Re: [PATCH 0/6] support "dataplane" mode for nohz_full

* Andrew Morton [off-list ref] wrote:

On Fri, 8 May 2015 19:11:10 -0400 Chris Metcalf [off-list ref] wrote:

quoted

On 5/8/2015 5:22 PM, Steven Rostedt wrote:

quoted

On Fri, 8 May 2015 14:18:24 -0700
Andrew Morton [off-list ref] wrote:

quoted

On Fri, 8 May 2015 13:58:41 -0400 Chris Metcalf [off-list ref] wrote:

quoted

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?

That's what it's really about: user-space wants to run exclusively, in 
pure user-mode, without any interrupts.

So I don't like 'NO_HZ_NO_INTERRUPTS' for a couple of reasons:

 - It is similar to a term we use in perf: PERF_PMU_CAP_NO_INTERRUPT.

 - Another reason is that 'NO_INTERRUPTS', in most existing uses in 
   the kernel generally relates to some sort of hardware weakness, 
   limitation, a negative property: that we try to limp along without 
   having a hardware interrupt and have to poll. In other driver code
   that uses variants of NO_INTERRUPT it appears to be similar. So I 
   think there's some confusion potential here.

 - Here the fact that we don't disturb user-space is an absolutely
   positive property, not a limitation, a kernel feature we work hard 
   to achieve. NO_HZ_PURE would convey that while NO_HZ_NO_INTERRUPTS 
   wouldn't.

 - NO_HZ_NO_INTERRUPTS has a double negation, and it's also too long,
   compared to NO_HZ_FULL or NO_HZ_PURE ;-) The term 'no HZ' already 
   expresses that we don't have periodic interruptions. We just 
   duplicate that information with NO_HZ_NO_INTERRUPTS, while 
   NO_HZ_FULL or NO_HZ_PURE qualifies it, makes it a stronger
   property - which is what we want I think.

So I think we should either rename NO_HZ_FULL to NO_HZ_PURE, or keep 
it at NO_HZ_FULL: because the intention of NO_HZ_FULL was always to be 
such a 'zero overhead' mode of operation, where if user-space runs, it 
won't get interrupted in any way.

There's no need to add yet another Kconfig variant - lets just enhance 
the current stuff and maybe rename it to NO_HZ_PURE to better express 
its intent.

Thanks,

	Ingo