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

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Message-ID: <20101124002411.GA29158@Krystal>
Date:	Tue, 23 Nov 2010 19:24:11 -0500
From:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	Jason Baron <jbaron@...hat.com>, mingo@...e.hu,
	peterz@...radead.org, 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

* Steven Rostedt (rostedt@...dmis.org) wrote:
> On Tue, 2010-11-23 at 18:43 -0500, Mathieu Desnoyers wrote:
> > * Jason Baron (jbaron@...hat.com) wrote:
> > [...]
> > > +static void update_jump_label_module(struct module *mod)
> > > +{
> > > +	struct hlist_head *head;
> > > +	struct hlist_node *node, *node_next, *module_node, *module_node_next;
> > > +	struct jump_label_entry *e;
> > > +	struct jump_label_module_entry *e_module;
> > > +	struct jump_entry *iter;
> > > +	int i, count;
> > > +
> > > +	/* if the module doesn't have jump label entries, just return */
> > > +	if (!mod->num_jump_entries)
> > > +		return;
> > > +
> > > +	for (i = 0; i < JUMP_LABEL_TABLE_SIZE; i++) {
> > > +		head = &jump_label_table[i];
> > > +		hlist_for_each_entry_safe(e, node, node_next, head, hlist) {
> > > +			if (!e->enabled)
> > > +				continue;
> > > +			hlist_for_each_entry_safe(e_module, module_node,
> > > +						  module_node_next,
> > > +						  &(e->modules), hlist) {
> > > +				if (e_module->mod != mod)
> > > +					continue;
> > 
> > Ouch.
> > 
> > Could you iterate on the loaded/unloaded module jump labels and do hash
> > table lookups rather than doing this O(n^3) iteration ?
> 
> This does not look O(n^3) to me.
> 
> It's a hash table, thus the first two loops is just iterating O(n) items
> in the hash.

Good point.

> 
> And the third loop is all the modules that use a particular event.
> 
> So it is O(n*m)  where n is the number of events, and m is the number of
> modules attached to the events. And that's only if all events are used
> by those modules. The actual case is much smaller.

Still, I wonder if the O(n) iteration on all events could be shrinked to
an interation on only the events present in the loaded/unloaded module ?
I think I did something like that for immediate values already. It might
apply (or not) here, just a thought.

Thanks,

Mathieu


-- 
Mathieu Desnoyers
Operating System Efficiency R&D Consultant
EfficiOS Inc.
http://www.efficios.com
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