linux-kernel - Re: [PATCH 0/6] support "dataplane" mode for nohz

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Date:	Sat, 9 May 2015 00:19:05 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	Ingo Molnar <mingo@...nel.org>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Chris Metcalf <cmetcalf@...hip.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Gilad Ben Yossef <giladb@...hip.com>,
	Ingo Molnar <mingo@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Rik van Riel <riel@...hat.com>, Tejun Heo <tj@...nel.org>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	"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-doc@...r.kernel.org>,
	Linux API <linux-api@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 0/6] support "dataplane" mode for nohz_full

On Sat, May 9, 2015 at 12:05 AM, Ingo Molnar <mingo@...nel.org> wrote:
>
> * Andrew Morton <akpm@...ux-foundation.org> wrote:
>
>> On Fri, 8 May 2015 19:11:10 -0400 Chris Metcalf <cmetcalf@...hip.com> wrote:
>>
>> > On 5/8/2015 5:22 PM, Steven Rostedt wrote:
>> > > On Fri, 8 May 2015 14:18:24 -0700
>> > > Andrew Morton <akpm@...ux-foundation.org> wrote:
>> > >
>> > >> On Fri, 8 May 2015 13:58:41 -0400 Chris Metcalf <cmetcalf@...hip.com> wrote:
>> > >>
>> > >>> A prctl() option (PR_SET_DATAPLANE) is added
>> > >> Dumb question: what does the term "dataplane" mean in this context?  I
>> > >> can't see the relationship between those words and what this patch
>> > >> does.
>> > > I was thinking the same thing. I haven't gotten around to searching
>> > > DATAPLANE yet.
>> > >
>> > > I would assume we want a name that is more meaningful for what is
>> > > happening.
>> >
>> > The text in the commit message and the 0/6 cover letter do try to explain
>> > the concept.  The terminology comes, I think, from networking line cards,
>> > where the "dataplane" is the part of the application that handles all the
>> > fast path processing of network packets, and the "control plane" is the part
>> > that handles routing updates, etc., generally slow-path stuff.  I've probably
>> > just been using the terms so long they seem normal to me.
>> >
>> > That said, what would be clearer?  NO_HZ_STRICT as a superset of
>> > NO_HZ_FULL?  Or move away from the NO_HZ terminology a bit; after all,
>> > we're talking about no interrupts of any kind, and maybe NO_HZ is too
>> > limited in scope?  So, NO_INTERRUPTS?  USERSPACE_ONLY?  Or look
>> > to vendors who ship bare-metal runtimes and call it BARE_METAL?
>> > Borrow the Tilera marketing name and call it ZERO_OVERHEAD?
>> >
>> > Maybe BARE_METAL seems most plausible -- after DATAPLANE, to me,
>> > of course :-)
>
> 'baremetal' has uses in virtualization speak, so I think that would be
> confusing.
>
>> I like NO_INTERRUPTS.  Simple, direct.
>
> NO_HZ_PURE?
>

Naming aside, I don't think this should be a per-task flag at all.  We
already have way too much overhead per syscall in nohz mode, and it
would be nice to get the per-syscall overhead as low as possible.  We
should strive, for all tasks, to keep syscall overhead down *and*
avoid as many interrupts as possible.

That being said, I do see a legitimate use for a way to tell the
kernel "I'm going to run in userspace for a long time; stay away".
But shouldn't that be a single operation, not an ongoing flag?  IOW, I
think that we should have a new syscall quiesce() or something rather
than a prctl.

--Andy
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