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

Ingo Molnar
2009-06-30T21:50:53Z

* 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.

Ingo