linux-kernel - Re: [PATCH 1/3] jump label: add enabled/disabled state to jump label key entries

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Date:	Wed, 24 Nov 2010 16:24:05 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Jason Baron <jbaron@...hat.com>
Cc:	rostedt@...dmis.org, mingo@...e.hu, mathieu.desnoyers@...ymtl.ca,
	hpa@...or.com, tglx@...utronix.de, andi@...stfloor.org,
	roland@...hat.com, rth@...hat.com, masami.hiramatsu.pt@...achi.com,
	fweisbec@...il.com, avi@...hat.com, davem@...emloft.net,
	sam@...nborg.org, ddaney@...iumnetworks.com,
	michael@...erman.id.au, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/3] jump label: add enabled/disabled state to jump
 label key entries

On Wed, 2010-11-24 at 10:19 -0500, Jason Baron wrote:
> On Wed, Nov 24, 2010 at 04:11:18PM +0100, Peter Zijlstra wrote:
> > On Wed, 2010-11-24 at 09:54 -0500, Jason Baron wrote:
> > > On Wed, Nov 24, 2010 at 09:20:09AM +0100, Peter Zijlstra wrote:
> > > > On Tue, 2010-11-23 at 16:27 -0500, Jason Baron wrote:
> > > > >         struct hlist_head modules;
> > > > >         unsigned long key;
> > > > > +       u32 nr_entries : 31,
> > > > > +              enabled :  1;
> > > > >  }; 
> > > > 
> > > > I still don't see why you do this, why not simply mandate that the key
> > > > is of type atomic_t* and use *key as enabled state?
> > > > 
> > > 
> > > Because I want to use *key as a pointer directly to 'struct jump_label_entry'.
> > > In this way jump_label_enable(), jump_label_disable(), become O(1) operations.
> > > That way we don't need any hashing.
> > 
> > But but but, you're doing a friggin stop_machine to poke text, that's
> > way more expensive than anything else.
> > 
> 
> Yes, but other arches do not require stop_machine(). Also, there is work
> for x86 to make the code patching happen without stop_machine().

Even without stop machine you're sending IPIs to all CPUs, that's not
free either.

And I think the only arch where you can do text pokes without cross-cpu
synchronization is one that doesn't have SMP support.


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