linux-kernel - Re: [PATCH 4/6] nohz: support PR_DATAPLANE

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Date:	Tue, 12 May 2015 11:50:30 +0200
From:	Ingo Molnar <mingo@...nel.org>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Chris Metcalf <cmetcalf@...hip.com>,
	Gilad Ben Yossef <giladb@...hip.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Ingo Molnar <mingo@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Rik van Riel <riel@...hat.com>, Tejun Heo <tj@...nel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Frederic Weisbecker <fweisbec@...il.com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Christoph Lameter <cl@...ux.com>,
	"Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>,
	linux-doc@...r.kernel.org, linux-api@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 4/6] nohz: support PR_DATAPLANE_QUIESCE


* Peter Zijlstra <peterz@...radead.org> wrote:

> On Fri, May 08, 2015 at 01:58:45PM -0400, Chris Metcalf wrote:
> > This prctl() flag for PR_SET_DATAPLANE sets a mode that requires the
> > kernel to quiesce any pending timer interrupts prior to returning
> > to userspace.  When running with this mode set, sys calls (and page
> > faults, etc.) can be inordinately slow.  However, user applications
> > that want to guarantee that no unexpected interrupts will occur
> > (even if they call into the kernel) can set this flag to guarantee
> > that semantics.
> 
> Currently people hot-unplug and hot-plug the CPU to do this. 
> Obviously that's a wee bit horrible :-)
> 
> Not sure if a prctl like this is any better though. This is a CPU 
> properly not a process one.

So if then a prctl() (or other system call) could be a shortcut to:

 - move the task to an isolated CPU
 - make sure there _is_ such an isolated domain available

I.e. have some programmatic, kernel provided way for an application to 
be sure it's running in the right environment. Relying on random 
administration flags here and there won't cut it.

Thanks,

	Ingo
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