Re: [PATCH v4 5/5] perf/sdt: Add support to perf record to trace SDT events

[Namhyung Kim <namhyung at kernel dot org>]

Hi Hemant,

As you know, you need to keep an eye on how (kprobes) event cache
patchset from Masami settles down.  For those who aren't CC'ed, please
see the link below:

  https://lkml.org/lkml/2014/10/31/207


On Sun, 02 Nov 2014 16:26:28 +0530, Hemant Kumar wrote:
> This patch adds support to perf to record SDT events. When invoked,
> the SDT event is looked up in the sdt-cache. If its found, an entry is
> made silently to uprobe_events file and then recording is invoked, and
> then the entry for the SDT event in uprobe_events is silently discarded.
>
> The SDT events are already stored in a cache file
> (/var/cache/perf/perf-sdt-file.cache).
> 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.

It likely to be ended up with per-file or per-buildid cache files under
~/.debug directory.  In this case we also need to have the (central)
event-to-cache table anyway IMHO.


>
> 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.

Isn't it enough just to use provide name?  I guess the provider names
are (should be?) unique among a system although there's no absolute
guarantee for that.


>
>             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.
>
> An example usage:
>
> # perf record -e %libc:setjmp -aR sleep 10
> [ perf record: Woken up 1 times to write data ]
> [ perf record: Captured and wrote 0.277 MB perf.data (~12103 samples) ]
>
> # perf report --stdio
> # To display the perf.data header info, please use --header/--header-only
> #
> # Samples: 1  of event 'libc:setjmp'
> # Event count (approx.): 1
> #
> # Overhead  Command  Shared Object  Symbol
> # ........  .......  .............  ...............
> #
>    100.00%  sleep    libc-2.16.so   [.] __sigsetjmp
>
>
> Signed-off-by: Hemant Kumar <hemant@linux.vnet.ibm.com>


[SNIP]
> +/* Session specific to SDT tracing */
> +struct record_sdt {
> +	bool    sdt;    /* is this SDT event tracing? */
> +	char    *str;   /* hold the event name */
> +} rec_sdt;
> +
> +int trace_sdt_event(const char *str)
> +{
> +	int ret = 0;
> +
> +	rec_sdt.sdt = true;
> +	rec_sdt.str = strdup(str);
> +	if (!rec_sdt.str)
> +		return -ENOMEM;
> +	ret = event_hash_list__lookup(str);
> +	return ret;
> +}

Hmm.. does it support multiple SDT events recorded in a session like:

  # perf record -e %libc:setjmp -e %libc:longjmp -aR sleep 10

?


[SNIP]
> +/* Parse an SDT event */
> +static int parse_perf_sdt_event(struct perf_sdt_event *sev,
> +				struct perf_probe_event *pev)
> +{
> +	struct perf_probe_point *pp = &pev->point;
> +
> +	pev->uprobes = true;
> +	pev->sdt = true;
> +	pev->event = strdup(sev->note->name);
> +	if (pev->event == NULL)
> +		return -ENOMEM;
> +	pev->group = strdup(sev->note->provider);
> +	if (pev->event == NULL)

s/event/group/

> +		return -ENOMEM;
> +
> +	pp->file = strdup(sev->file_name);
> +	if (pp->file == NULL)
> +		return -ENOMEM;
> +
> +	pp->function = strdup(sev->note->name);

Missing NULL check.  Also you need to free prior allocations in case of
error.

Thanks,
Namhyung


> +	pp->offset = sev->note->addr.a64[0];
> +	return 0;
> +}