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    SubjectRe: [PATCH 0/13] maps: pagemap, kpagemap, and related cleanups
    On Thu, Apr 12, 2007 at 09:23:45PM -0500, 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.
    > > >>Maybe. How about LRU? Reclaim performance is bad, and you want to work out
    > > >>which pages keep going off the end of it, or which pages keep getting
    > > >>written out via it, or who's pages are on the active list, forcing mine
    > > >>out.
    > > >
    > > >
    > > >Those are actually probably a good match for systemtap as they're all
    > > >events.
    > >
    > > Traverse the LRU? Which files do they belong to? What process maps them?
    > -ENOPARSE.

    For non-event based data gathering using kprobes we can have a debugfs file
    like /debug/kprobes/snapshot_probe and write a kprobe module with probe at
    ->write() function and then the user space can trigger the data collection

    echo "1" > /debug/kprobes/snapshot_probe

    Thus, the actual data collection code can reside in a separate
    module or a systemtap script which provides very good post-processing
    capabalities, and can be used without recompiling or rebooting the kernel.


    Maneesh Soni
    Linux Technology Center,
    IBM India Systems and Technology Lab,
    Bangalore, India
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