Re: [PATCH 0/13] maps: pagemap, kpagemap, and related cleanups

[Ananth N Mavinakayanahalli <ananth@in.ibm.com>]

On Fri, Apr 13, 2007 at 12:54:36PM +1000, Nick Piggin wrote:
> Matt Mackall wrote:
> >On Fri, Apr 13, 2007 at 12:21:25PM +1000, Nick Piggin wrote:
> >
> >>Matt Mackall wrote:
> >>
> >>>On Fri, Apr 13, 2007 at 11:42:29AM +1000, Nick Piggin wrote:
> >>
> >>>>If kprobes is simply crappy and doesn't work properly for this, then I
> >>>>could accept that. I'm not someone trying to get this info. So why can't
> >>>>it be used? (not just for kpagemap, but for clear_refs and all that gunk
> >>>>too).
> >>>
> >>>
> >>>kprobes is good for looking at events, but bad for looking at state.
> >>>Especially metric shitloads of state.
> >>
> >>Why? Why is a kprobes trap significantly more expensive than a read
> >>syscall?
> >
> >
> >I guess I'm not clear on what you're proposing. From my understanding
> >of kprobes (admittedly not an expert), this is hard to do and not a
> >very good match.
> 
> But you have an idea that it is bad for exposing lots of data. Why?
> (I'm not a kprobes expert either, these are not rhetorical questions)

You could tie your kprobe module to use relay channels. Kprobe handlers
run lockless and using the per-cpu relay channels will provide a fast
transport mechanism for exposing lots of data.

http://relayfs.sourceforge.net/examples.html#tprintk_kprobes is an
example using the earlier relayfs interface. It shouldn't be that hard
to change it to use the newer relay stuff.

AFAIK acme is using a similar mechanism for ctracer
(http://oops.ghostprotocols.net:81/blog/?p=50)

Ananth