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

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Date:	Thu, 29 Sep 2011 21:40:57 -0700
From:	Jeremy Fitzhardinge <jeremy@...p.org>
To:	Steven Rostedt <rostedt@...dmis.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 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.

> 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.

    J
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