Re: [PATCH perf/core v4 14/19] perf buildid-cache: Scan and import user SDT events to probe cache

[Masami Hiramatsu]

On Wed, 27 Apr 2016 12:28:16 -0300
Arnaldo Carvalho de Melo <acme@xxxxxxxxxx> wrote:

> Em Wed, Apr 27, 2016 at 08:49:08PM +0530, Hemant Kumar escreveu:
> > 
> > 
> > On 04/26/2016 02:34 PM, Masami Hiramatsu wrote:
> > >From: Masami Hiramatsu <masami.hiramatsu.pt@xxxxxxxxxxx>
> > >
> > >perf buildid-cache --add <binary> scans given binary and add
> > >the SDT events to probe cache. "sdt_" prefix is appended for
> > >all SDT providers to avoid event-name clash with other pre-defined
> > >events. It is possible to use the cached SDT events as other cached
> > >events, via perf probe --add "sdt_<provider>:<event>=<event>".
> > >
> > >e.g.
> > >   ----
> > >   # perf buildid-cache --add /lib/libc-2.17.so
> > >   # perf probe --cache --list | head -n 5
> > >   /usr/lib/libc-2.17.so (a6fb821bdf53660eb2c29f778757aef294d3d392):
> > >   sdt_libc:setjmp=setjmp
> > >   sdt_libc:longjmp=longjmp
> > >   sdt_libc:longjmp_target=longjmp_target
> > >   sdt_libc:memory_heap_new=memory_heap_new
> > >   # perf probe -x /usr/lib/libc-2.17.so \
> > >     -a sdt_libc:memory_heap_new=memory_heap_new
> > >   Added new event:
> > >     sdt_libc:memory_heap_new (on memory_heap_new
> > >    in /usr/lib/libc-2.17.so)
> > >
> > >   You can now use it in all perf tools, such as:
> > >
> > >           perf record -e sdt_libc:memory_heap_new -aR sleep 1
> > >
> > >   # perf probe -l
> > >     sdt_libc:memory_heap_new (on new_heap+183 in /usr/lib/libc-2.17.so)
> > >   ----
> > 
> > Patch looks good to me. Have a few questions below :
> > 
> > What about the same binary path having different build-ids. For e.g
> > a binary say, "test" has some markers, we add it to the cache. And,
> > then the file gets rebuilt again with different build-id now. And we try
> > to add to the cache again. It shows multiple entries in the cache :
> > # perf probe --cache --list
> > /home/hemant/test (157380727e2b3854395aa915dfc91dbccc02058b):
> > sdt_user_test:marker1=marker1
> > /home/hemant/test (64c0a018636e6d5145b09fc65839c1a4a7899f18):
> > sdt_user_test:marker1=marker1
> > /home/hemant/test (c9e34759ae95b68fa385831041c5d9e0dd1697fb):
> > sdt_user_test:marker1=marker1
> > ...
> > 
> > But, perf list sdt shows only one entry (which it should) :
> > # perf list sdt
> > 
> > List of pre-defined events (to be used in -e):
> > 
> >   sdt_user_test:marker1                                [SDT event]
> > 
> > perf probe also works as expected :
> > # perf probe -x /home/hemant/test %sdt_user_test:marker1
> > Added new event:
> >   sdt_user_test:marker1  (on %marker1 in /home/hemant/test)
> > 
> > You can now use it in all perf tools, such as:
> > 
> >     perf record -e sdt_user_test:marker1 -aR sleep 1
> > 
> > So, the question is, do we delete the previous entries for "test" from
> > the cache once we get a newer version of "test"?
> 
> No, we shouldn't, since those entries may be used for other tasks that
> involves using the exact DSO used for a particular perf.data session.

Agreed. And you can do it by using perf buildid-cache as below;

# perf buildid-cache --purge /home/hemant/test
# perf buildid-cache --add /home/hemant/test

This actually removes all old binary caches, and it maybe not
what you want. Or, you can try removing cached events too.

# perf probe --cache -d %sdt_user_test:\*
# perf buildid-cache --add /home/hemant/test

> Humm, but you are talking about what cache? The "probe cache" or the
> "build-id cache"? My previous statement was about the build-id cache.
> 
> For the probe cache, humm, probably we want to keep it as well, we may
> have moved that 'test' file to some other place, renamed it, etc, but it
> continues being accessible by its content-based identifier (the
> build-id) and could be used in ways we don't envision right now.
> 
> I.e. the same principle used for the build-id cache should be used for
> this probe cache, where we store things by build-id.
> 
> We need to prune this from time to time and for this we have:
> 
>   perf buildid-cache purge
> 
> But that right now is unflexible, we should have a way to ask to control
> how much is purged :-\

Agreed, current buildid-cache --update does just rescan current
binary, but maybe it should also remove old caches.

> > [SNIP]
> > >+	list_for_each_entry(note, &sdtlist, note_list) {
> > >+		ret = snprintf(sdtgrp, 64, "sdt_%s", note->provider);
> > >+		if (ret < 0)
> > >+			break;
> > >+		/* Try to find same-name entry */
> > >+		entry = probe_cache__find_by_name(pcache, sdtgrp, note->name);
> > 
> > Wouldn't it be better to compare the build-id rather than the event
> > name? So, if there is a new sdt event added to a binary, its build-id will
> > change. And, if there is no change, the build-id remains the same.

This is not such purpose, but just for folding same name SDTs(but different
addresses) on same binary.

> > 
> > Only if there is a change in the build-id, we can go for searching the
> > event name. This two level check can help optimizing the search.

That have been done in build-id.c :)

Thank you,

-- 
Masami Hiramatsu <mhiramat@xxxxxxxxxx>