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

Masami Hiramatsu
2009-06-30T23:10:39Z

Ingo Molnar wrote:
> * Masami Hiramatsu <mhiramat@redhat.com> wrote:
>
>> Ingo Molnar wrote:
>>> * Masami Hiramatsu <mhiramat@redhat.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@redhat.com