502812431.1349.91.camel@gmx.de>
Subject: Re: [RFC PATCH 7/9] housekeeping: Use own boot option, independant
 from nohz
From: Mike Galbraith <efault@gmx.de>
To: paulmck@linux.vnet.ibm.com
Cc: Frederic Weisbecker <fweisbec@gmail.com>,
        Luiz Capitulino <lcapitulino@redhat.com>,
        LKML <linux-kernel@vger.kernel.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Chris Metcalf <cmetcalf@mellanox.com>,
        Thomas Gleixner <tglx@linutronix.de>, Christoph Lameter <cl@linux.com>,
        Ingo Molnar <mingo@kernel.org>, Rik van Riel <riel@redhat.com>,
        Wanpeng Li <kernellwp@gmail.com>
Date: Tue, 15 Aug 2017 17:53:51 +0200
In-Reply-To: <20170815153005.GJ7017@linux.vnet.ibm.com>
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         <20170814133440.3dc31bad@redhat.com> <1502735386.31351.61.camel@gmx.de>
         <20170815130712.GA16627@lerouge> <1502810123.1349.71.camel@gmx.de>
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On Tue, 2017-08-15 at 08:30 -0700, Paul E. McKenney wrote:
> On Tue, Aug 15, 2017 at 05:15:23PM +0200, Mike Galbraith wrote:
> > On Tue, 2017-08-15 at 15:07 +0200, Frederic Weisbecker wrote:
> > > On Mon, Aug 14, 2017 at 08:29:46PM +0200, Mike Galbraith wrote:
> > > > On Mon, 2017-08-14 at 13:34 -0400, Luiz Capitulino wrote:
> > > > > On Mon, 14 Aug 2017 19:01:09 +0200
> > > > > 
> > > > > > What is the source of the load balancing inducing such latency when a single
> > > > > > task is affine to a CPU? If this is idle load balancing, it is now affine to
> > > > > > housekeepers. If this is task wakeup then it's suprising because select_task_rq()
> > > > > > is optimized toward single CPU affinity.
> > > > > 
> > > > > I guess it was idle load balancing, but I don't remember because this
> > > > > was a few years ago. I think this might be reproducible without using
> > > > > isolcpus=. I'll give it a try shortly and let you know.
> > > > 
> > > > idle_balance() can swamp other noise by a couple orders of magnitude,
> > > 
> > > Ah I missed that one. Is there any way we can also lower the overhead there?
> > 
> > Why?  HPC proggies won't benefit from a partially filled pothole any
> > more that a ~zero ground clearance formula 1 car would.  The pothole
> > intolerant either isolate, killing (most) LB, or they meet a wall.
> 
> Don't the HPC guys just disable idle_balance(), or am I out of date again?

They could do just that if what they're doing is not really critical. 
I'm not an HPC guy, so can only speculate.  I don't see much difference
between HPC and RT though, the rules of the game seem to be about the
same (them both being HPC;).. what you can control, you do control.

	-Mike