[ltt-dev] LTTng 0.87 improved page fault tracing

[Mathieu Desnoyers]

* KOSAKI Motohiro (kosaki.motohiro at jp.fujitsu.com) wrote:
> Hi
> 
> interesting..
> 
> > I just combined the 4 page fault handler events that were in the tracing
> > hot path of LTTng into 2 :
> > 
> > kernel page_fault_entry
> > kernel page_fault_exit
> > 
> > They take as parameter the combination of what was available in the
> > trap_entry/exit events and handle_mm_fault entry/exit events. This
> > should lessen the performance impact of the tracer when it's active.
> > I did the related modifications in LTTV 0.12.8.
> 
> Just question.
> 
> As far as I know, customer has two different requeremtn of the page fault.

    1 a)
>   (1) collect number of all page fault
>       -> if it is too large, too many interrupt decrease performance.
        (a single event is required for this)

    1 b) the user may also want to know the time spent in the page fault
    handler to service those faults, therefore involving page fault
    entry and exit events.


>   (2) collect number of major page fault
>       -> major page fault indicate to increase random access I/O,
>          then, some customer want to collect major page fault
>          (don't include minor page fault)

Yes, the trace_page_fault_exit takes the "fault" parameter returned by
handle_mm_fault (which is recorded to the trace as the "res" event
field). Using

res & VM_FAULT_MAJOR

will give only the major page faults. Note that some knowledge of the
bitmask is required to interpret the "res" bitfield. This could be done
by a specific analysis module. I would ideally like to create a LTTng
module to export tables including those bitfields so we can keep the
bitfield interpretation in sync with the kernel code changes more or
less automatically.

> 
> Is this patch fill (2) requirement?
> 

Yes.

Mathieu

> 
> 

-- 
Mathieu Desnoyers
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