linux-kernel - Re: [PATCH RFC 0/8] jump-label: allow early jump_label

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Message-ID: <1317396484.4588.52.camel@gandalf.stny.rr.com>
Date:	Fri, 30 Sep 2011 11:28:00 -0400
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Jeremy Fitzhardinge <jeremy@...p.org>
Cc:	"David S. Miller" <davem@...emloft.net>,
	David Daney <david.daney@...ium.com>,
	Michael Ellerman <michael@...erman.id.au>,
	Jan Glauber <jang@...ux.vnet.ibm.com>,
	Jason Baron <jbaron@...hat.com>,
	the arch/x86 maintainers <x86@...nel.org>,
	Xen Devel <xen-devel@...ts.xensource.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Jeremy Fitzhardinge <jeremy.fitzhardinge@...rix.com>
Subject: Re: [PATCH RFC 0/8] jump-label: allow early jump_label_enable()

On Thu, 2011-09-29 at 21:40 -0700, Jeremy Fitzhardinge wrote:
> On 09/29/2011 05:52 PM, Steven Rostedt wrote:
> > On Thu, 2011-09-29 at 16:26 -0700, Jeremy Fitzhardinge wrote:
> >> From: Jeremy Fitzhardinge <jeremy.fitzhardinge@...rix.com>
> >>
> >> One big question which arises is whether the _early() function is
> >> necessary at all.  All the stop_machine/mutex/etc stuff that
> >> arch_jump_label_transform() ends up doing is redundant pre-SMP, but it
> >> shouldn't hurt.  Maybe we can just drop the _early function?  It works
> >> on x86, at least, because jump_label_enable() works, which uses the full
> >> form.  And dropping it would reduce this to a very much smaller series.
> > It does slow down the boot process, which is not a good thing when
> > everyone is pushing for the fastest restarts.
> 
> Would it really though?  stop_machine() doesn't do very much when there
> are no other cpus.
> 
> Not that I measured or anything, but there was no obvious big lag at boot.

Just bringing up the point, but without measurements, its all hand
waving. It may not be a big deal, and simpler code is always better if
it doesn't harm anything else.

> 
> > What we should probably do is have a global read_mostly variable called,
> > smp_activated or something, then things that can be called before and
> > after can read this variable to determine if it can skip certain
> > protections.
> 
> Could do that if it turns out to be a problem.
> 
> > While we're at it, perhaps we could add a memory_initialized for things
> > like tracers that want to trace early but need to wait till it can
> > allocate buffers. If we had this flag, it could instead do an early
> > memory init to create the buffers.
> 
> That seems orthogonal to the jump_label changes.

It is. But it's something that bugs me and this just reminded me of
it ;)

-- Steve


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