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

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Date:	Fri, 30 Sep 2011 09:09:40 -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 09/30/2011 08:28 AM, Steven Rostedt wrote:
> 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.

I think the simplest thing is to make stop_machine() well-defined in a
pre-smp environment, where it just directly calls the callback:

diff --git a/kernel/stop_machine.c b/kernel/stop_machine.c
index ba5070c..b6ad9b3 100644
--- a/kernel/stop_machine.c
+++ b/kernel/stop_machine.c
@@ -485,6 +485,11 @@ int __stop_machine(int (*fn)(void *), void *data, const struct cpumask *cpus)
 					    .num_threads = num_online_cpus(),
 					    .active_cpus = cpus };
 
+	if (smdata.num_threads == 1) {
+		(*fn)(data);
+		return 0;
+	}
+
 	/* Set the initial state and stop all online cpus. */
 	set_state(&smdata, STOPMACHINE_PREPARE);
 	return stop_cpus(cpu_online_mask, stop_machine_cpu_stop, &smdata);

so that its guaranteed safe to use at any point.

    J
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