linux-kernel - Re: [PATCH 4/4] nohz: Set isolcpus when nohz

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Date:	Thu, 21 May 2015 07:14:09 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Afzal Mohammed <afzal.mohd.ma@...il.com>
Cc:	Frederic Weisbecker <fweisbec@...il.com>,
	Mike Galbraith <umgwanakikbuti@...il.com>,
	Sasha Levin <levinsasha928@...il.com>,
	Ingo Molnar <mingo@...nel.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Chris Metcalf <cmetcalf@...hip.com>,
	"Rafael J . Wysocki" <rafael.j.wysocki@...el.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Dave Jones <davej@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Oleg Nesterov <oleg@...hat.com>,
	Ingo Molnar <mingo@...hat.com>, Rik van Riel <riel@...hat.com>,
	Martin Schwidefsky <schwidefsky@...ibm.com>
Subject: Re: [PATCH 4/4] nohz: Set isolcpus when nohz_full is set

On Thu, May 21, 2015 at 06:59:05PM +0530, Afzal Mohammed wrote:
> Hi,
> 
> On Thu, May 21, 2015 at 03:06:23PM +0200, Frederic Weisbecker wrote:
> > On Thu, May 21, 2015 at 05:57:59AM -0700, Paul E. McKenney wrote:
> 
> > > Indeed, NO_HZ_FULL is special purpose.  You normally would select
> > > NO_HZ_FULL_ALL only on a system intended for heavy compute without
> > > normal-workload distractions or for some real-time systems.  For mixed
> > > workloads, you would build with NO_HZ_FULL (but not NO_HZ_FULL_ALL) and
> > > use the boot parameters to select which CPUs are to be running the
> > > specialized portion of the workload.
> > > 
> > > And you would of course need to lead enough CPUs running normally to
> > > handle the non-specialized portion of the workload.
> > > 
> > > This sort of thing has traditionally required specialized kernels,
> > > so the cool thing here is that we can make Linux do it.  Though, as
> > > you noticed, careful configuration is still required.
> > > 
> > > Seem reasonable?
> 
> Yes, thanks, some dots got connected :)
> 
> > That said if he saw a big performance regression after applying these patches,
> > then there is likely a problem in the patchset. Well it could be due to that mode
> > which loops on full dynticks before resuming to userspace. Indeed when that is
> > enabled, I expect real throughput issues on workloads doing lots of kernel <->
> > userspace roundtrips. We just need to make sure this thing only works when requested.
> 
> With this change (& having NO_HZ_FULL_ALL), hackbench was being served
> only by the boot cpu, while w/o this change, all 8 (this is a quad
> core HT processor) was being used - observation based on 'top'.

Good to know!  And it will be interesting to see what Frederic decides
based on his review of the patchset.

							Thanx, Paul

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