Tracking vm activity

[William Cohen]

Jun Koi wrote:
> 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

It is quite possible that more than one application has outstanding page faults. 
By definition a context switch happens when a major page fault occurs. However, 
associative arrays can be used to keep that kind of matching information 
straight. Thus, this will not be a problem.

-Will