Tracking vm activity

[Jun Koi]

On 4/24/07, William Cohen <wcohen@redhat.com> wrote:
> Hi,
>
> I am playing around with getting information about page faults and I noticed
> that there is a probe alias for the entry for the pagefault code. However, there
> is no matching probe point for the return. It is useful to look at the return
> value to determine what kind of page fault occurred (major or minor). The
> attached patch provides a similar probe point for the return point. any comments
> on the patch?
>
> This would be useful for the following senario. A probe on vm.pagefault could
> get the address and a probe on vm.pagefault.return could get information how it
> was handled. The analysis could then do things like which addresses cause major
> page faults (real disk accesses). One could write out a log of the major page
> faults (and the mmap operations) and track which files caused the page faults.
> Probably don't want to walk through the mm structures when taking a page fault
> to determine which file it came from.
>

Hi Will,

I found this idea interesting. However, what do you think if the
machine is SMP, or kernel is preemptive capable?

Just imagine a scenario like this with 2 kernel threads A and B:
- A causes page fault, then enters page fault handler (1)
- B preempts A, causes fault, then enters page fault handler  (2)
- A exits page fault handler (3)
- B exits page fault handler (4)

In this case, you get the exit value at (3), but it is not relevant to
the parameters collected in the above step (2).

I cannot figure out how to correlate "enter" and "exit" function
events in SMP or preemptive case.

Any idea?

Thanks,
Jun