Re: [RFC PATCH -tip 2/2 v2] add a scripts for pagecache usage per process

On Mon, Feb 01, 2010 at 02:17:35AM -0600, Tom Zanussi wrote:

Here's one way, using the tracepoint filters - it does make a big
difference in this case.

Before (using the new -P option, which includes perf in the trace
data):  

root@tropicana:~# perf record -c 1 -f -a -M -R -e filemap:add_to_page_cache -e filemap:find_get_page -e filemap:remove_from_page_cache -P sleep 5
[ perf record: Woken up 0 times to write data ]
[ perf record: Captured and wrote 71.201 MB perf.data (~3110815 samples) ]

After (filters out events generated by perf):

root@tropicana:~# perf record -c 1 -f -a -M -R -e filemap:add_to_page_cache -e filemap:find_get_page -e filemap:remove_from_page_cache sleep 5
[ perf record: Woken up 1 times to write data ]
[ perf record: Captured and wrote 0.309 MB perf.data (~13479 samples) ]

Tom

[PATCH] perf record: filter out perf process tracepoint events

The perf process itself can generate a lot of trace data, which most
of the time isn't of any interest.  This patch adds a predicate to the
kernel tracepoint filter of each recorded event type which effectively
screens out any event generated by perf.

Assuming the common case would be to ignore perf, this makes it the
default; the old behavior can be selected by using 'perf record -P'.

I think filtering out perf from the instrumentation is a very
desirable features.

But I see two drawbacks with this patch.
First of all, we want to keep perf as a part of the instrumentation
as a default behaviour I think, as it is a true part of the system
wide load. So I would rather suggest to keep it as a default and
have an exclude_perf option instead of include_perf.

The other downside is that this filtering only applies to ftrace events
and not to other perf events. I would expect an exclude_perf option
to apply to every events, not just a family of them.

This is not that easy though. It's trivial for a process bound
instrumentation as we only need to use enable_on_exec for that
(assuming we create the targeted process from perf).

Otherwise we need the cpu events to filter out a given context, which
needs to be done from the kernel, on events scheduling time.
It's just an idea, I'm adding more interested parties in Cc.

Thanks.

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