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 19:12:49 -0400
From:	Masami Hiramatsu <mhiramat@...hat.com>
To:	Ingo Molnar <mingo@...e.hu>
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

Ingo Molnar wrote:
> * 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.

Hi Ingo,

Would I might be misunderstood?

struct list_head {
        struct list_head *next, *prev;
};

struct hlist_node {
        struct hlist_node *next, **pprev;
};

Both of list_head and hlist_node are the same size...


-- 
Masami Hiramatsu

Software Engineer
Hitachi Computer Products (America), Inc.
Software Solutions Division

e-mail: mhiramat@...hat.com

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