linux-kernel - Re: [PATCH -tip 3/3] kprobes: cleanup: use list instead of hlist for insn

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Date:	Tue, 30 Jun 2009 23:50:23 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Masami Hiramatsu <mhiramat@...hat.com>
Cc:	Ananth N Mavinakayanahalli <ananth@...ibm.com>,
	lkml <linux-kernel@...r.kernel.org>,
	systemtap <systemtap@...rces.redhat.com>,
	DLE <dle-develop@...ts.sourceforge.net>,
	Jim Keniston <jkenisto@...ibm.com>
Subject: Re: [PATCH -tip 3/3] kprobes: cleanup: use list instead of hlist
	for insn_pages


* Masami Hiramatsu <mhiramat@...hat.com> wrote:

> Ingo Molnar wrote:
> > * Masami Hiramatsu <mhiramat@...hat.com> wrote:
> > 
> >> Use struct list instead of struct hlist for managing insn_pages, 
> >> because insn_pages doesn't use hash table.
> > 
> >>  struct kprobe_insn_page {
> >> -	struct hlist_node hlist;
> >> +	struct list_head list;
> > 
> > Hm, you know that this increases the size of kprobe_insn_page by 4/8 
> > bytes, right?
> 
> Sure, that will increase size.
> 
> > hlists are not just used for hashes - but also when we want a more 
> > compact / smaller list head.
> 
> Oh, I thought hlists are used for hash tables...

... because they are smaller, hence the hash table of list 
heads becomes twice as dense as with list_head.

But otherwise it's an (almost) equivalent primitive to list_head, 
with a slightly higher runtime cost versus better RAM footprint.

> > How many kprobe_insn_page's can be allocated in the system, 
> > maximally?
> 
> It's depends on how many probes you will use, but logically, 1 
> kprobe_insn_pages is allocated per 4096/16 = 256 probes. So, if 
> you use 25,600 probes on your system, memory consumption will 
> increase 400/800 bytes.

it's your call really - just wanted to react on the 'because it 
should be used for hash tables' comment in the changelog.

	Ingo
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