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Re: [PATCH v3 5/5] perf/sdt: Add support to perf record to trace SDT events
- From: Hemant Kumar <hemant at linux dot vnet dot ibm dot com>
- To: Masami Hiramatsu <masami dot hiramatsu dot pt at hitachi dot com>
- Cc: linux-kernel at vger dot kernel dot org, srikar at linux dot vnet dot ibm dot com, peterz at infradead dot org, oleg at redhat dot com, hegdevasant at linux dot vnet dot ibm dot com, mingo at redhat dot com, anton at redhat dot com, systemtap at sourceware dot org, namhyung at kernel dot org, aravinda at linux dot vnet dot ibm dot com, penberg at iki dot fi
- Date: Wed, 22 Oct 2014 13:50:22 +0530
- Subject: Re: [PATCH v3 5/5] perf/sdt: Add support to perf record to trace SDT events
- Authentication-results: sourceware.org; auth=none
- References: <20141010104402 dot 15506 dot 73285 dot stgit at hemant-fedora> <20141010105914 dot 15506 dot 84827 dot stgit at hemant-fedora> <54475292 dot 20409 at hitachi dot com>
Hi Masami,
On 10/22/2014 12:15 PM, Masami Hiramatsu wrote:
Hi Hemant,
(2014/10/10 19:59), Hemant Kumar wrote:
The SDT events are already stored in a cache file
(/var/cache/perf/perf-sdt-file.cache).
Please describe what this patch does at first.
Sure, will do that.
Although the file_hash table helps in addition or deletion of SDT events from the
cache, its not of much use when it comes to probing the actual SDT event,
because the key to this hash list is a file name and not the SDT event name
(which is given as an argument to perf record). So, we won't be able to hash
into it.
To avoid this problem, we can create another hash list "event_hash" list which
will be maintained along with the file_hash list.
Whenever a user invokes 'perf record -e %provider:event, perf should initialize
the event_hash list and the file_hash list.
The key to event_hash list is calculated from the event name and its
provider name.
event_hash sdt_note
|---------| ----------------
| | | file_ptr |==> container file_sdt_ent
key = 129 =>| hlist ==|===|=> event_list=|==> to sdt notes hashed to
| | | name | same entry
|---------| | provider |
| | | note_list==|==> to other notes in the
key = 130 =>| hlist | --------------- same file
|---------|
The entry at that key in event_hash contains a list of SDT notes hashed to the
same entry. It compares the name and provider to see if that is the SDT note we
are looking for. If yes, find out the file that contains this SDT note. There is
a file_ptr pointer embedded in this note which points to the struct file_sdt_ent
contained in the file_hash. From "file_sdt_ent" we will find out the file name.
Convert this sdt note into a perf event and then write this into uprobe_events
file to be able to record the event.
Then, corresponding entries are added to uprobe_events file for
the SDT events.
After recording is done, these events are silently deleted from uprobe_events
file. The uprobe_events file is present in debugfs/tracing directory.
To support the addition and deletion of SDT events to/from uprobe_events
file, a record_sdt struct is maintained which has the event data.
OK, I have some comments on this.
An example usage:
# ./perf record -e %user_app:fun_start -aR /home/user_app
At first, I'd like to add SDT support for adding probes too, like below;
./perf probe -a '%user_app:fun_start $vars'
But I think, previously we discussed that we won't be having "perf
probe" for SDT events.
We list them and probe/trace them using "perf record" directly.
So, maybe we don't need to remove the SDT-based events silently, nor
hide it from users. I think you just need to add new sdt events and
verify it if there is.
BTW, for silently adding event, I'll introduce --quite(-q) option for
perf probe. So you'll just need to set silent flag with that.
Thank you,
--
Thanks,
Hemant Kumar